10/09/2007
Anybody say Amen!
When you see it!
9/14/2007
(from SQLMAG) Avoid Red Zone
Have your customers or managers ever asked you how much their databases grew during the past year? Have you needed to plan how much disk capacity you'll need for the next year based on your database's average growth rate during the past 12 months? How long will your existing unallocated disk space last based on your current growth rate? To answer these kinds of database-growth questions or similar disk-space questions, you need some historical space-usage information about your databases. I've developed a process that you can use to automatically collect space-usage statistics for each of your databases. You can then use the collected space information to perform a simple growth-rate calculation.
Several months ago, I decided to build a process to capture space-usage information for each database on a system so that I could track disk-space consumption over time. I wanted to find the amount of space allocated and used for both the data and the log files. I was looking for the same information that you see in Enterprise Manager's Database Details pane when you're viewing Space Allocated information, but I needed the information to be available to T-SQL code. Using SQL Server Profiler, I discovered that Enterprise Manager obtains space-allocated information by using two DBCC statements. One of the statements, SQLPERF, is documented; the other DBCC statement, SHOWFILESTATS, isn't. By manually running DBCC SHOWFILESTATS on each database and comparing the output with what Enterprise Manager displayed, I determined that this command would provide me used disk space information by database.
Both SQL Server 2000 and SQL Server 7.0 use these DBCC statements to populate Enterprise Manager's Space Allocated display. The DBCC SQLPERF(LOGSPACE) statement returns transaction log space information—the allocated log size for each database in megabytes and the percentage of log space used for each database—for all databases. With some simple math, you can easily convert the percentage of log space used into megabytes. This DBCC statement helped me obtain the log file space information I wanted to track.
I used the undocumented DBCC SHOWFILESTATS statement, which returns space-usage information for one database's data, to obtain the rest of the disk-space statistics I wanted. This statement returns one record per physical data file. Each statistics record returned appears in six columns: Fileid, FileGroup, TotalExtents, UsedExtents, Name, and FileName. You can use the TotalExtents column to determine the total space allocated to data and the UsedExtents column to determine the total space used for data. By summing the TotalExtents and UsedExtents values of all files within a database, then converting the number of extents into megabytes, I calculated the total space allocated and total space used for data. These calculations gave me the data space usage information I wanted to track over time. Figure 1 shows sample output of the DBCC SHOWFILESTATS command after you run it against the master database.
I've built these two DBCC statements into a process that automatically collects space information by database. This process runs periodically and saves space-usage statistics in a database table. The process consists of a SQL Server Agent job that contains two steps. The first step executes a stored procedure named usp_get_dbstats, which generates a T-SQL script. The resulting script consists of a DBCC SQLPERF(LOGSPACE) statement to gather the log information for all databases, a DBCC SHOWFILESTATS statement for each database, and some code to manipulate the DBCC data into the right format for saving the space-usage information in. The second step executes the T-SQL script that the first step generates. After extracting the space-usage information from SQL Server and formatting the data, this script populates a permanent database table with the current data and log space-usage information. You can then use this permanent table to answer a wealth of disk space allocation questions.
This process of gathering space-usage statistics is an example of using T-SQL code to generate T-SQL code. I used this two-step process to minimize the complexity of writing a stored procedure that would need to issue a USE statement to let me run the DBCC SHOWFILESTATS command against each database. Now, let's look at my homegrown disk-space collection process in a little more detail.
The Stored Procedure
The usp_get_dbstats stored procedure, which Listing 1 shows, is the main body of the space-usage statistics-gathering process. The stored procedure queries the system tables and programmatically generates and executes PRINT statements to produce a T-SQL script that, when executed, uses two DBCC statements to extract current space-usage information. Let's walk through this stored procedure one section at a time.
The code at callout A in Listing 1 gathers the log-space usage information. This block of code, like the others in Listing 1, dynamically generates and executes a series of PRINT statements that become the T-SQL script that gathers the space-usage statistics. The code at callout A produces a set of T-SQL statements that create a temporary table called #tmplg, then populates the table with the output from DBCC SQLPERF(LOGSPACE). The INSERT INTO statement that has the EXECUTE option puts the DBCC statement's output into the #tmplg table, which will eventually contain one record for each database on the server. Each record will contain information that goes into columns labeled DBName, LogSize, LogSpaceUsed, and Status. You can find the definitions of each of these columns in SQL Server Books Online (BOL) under the heading "DBCC SQLPERF."
Callout B shows the code that creates the #tmp_stats temporary table. Each record in this table will hold both the data and log space-usage information for a database. Later code blocks will populate and use this temporary table. This section of the code executes a series of PRINT statements to append to the T-SQL script that the code at callout A started.
The code at callout C generates the DBCC SHOWFILESTATS statement for each database. This chunk of code also generates the T-SQL statements that merge the DBCC SHOWFILESTATS information with records in the #tmplg table for each database to produce one record per database containing data and log disk-space statistics. Again, the code uses PRINT statements to generate T-SQL code that will gather DBCC SHOWFILESTATS information.
Remember that the DBCC SQLPERF statement generates log-size information for all databases, whereas the DBCC SHOWFILESTATS statement gathers the data sizes for only the current database. This scope limitation of the DBCC SHOWFILESTATS command requires that the stored procedure generate code that will execute the DBCC SHOWFILESTATS statement against each database on the SQL Server box.
The code at callout C uses a cursor, AllDatabases, to hold a list of the databases on the system. This cursor lets the stored procedure iterate through the list of databases inside a WHILE loop to generate a DBCC SHOWFILESTATS statement for each database. Inside the WHILE loop, the stored procedure generates code to create a temporary table, #tmp_sfs, to hold the output of the DBCC SHOWFILESTATS statement that's followed by an INSERT INTO statement. Again, I used the INSERT INTO statement with the EXECUTE option to insert the DBCC SHOWFILESTATS statement information into the temporary table. The last several lines of code in this section generate the code that will convert the LogSpaceUsed column in table #tmplg from a percentage of used space into a megabyte value. Then, the code populates the #tmp_stats table with current data and log space-usage statistics for the current database.
The code at callout D generates the T-SQL statements to put the data and log space-usage statistics into a permanent table. This section uses a simple INSERT INTO statement to populate a permanent table, DBSTATS, with the current calculated database space-usage statistics that the temporary table #tmp_stats holds.
Web Listing 1 shows a sample of what the T-SQL script would look like if you executed usp_get_dbstats on a server that had only a few databases. This output was produced on a system that had only the standard SQL Server installed databases (master, model, msdb, Northwind, Pubs, and tempdb), plus one user-defined database (DBA). Note that in this listing, one chunk of code collects transaction log space information by using the DBCC SQLPERF(LOGSIZE) command. Seven sections of similar code, one for each database, use the DBCC SHOWFILESTATS statement to gather data space-usage statistics.
The Permanent Table
The usp_get_dbstats stored procedure assumes that the space-usage statistics it gathers will be stored in a permanent table called DBSTATS. So before executing the T-SQL script that usp_get_dbstats generates, you'll need to create the DBSTATS table. Running the code that Listing 2, page 30, shows creates the DBSTATS permanent table that will hold all the historic database space-usage information. Records are appended to the DBSTATS table each time you execute the commands that usp_get_
dbstats generates.
In my shop, we have a DBA database that contains the DBSTATS table and the usp_get_dbstats stored procedure. If your shop has a database that your DBAs use to hold stored procedures and tables such as DBSTATS, you can change the default database at the beginning of usp_get_dbstats (variable @DBSTATS_DB) to a database appropriate to your site. Note that if you do change the default database name, you need to change the @DBSTATS_DB declaration to match the size of your database name.
The SQL Server Agent Job
You could manually execute the usp_get_dbstats stored procedure to generate the T-SQL script to gather the space-usage statistics, then copy the generated T-SQL script into Query Analyzer to get the current statistics into the DBSTATS table. However, this manual approach would quickly become boring and waste your valuable time. Instead of generating your statistics manually, I recommend that you build a SQL Server Agent job like the Get DBSTATS job that Figure 2 shows.
The Get DBSTATS SQL Server Agent job has two steps. The first step, which Figure 3 shows, uses the osql command to execute the usp_get_dbstats stored procedure. Using osql lets the second step place the output from usp_get_dbstats into a file for execution. The -o option tells the usp_get_dbstats stored procedure to write output to a file called c:\temp\get_dbstats.sql. This file is the T-SQL script that the second step of the SQL Server Agent job will execute.
The second step of Get DBSTATS, which Figure 4 shows, executes the statements that usp_get_dbstats generated, extracting and saving disk-space usage information. Figure 4 shows the osql command that executes the script that the first step produced. The input (-i) parameter feeds into the osql process the T-SQL script that the first step built.
In my shop, I've scheduled the SQL Server Agent job to run once a week so that I can capture the database space-usage statistics and monitor the growth of our databases week by week. You need to determine how frequently you should gather space-usage statistics for your environment. Capturing disk-space usage lets me perform several kinds of disk-space usage analysis. I can track monthly and yearly disk usage, both by individual databases and overall, and how much additional disk space was used when we migrated data related to a particular project.
Growth-Rate Calculation
If you don't have any disk-space usage information, predicting an average database growth rate is extremely difficult. After you've implemented a disk-usage collection method such as the one I've outlined, you have statistics available to help you calculate a database's average growth rate. I produce a simple Microsoft Excel chart monthly that tracks our disk-space usage over time.
Figure 5 shows the monthly disk-space usage for one of our production servers, SQLPROD1. This graph represents the amount of disk space that all our production databases on SQLPROD1 were using on different dates over a period of 7 months. Note that I recorded several spikes in the graph. Over time, I can associate the peaks and valleys with specific events that cause unusual growth in our database, so I can better predict growth rates for upcoming database work. In Figure 5, you can see when we added DB_TEST: The used space on server SQLPROD1 grew almost 3GB.
Although this graph represents disk-space usage statistics starting only in July 2001, getting a picture of the average disk-space growth rate for a more recent or longer period on this server is easy. I can determine the monthly growth rate by using the following simple formula:
MONTHLY_GROWTH_RATE = (SPACE_USED_END
- SPACE_USED_BEGIN) / NUMBER_OF_MONTHS
The amount of disk space occupied on July 1, 2001 (SPACE_USED_BEGIN), was 6.5GB. By February 4, 2002 (SPACE_USED_END), the used disk space had grown to 7.66GB. The number of months between the July and February data points is a little more than 7. According to this formula, the monthly growth rate for our SQLPROD1 box is a little more than 0.16GB per month. Now that I can calculate the monthly growth rate, based on statistics, I can predict the number of months before our database growth consumes our available free disk space and I'll have time to acquire more disk space in advance.
Calculating a monthly growth rate for our SQLPROD1 server would be impossible without collecting statistics over time. This homegrown solution, using documented and undocumented DBCC statements, meets my organization's needs. Other organizations might find they need to collect more historical space-usage information, such as space usage by tables within a database. Whether you acquire canned software to track space usage or choose a homegrown solution, gathering database-growth information over time can give you valuable insight into the growth patterns of your databases. Without historical growth-rate information, you have no way to adequately understand a database's disk usage. Knowing the current growth rate of each database will help you more accurately plan for future disk acquisitions.
Figure 2
Figure 3
Figure 4
Figure 5
Figure 1
| FIGURE 1: Sample Output of DBCC SHOWFILESTATS | |||||
| Fileid | FileGroup | TotalExtents | UsedExtents | Name | FileName |
| 1 | 1 | 171 | 168 | master | g:\mssql7\data\master.mdf |
| DBCC execution completed. If DBCC printed error messages, contact your system administrator. | |||||
Listing 1
LISTING 1: The usp_get_dbstats Stored Procedure
IF EXISTS (SELECT * FROM sysobjects WHERE id =
object_id(N'[dbo].[usp_get_dbstats]') AND
OBJECTPROPERTY(id, N'IsProcedure') = 1)
DROP PROCEDURE [dbo].[usp_get_dbstats]
GO
SET QUOTED_IDENTIFIER OFF SET ANSI_NULLS ON
GO
CREATE PROCEDURE usp_get_dbstats AS
DECLARE @DBSTATS_DB char(3)
SET @DBSTATS_DB = 'DBA'
-- Begin callout A
PRINT 'DECLARE @cmd nvarchar(1024) '
PRINT 'IF EXISTS (SELECT * FROM tempdb..sysobjects
WHERE id = object_id(N' + char(39) +
'[tempdb]..[#tmplg]' + char(39) + '))'
PRINT 'DROP TABLE #tmplg'
PRINT 'CREATE TABLE #tmplg'
PRINT '('
PRINT 'DBName varchar(32),'
PRINT 'LogSize real,'
PRINT 'LogSpaceUsed real,'
PRINT 'Status int'
PRINT ')'
PRINT 'SELECT @cmd = ' + char(39) + 'dbcc sqlperf (logspace)' + char(39)
PRINT 'INSERT INTO #tmplg EXECUTE (@cmd)'
-- End callout A
-- Begin callout B
PRINT 'IF EXISTS (SELECT * FROM tempdb..sysobjects
WHERE id = object_id(N' + char (39) +
'[tempdb]..[#tmp_stats]' + char(39 ) + '))'
PRINT 'DROP TABLE #tmp_stats'
PRINT 'CREATE TABLE #tmp_stats ('
PRINT 'totalextents int, '
PRINT 'usedextents int,'
PRINT 'dbname varchar(40),'
PRINT 'logsize real,'
PRINT 'logspaceused real'
PRINT ')'
PRINT 'go'--End callout B
--Begin callout C
DECLARE AllDatabases CURSOR FOR
SELECT name FROM master..sysdatabases
OPEN AllDatabases
DECLARE @DB nvarchar(128)
FETCH NEXT FROM AllDatabases INTO @DB
WHILE (@@FETCH_STATUS = 0)
BEGIN
PRINT 'USE [' + @DB + ']'
PRINT 'GO'
PRINT 'IF EXISTS (SELECT * FROM tempdb..sysobjects
WHERE id = object_id(N' + char(39)
+ '[tempdb]..[#tmp_sfs]' + char(39) + '))'
PRINT 'DROP TABLE #tmp_sfs'
PRINT 'CREATE TABLE #tmp_sfs ('
PRINT 'fileid int,'
PRINT 'filegroup int, '
PRINT 'totalextents int, '
PRINT 'usedextents int,'
PRINT 'name varchar(1024),'
PRINT 'filename varchar(1024)'
PRINT ')'
PRINT 'go'
PRINT 'DECLARE @cmd nvarchar(1024)'
PRINT 'SET @cmd=' + char(39) + 'DBCC SHOWFILESTATS' + char(39)
PRINT 'INSERT INTO #tmp_sfs EXECUTE(@cmd)'
PRINT 'DECLARE @logsize real '
PRINT 'DECLARE @logspaceused real '
PRINT 'SELECT @logsize= logsize FROM #tmplg
WHERE dbname = ' + char(39) + @DB +
char(39)
PRINT 'SELECT @logspaceused = (logsize*logspaceused)/100.0'
PRINT ' FROM #tmplg WHERE dbname = ' + char(39) + @DB + char(39)
PRINT 'SET @cmd = ' + char(39) + ' INSERT INTO #tmp_stats' + char(39) + ' +'
PRINT ' ' + char(39) + '(totalextents,usedextents,
dbname,logsize,logspaceused)' + char(39) +
' +'
PRINT ' ' + char(39) + ' SELECT SUM(totalextents),
SUM (usedextents),' + char(39) + ' +
char(39) + ' + char(39) + @DB + char(39) + '+ char(39) + '
+ char(39) + ',' + char(39) + ' + '
PRINT ' CAST(@logsize AS varchar) + ' + char(39) + ','
+ char(39) + ' + CAST (@logspaceused
AS varchar) +'
PRINT ' ' + char(39) + ' FROM #tmp_sfs' + char(39)
PRINT 'EXEC sp_executesql @cmd'
FETCH NEXT FROM AllDatabases INTO @DB
END --(@@FETCH_STATUS = 0)
--End callout C
--Begin callout D
PRINT 'INSERT INTO ' + @DBSTATS_DB + '.dbo.DBSTATS '
PRINT ' (RECORD_TYPE, DBNAME, DATA_SIZE, DATA_USED, LOG_SIZE, LOG_USED)'
PRINT ' SELECT 1,dbname,totalextents*64/1024 , usedextents*64/1024 ,'
PRINT ' logsize ,logspaceused FROM #tmp_stats'
--End callout D
CLOSE AllDatabases
DEALLOCATE AllDatabases
GO
SET QUOTED_IDENTIFIER OFF SET ANSI_NULLS ON
GO
Listing 2
LISTING 2: Code That Creates the Permanent Table DBSTATS
IF EXISTS (SELECT * FROM sysobjects WHERE id =
object_id(N'[dbo].[DBSTATS]') AND
OBJECTPROPERTY(id, N'IsUserTable') = 1)
DROP TABLE [dbo].[DBSTATS]
GO
CREATE TABLE [dbo].[DBSTATS] (
[ID] [int] IDENTITY (1, 1) NOT NULL ,
[RECORD_TYPE] [int] NOT NULL ,
[DBNAME] [char] (50) NOT NULL ,
[DATA_SIZE] [decimal](9, 2) NULL ,
[DATA_USED] [decimal](9, 2) NULL ,
[LOG_SIZE] [decimal](9, 2) NULL ,
[LOG_USED] [decimal](9, 2) NULL ,
[STAT_DATE] [datetime] NULL
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[DBSTATS] WITH NOCHECK ADD
CONSTRAINT [DF_DBSTATS_STAT_DATE] DEFAULT
(getdate()) FOR [STAT_DATE]
GO
9/13/2007
转载 --500强企业面试时 的英语提问
500强企业面试时，常用英语提出问题。宏威职业顾问经过归纳总结，将500强面试的问题基本上是按前面我们所说的能力素质模型，分成3类16条，每一条里有几个不同的问题，大体如下：
一．分析判断能力：
1．分析能力：
1) 请给出一个事例，表明你在面对情况非常复杂的局面的时候是如何分析和评估的。
Tell me about a complex situation which you have had to analyze and assess.
2) 当你面对一个有矛盾冲突的问题时，你会怎么做？
When you have been faced with conflicting information about a problem, what have you done?
3) 请列举一个你在工作中面对一种微妙而又困难的局面时，能够成功地保持客观的分析能力的例子。
Tell me about an occasion where you successfully maintained your objectivity in addressing a sensitive and difficult situation at work.
2．市场敏感度：
1) 请给出一个你满足消费者或客户需求变化的例子。
Tell me about a time when you have addressed a change in consumers’ or customers’ needs.
2) 你最近的一次创新活动是什么？
What is the most recent initiative you have involved?
3) 是否曾经将一些想法或实践方法从外部世界中带入到你的工作或学习中去？
What ideas or practices have you brought into your work/ study from the external world?
3．创造力：
1) 请给出一个你用创造性的方案解决企业问题的例子。
Tell me about creative solutions to business problems that you have come up with.
2) 请描述一下你最近创新或新开发的事物。
Describe a recent innovation or new development that you have introduced.
3) 是否可以给出一个你提供给其他同事的不同的方式方法或看待问题的角度的例子？
Can you give me an example of when you had a different approach or business perspective to your colleagues?
4．清晰的目的性：
1) 请给出一个你成功地解决一件非常复杂的工作的事例，并说明你是如何确定哪些工作需要完成的。
Tell me about a time when you successfully tacked complex work issue and how you worked out what needed to be done.
2) 什么时候你处理过一个在最终期限前你无法完成的项目？
When have you worked on a project where the deadline couldn’t be met?
3) 请给出一个你必须在同一时间完成很多项任务的事例。
Tell me about a time when you have had to manage a large number of activities at the same time.
5．学习能力：
1) 请给出一个最近你从别人的错误中有所学习的例子。
Tell me about the last time you learned from someone else’s mistake.
2) 请给出一个最近你从事的项目或任务的例子，在该项目或任务中，（最起码刚开始）的表现不尽人意。
Please describe a recent task or project in which, at least initially, your performance was less than satisfactory.
3) 你认为你最需要的专业发展的方面是什么？
What do you believe are the areas in which you require the most professional development?
6．结构化的思维能力：
1) 某快速消费品公司最近遇到了利润下降的问题，请你分析一下可能的原因。
The profit of a FMCG company decreases recently. Please analyze possible reasons of the profit decrease.
2) 某美国地铁公司希望投资中国地铁，希望五年可收回投资，请你做一个可行性分析。
A certain US based Subway Company planed to invest on Subway of China, and they hope the investment can be returned in five years. Please conduct a feasibility analysis.
二．人际交往能力：
1．领导能力：
1) 请给出一个你必须展示你的领导能力的例子。
Tell me of an occasion where you had to demonstrate business leadership.
2) 什么情况下，你会不得不出面对职责进行进一步的界定或解决团队的问题，以保持绩效水平？
When have you had to intervene to clarify roles or resolve team issues to obtain performance ?
3) 你怎样运用目标和目的来驱动团队达到卓越的绩效？
How have you used targets or goals to drive outstanding performance?
2．沟通影响力：
1) 请给出一个最近你培训和指导他人的例子。
Tell me of a recent occasion when you contributed to the training or education of another person.
2) 你具备哪些辅导或给予他人反馈的经验？
What experience in coaching and giving feedback have you had ?
3) 你为下属创造过什么样的学习或发展机会？
What development or learning opportunities have you created ?
3．团队合作能力：
1) 在与你最为亲密的同事相处时，为了确保团队目标能够完成，你在其中扮演什么角色？
Amongst your immediate colleagues , what part do you play in ensuring that the team meets its objectives ?
2) 请给出一个你在非常有效的团队内工作的例子。
Tell me about a time when you worked in a really effective team.
3) 什么时候你的目标和你的团队的目标不是很吻合？
When have your objectives been at odds with those of the team?
4．客户服务能力
1) 请给出一个你主动了解客户需求，从而提供服务并获得认可的例子。
Give me a case that you proactively worked out customers’ needs and your service finally gained customers’ recognition.
2) 请给出一个你虽然遇到困难但仍然有效为客户解决问题的事例。
Tell me about a case that you effectively solved customers’ problem although you met difficulties when solving the problem.
三．工作态度：
1．开拓能力：
1) 请给出一个你面对非常具有挑战性的目标，但是仍然通过个人的努力最终达成目标的例子。
Give me a case that you got a very challenging target and you finally achieved the target through your hard work.
2) 请给出一个你改进现有工作方法或流程的事例。
Tell me about a case that you improved the current work process.
2．诚信正直：
1) 请给出一个你坚持你认为正确的事情的例子。
Tell me about a time when you stood up for what you believed was the right thing to do .
2) 你是怎样理解职业道德标准的？
What do you understand by business ethics?
3) 你是否在某种情况下有理由挑战你的老板，甚至老板的老板？
Have you ever had course to challenge your bosses or bosses’ boss?
3．职业化的行为：
1) 请给出一个你与客户发生冲突的例子，你是如何处理的？
Tell a case that you have conflicts with clients, how did you deal with it?
2) 请给出一个你认为你的客户或同事的想法不值一提的例子。
Please describe an example that you feel that your colleagues or clients’ thinking doesn’t make any sense.
3) 在什么情况下，你会愿意改变你的想法？
In what kind of situation will you decide to change your mind?
4．高效的工作能力：
1) 如果某件事情预计需要三天的时间，而其实只需要一天就能完成，你会如何处理？
What arrangement will you make if you have 3 days to do something while you only need 1 day to complete it?
2) 请给出一个你管理项目的例子。
Please describe an example that you manage a project.
5．计划与自我管理能力：
1) 请给出一个你同时面临几项工作，但经过合理规划最终有效完成的事例。
Tell me about a case that you handled several projects in the same time but you successfully completed all your work through effective planning.
2) 你在工作中拥有哪些资源？你怎样合理规划利用这些资源？
What kind of resources do you have in your work? How do you effectively plan and utilize these resources?
6．充满工作激情：
1) 请给出一个最近你运用创新的方法来改善工作绩效的事例。
Tell me about your recent initiatives to improve work results.
2) 你在工作中遇到过什么样的挫折？
What have you faced a major setback in your work?
3) 你上一次在工作中进行的一个权衡利弊后冒的风险是什么？
What was the last calculated risk you took at work?
8/25/2007
有用的而且没有文档的dbcc命令
Some Useful Undocumented SQL Server 7.0 and 2000 DBCC Commands
By : Alexander Chigrik
In this article, I want to tell you about some useful undocumented DBCC commands, and how you can use these commands in SQL Server 7.0 and 2000 for administering and monitoring.DBCC is an abbreviation for Database Console Command. DBCC commands are generally used to check the physical and logical consistency of a database, although they are also used for a variety of miscellaneous tasks, as you will see here.
Note, the command:
DBCC TRACEON (3604)
is issued before each of the following DBCC examples in order to better demonstrate the effects of the command by displaying a trace of the output of the DBCC command. It is not actually required to run the DBCC commands examined below. If you run any of the DBCC commands below without the above option, the command runs, but you don't see what it is doing.
DBCC BUFFER This command can be used to display buffer headers and pages from the buffer cache.Syntax:dbcc buffer ([dbiddbname] [,objidobjname] [,nbufs], [printopt])
where:
dbiddbname - database iddatabase name
objidobjname - object idobject name
nbufs - number of buffers to examine
printopt - print option, which includes:
0 - print out only the buffer header and page header (default) 1 - print out each row separately and the offset table 2 - print out each row as a whole and the offset table
This is an example:DBCC TRACEON (3604)DBCC buffer(master,'sysobjects')
DBCC BYTES This command can be used to dump out bytes from a specific address.Syntax:
dbcc bytes (startaddress, length)
where:
startaddress - starting address to dump
length - number of bytes to dumpThis is an example:DBCC TRACEON (3604)DBCC bytes (10000000, 100)
DBCC DBINFO Displays DBINFO structure for the specified database.
Syntax:
DBCC DBINFO [(dbname)]
where:
dbname - is the database nameThis is an example:DBCC TRACEON (3604)DBCC DBINFO (master)
DBCC DBTABLE This command displays the contents of the DBTABLE structure.Syntax:
DBCC DBTABLE ({dbiddbname})
where:
dbiddbname - database name or database IDThis is an example:DBCC TRACEON (3604)DBCC DBTABLE (master)
The DBTABLE structure has an output parameter called dbt_open. This parameter keeps track of how many users are in the database.
DBCC DESPrints the contents of the specified DES (descriptor).Syntax:
dbcc des [([dbiddbname] [,objidobjname])]
where:
dbiddbname - database id or the database nameobjidobjname - object id or the object nameThis is an example:DBCC TRACEON (3604)DBCC DES
DBCC HELP DBCC HELP returns syntax information for the specified DBCC statement. In comparison with DBCC HELP command in version 6.5, it returns syntax information only for the documented DBCC commands.Syntax:DBCC HELP ('dbcc_statement' @dbcc_statement_var '?')
This is an example:DBCC TRACEON (3604)DECLARE @dbcc_stmt sysnameSELECT @dbcc_stmt = 'CHECKTABLE'DBCC HELP (@dbcc_stmt)
DBCC IND Shows all pages in use by indexes of the specified table.Syntax:dbcc ind(dbiddbname, objidobjname, printopt = {-2-10123})
where:
dbiddbname - database id or the database name
objidobjname - object id or the object name
printopt - print optionThere is change in this command in how it is used in SQL Server 7.0, in that the printopt parameter is now no longer optional.This is an example:DBCC TRACEON (3604)DBCC IND (master, sysobjects, 0)
DBCC LOGThis command is used to view the transaction log for the specified database.Syntax:DBCC log ({dbiddbname}, [, type={-101234}])
where:
dbid or dbname - Enter either the dbid or the name of the databasetype - is the type of output, and includes these options:0 - minimum information (operation, context, transaction id)1 - more information (plus flags, tags, row length, description)2 - very detailed information (plus object name, index name, page id, slot id)3 - full information about each operation4 - full information about each operation plus hexadecimal dump of the current transaction log's row.-1 - full information about each operation plus hexadecimal dump of the current transaction log's row, plus Checkpoint Begin, DB Version, Max XDESIDby default, type = 0To view the transaction log for the master database, run the following command:DBCC log (master)
DBCC PAGE You can use this command to view the data page structure.
Syntax:
DBCC PAGE ({dbiddbname}, pagenum [,print option] [,cache] [,logical])
where:
dbiddbname - Enter either the dbid or the name of the databasepagenum - Enter the page number of the SQL Server page that is to be examinedprint option - (Optional) Print option can be either 0, 1, or 2
0 - (Default) This option causes DBCC PAGE to print out only the page header information.
1 - This option causes DBCC PAGE to print out the page header information, each row of information from the page, and the page's offset table. Each of the rows printed out will be separated from each other.
2 - This option is the same as option 1, except it prints the page rows as a single block of information rather than separating the individual rows. The offset and header will also be displayed.
cache - (Optional) This parameter allows either a 1 or a 0 to be entered
0 - This option causes DBCC PAGE to retrieve the page number from disk rather than checking to see if it is in cache.
1 - (Default) This option takes the page from cache if it is in cache rather than getting it from disk only.
logical - (Optional) This parameter is for use if the page number that is to be retrieved is a virtual page rather then a logical page. It can be either 0 or 1.
0 - If the page is to be a virtual page number.
1 - (Default) If the page is the logical page number.
This is an example:DBCC TRACEON (3604)DBCC PAGE (master, 1, 1)
DBCC PROCBUFThis command displays procedure buffer headers and stored procedure headers from the procedure cache.Syntax:DBCC procbuf([dbiddbname], [objidobjname], [nbufs], [printopt = {01}])
where:
dbiddbname - database id or the database name
objidobjname - object id or the object name
nbufs - number of buffers to print
printopt - print option
(0 - print out only the proc buff and proc header (default), 1 - print out proc buff, proc header, and contents of buffer)This is an example:DBCC TRACEON (3604)DBCC procbuf(master,'sp_help',1,0)
DBCC PRTIPAGEThis command prints the page number pointed to by each row on the specified index page.Syntax:DBCC prtipage(dbid, objid, indexid, indexpage)
where:
dbid - database ID
objid - object ID
indexid - index ID
indexpage - the logical page number of the index page to dump
This is an example:DBCC TRACEON (3604)DECLARE @dbid int, @objectid intSELECT @dbid = DB_ID('master')SELECT @objectid = object_id('sysobjects') DBCC prtipage(@dbid,@objectid,1,0)
DBCC PSSThis command shows info about processes currently connected to the server.Syntax:DBCC pss(suid, spid, printopt = { 1 0 })
where:
suid - server user ID
spid - server process ID
printopt - print option (0 standard output, 1 all open DES's and current sequence tree)This is an example:DBCC TRACEON (3604)DBCC pss
DBCC RESOURCEThis command shows the server's level RESOURCE, PERFMON and DS_CONFIG information. RESOURCE shows addresses of various data structures used by the server. PERFMON structure contains master..spt_monitor field info. DS_CONFIG structure contains master..syscurconfigs field information.Syntax:
DBCC resource
This is an example:DBCC TRACEON (3604)DBCC resource
DBCC TAB You can use the following undocumented command to view the data pages structure (in comparison with DBCC PAGE, this command will return information about all data pages for viewed table, not only for particular number)Syntax:DBCC tab (dbid, objid)
where:
dbid - is the database id
objid - is the table idThis is an example:DBCC TRACEON (3604)DECLARE @dbid int, @objectid intSELECT @dbid = DB_ID('master')SELECT @objectid = object_id('sysdatabases')DBCC TAB (@dbid,@objectid)
Published with the explicit written permission of the author. Copyright 200
8/15/2007
Oracle interview questions and answers
1. Describe the difference between a procedure, function and anonymous pl/sql block. Level: Low
Expected answer : Candidate should mention use of DECLARE statement, a function must return a value while a procedure doesn't have to.
2. What is a mutating table error and how can you get around it?
Level: Intermediate
Expected answer: This happens with triggers. It occurs because the trigger is trying to update a row it is currently using. The usual fix involves either use of views or temporary tables so the database is selecting from one while updating the other.
3. Describe the use of %ROWTYPE and %TYPE in PL/SQL
Level: Low
Expected answer: %ROWTYPE allows you to associate a variable with an entire table row. The %TYPE associates a variable with a single column type.
4. What packages (if any) has Oracle provided for use by developers?
Level: Intermediate to high
Expected answer: Oracle provides the DBMS_ series of packages. There are many which developers should be aware of such as DBMS_SQL, DBMS_PIPE, DBMS_TRANSACTION, DBMS_LOCK, DBMS_ALERT, DBMS_OUTPUT, DBMS_JOB, DBMS_UTILITY, DBMS_DDL, UTL_FILE. If they can mention a few of these and describe how they used them, even better. If they include the SQL routines provided by Oracle, great, but not really what was asked.
5. Describe the use of PL/SQL tables
Level: Intermediate
Expected answer: PL/SQL tables are scalar arrays that can be referenced by a binary integer. They can be used to hold values for use in later queries or calculations. In Oracle they will be able to be of the %ROWTYPE designation, or RECORD.
6. When is a declare statement needed ?
Level: Low
The DECLARE statement is used in PL/SQL anonymous blocks such as with stand alone, non-stored PL/SQL procedures. It must come first in a PL/SQL stand alone file if it is used.
7. In what order should a open/fetch/loop set of commands in a PL/SQL block be implemented if you use the %NOTFOUND cursor variable in the exit when statement? Why?
Level: Intermediate
Expected answer: OPEN then FETCH then LOOP followed by the exit when. If not specified in this order will result in the final return being done twice because of the way the %NOTFOUND is handled by PL/SQL.
8. What are SQLCODE and SQLERRM and why are they important for PL/SQL developers?
Level: Intermediate
Expected answer: SQLCODE returns the value of the error number for the last error encountered. The SQLERRM returns the actual error message for the last error encountered. They can be used in exception handling to report, or, store in an error log table, the error that occurred in the code. These are especially useful for the WHEN OTHERS exception.
9. How can you find within a PL/SQL block, if a cursor is open?
Level: Low
Expected answer: Use the %ISOPEN cursor status variable.
10. How can you generate debugging output from PL/SQL?
Level:Intermediate to high
Expected answer: Use the DBMS_OUTPUT package. Another possible method is to just use the SHOW ERROR command, but this only shows errors. The DBMS_OUTPUT package can be used to show intermediate results from loops and the status of variables as the procedure is executed. The new package UTL_FILE can also be used.
11. What are the types of triggers?
Level:Intermediate to high
Expected Answer: There are 12 types of triggers in PL/SQL that consist of combinations of the BEFORE, AFTER, ROW, TABLE, INSERT, UPDATE, DELETE and ALL key words: BEFORE ALL ROW INSERT AFTER ALL ROW INSERT BEFORE INSERT AFTER INSERT etc.
[Edit section] DBA
1. Give one method for transferring a table from one schema to another:
Level:Intermediate
Expected Answer: There are several possible methods, export-import, CREATE TABLE... AS SELECT, or COPY.
2. What is the purpose of the IMPORT option IGNORE? What is it's default setting?
Level: Low
Expected Answer: The IMPORT IGNORE option tells import to ignore "already exists" errors. If it is not specified the tables that already exist will be skipped. If it is specified, the error is ignored and the tables data will be inserted. The default value is N.
3. You have a rollback segment in a version 7.2 database that has expanded beyond optimal, how can it be restored to optimal?
Level: Low
Expected answer: Use the ALTER TABLESPACE ..... SHRINK command.
4. If the DEFAULT and TEMPORARY tablespace clauses are left out of a CREATE USER command what happens? Is this bad or good? Why?
Level: Low
Expected answer: The user is assigned the SYSTEM tablespace as a default and temporary tablespace. This is bad because it causes user objects and temporary segments to be placed into the SYSTEM tablespace resulting in fragmentation and improper table placement (only data dictionary objects and the system rollback segment should be in SYSTEM).
5. What are some of the Oracle provided packages that DBAs should be aware of? Level: Intermediate to High
Expected answer: Oracle provides a number of packages in the form of the DBMS_ packages owned by the SYS user. The packages used by DBAs may include: DBMS_SHARED_POOL, DBMS_UTILITY, DBMS_SQL, DBMS_DDL, DBMS_SESSION, DBMS_OUTPUT and DBMS_SNAPSHOT. They may also try to answer with the UTL*.SQL or CAT*.SQL series of SQL procedures. These can be viewed as extra credit but aren?t part of the answer.
6. What happens if the constraint name is left out of a constraint clause?
Level: Low
Expected answer: The Oracle system will use the default name of SYS_Cxxxx where xxxx is a system generated number. This is bad since it makes tracking which table the constraint belongs to or what the constraint does harder.
7. What happens if a tablespace clause is left off of a primary key constraint clause?
Level: Low
Expected answer: This results in the index that is automatically generated being placed in then users default tablespace. Since this will usually be the same tablespace as the table is being created in, this can cause serious performance problems.
8. What is the proper method for disabling and re-enabling a primary key constraint?
Level: Intermediate
Expected answer: You use the ALTER TABLE command for both. However, for the enable clause you must specify the USING INDEX and TABLESPACE clause for primary keys.
9. What happens if a primary key constraint is disabled and then enabled without fully specifying the index clause?
Level: Intermediate
Expected answer: The index is created in the user?s default tablespace and all sizing information is lost. Oracle doesn?t store this information as a part of the constraint definition, but only as part of the index definition, when the constraint was disabled the index was dropped and the information is gone.
10. (On UNIX) When should more than one DB writer process be used? How many should be used?
Level: High
Expected answer: If the UNIX system being used is capable of asynchronous IO then only one is required, if the system is not capable of asynchronous IO then up to twice the number of disks used by Oracle number of DB writers should be specified by use of the db_writers initialization parameter.
11. You are using hot backup without being in archivelog mode, can you recover in the event of a failure? Why or why not?
Level: High
Expected answer: You can?t use hot backup without being in archivelog mode. So no, you couldn?t recover.
12. What causes the "snapshot too old" error? How can this be prevented or mitigated?
Level: Intermediate
Expected answer: This is caused by large or long running transactions that have either wrapped onto their own rollback space or have had another transaction write on part of their rollback space. This can be prevented or mitigated by breaking the transaction into a set of smaller transactions or increasing the size of the rollback segments and their extents.
13. How can you tell if a database object is invalid?
Level: Low
Expected answer: By checking the status column of the DBA_, ALL_ or USER_OBJECTS views, depending upon whether you own or only have permission on the view or are using a DBA account.
14. A user is getting an ORA-00942 error yet you know you have granted them permission on the table, what else should you check?
Level: Low
Expected answer: You need to check that the user has specified the full name of the object (select empid from scott.emp; instead of select empid from emp;) or has a synonym that points to the object (create synonym emp for scott.emp;)
15. A developer is trying to create a view and the database won?t let him. He has the "DEVELOPER" role which has the "CREATE VIEW" system privilege and SELECT grants on the tables he is using, what is the problem? Level: Intermediate Expected answer: You need to verify the developer has direct grants on all tables used in the view. You can?t create a stored object with grants given through a role.
16. If you have an example table, what is the best way to get sizing data for the production table implementation?
Level: Intermediate
Expected answer: The best way is to analyze the table and then use the data provided in the DBA_TABLES view to get the average row length and other pertinent data for the calculation. The quick and dirty way is to look at the number of blocks the table is actually using and ratio the number of rows in the table to its number of blocks against the number of expected rows.
17. How can you find out how many users are currently logged into the database? How can you find their operating system id?
Level: high Expected answer: There are several ways. One is to look at the v$session or v$process views. Another way is to check the current_logins parameter in the v$sysstat view. Another if you are on UNIX is to do a "ps -ef|grep oracle|wc -l? command, but this only works against a single instance installation.
18. A user selects from a sequence and gets back two values, his select is:
SELECT pk_seq.nextval FROM dual; What is the problem?
Level: Intermediate
Expected answer: Somehow two values have been inserted into the dual table. This table is a single row, single column table that should only have one value in it.
19. How can you determine if an index needs to be dropped and rebuilt?
Level: Intermediate
Expected answer: Run the ANALYZE INDEX command on the index to validate its structure and then calculate the ratio of LF_BLK_LEN/LF_BLK_LEN+BR_BLK_LEN and if it isn?t near 1.0 (i.e. greater than 0.7 or so) then the index should be rebuilt. Or if the ratio BR_BLK_LEN/ LF_BLK_LEN+BR_BLK_LEN is nearing 0.3.
[Edit section] SQL/ SQLPlus
1. How can variables be passed to a SQL routine? Level: Low
Expected answer: By use of the & symbol. For passing in variables the numbers 1-8 can be used (&1, &2,...,&8) to pass the values after the command into the SQLPLUS session. To be prompted for a specific variable, place the ampersanded variable in the code itself: "select * from dba_tables where owner=&owner_name;" . Use of double ampersands tells SQLPLUS to resubstitute the value for each subsequent use of the variable, a single ampersand will cause a reprompt for the value unless an ACCEPT statement is used to get the value from the user.
2. You want to include a carriage return/linefeed in your output from a SQL script, how can you do this?
Level: Intermediate to high
Expected answer: The best method is to use the CHR() function (CHR(10) is a return/linefeed) and the concatenation function "||". Another method, although it is hard to document and isn?t always portable is to use the return/linefeed as a part of a quoted string.
3. How can you call a PL/SQL procedure from SQL?
Level: Intermediate
Expected answer: By use of the EXECUTE (short form EXEC) command.
4. How do you execute a host operating system command from within SQL?
Level: Low
Expected answer: By use of the exclamation point "!" (in UNIX and some other OS) or the HOST (HO) command.
5. You want to use SQL to build SQL, what is this called and give an example
Level: Intermediate to high
Expected answer: This is called dynamic SQL. An example would be: set lines 90 pages 0 termout off feedback off verify off spool drop_all.sql select ?drop user ?||username||? cascade;? from dba_users where username not in ("SYS?,?SYSTEM?); spool off Essentially you are looking to see that they know to include a command (in this case DROP USER...CASCADE;) and that you need to concatenate using the ?||? the values selected from the database.
6. What SQLPlus command is used to format output from a select?
Level: low
Expected answer: This is best done with the COLUMN command.
7. You want to group the following set of select returns, what can you group on? Max(sum_of_cost), min(sum_of_cost), count(item_no), item_no Level: Intermediate Expected answer: The only column that can be grouped on is the "item_no" column, the rest have aggregate functions associated with them.
8. What special Oracle feature allows you to specify how the cost based system treats a SQL statement?
Level: Intermediate to high Expected answer: The COST based system allows the use of HINTs to control the optimizer path selection. If they can give some example hints such as FIRST ROWS, ALL ROWS, USING INDEX, STAR, even better.
9. You want to determine the location of identical rows in a table before attempting to place a unique index on the table, how can this be done?
Level: High
Expected answer: Oracle tables always have one guaranteed unique column, the rowid column. If you use a min/max function against your rowid and then select against the proposed primary key you can squeeze out the rowids of the duplicate rows pretty quick. For example: select rowid from emp e where e.rowid > (select min(x.rowid) from emp x where x.emp_no = e.emp_no); In the situation where multiple columns make up the proposed key, they must all be used in the where clause.
10. What is a Cartesian product?
Level: Low
Expected answer: A Cartesian product is the result of an unrestricted join of two or more tables. The result set of a three table Cartesian product will have x * y * z number of rows where x, y, z correspond to the number of rows in each table involved in the join.
11. You are joining a local and a remote table, the network manager complains about the traffic involved, how can you reduce the network traffic?
Level: High
Expected answer: Push the processing of the remote data to the remote instance by using a view to pre-select the information for the join. This will result in only the data required for the join being sent across.
12. What is the default ordering of an ORDER BY clause in a SELECT statement?
Level: Low
Expected answer: Ascending
13. What is tkprof and how is it used?
Level: Intermediate to high Expected answer: The tkprof tool is a tuning tool used to determine cpu and execution times for SQL statements. You use it by first setting timed_statistics to true in the initialization file and then turning on tracing for either the entire database via the sql_trace parameter or for the session using the ALTER SESSION command. Once the trace file is generated you run the tkprof tool against the trace file and then look at the output from the tkprof tool. This can also be used to generate explain plan output.
14. What is explain plan and how is it used?
Level: Intermediate to high
Expected answer: The EXPLAIN PLAN command is a tool to tune SQL statements. To use it you must have an explain_table generated in the user you are running the explain plan for. This is created using the utlxplan.sql script. Once the explain plan table exists you run the explain plan command giving as its argument the SQL statement to be explained. The explain_plan table is then queried to see the execution plan of the statement. Explain plans can also be run using tkprof.
15. How do you set the number of lines on a page of output? The width?
Level: Low
Expected answer: The SET command in SQLPLUS is used to control the number of lines generated per page and the width of those lines, for example SET PAGESIZE 60 LINESIZE 80 will generate reports that are 60 lines long with a line width of 80 characters. The PAGESIZE and LINESIZE options can be shortened to PAGES and LINES.
16. How do you prevent output from coming to the screen?
Level: Low
Expected answer: The SET option TERMOUT controls output to the screen. Setting TERMOUT OFF turns off screen output. This option can be shortened to TERM.
17. How do you prevent Oracle from giving you informational messages during and after a SQL statement execution?
Level: Low
Expected answer: The SET options FEEDBACK and VERIFY can be set to OFF.
18. How do you generate file output from SQL?
Level: Low
Expected answer: By use of the SPOOL command
[Edit section] Tuning Questions
1. A tablespace has a table with 30 extents in it. Is this bad? Why or why not.
Level: Intermediate
Expected answer: Multiple extents in and of themselves aren?t bad. However if you also have chained rows this can hurt performance.
2. How do you set up tablespaces during an Oracle installation?
Level: Low
Expected answer: You should always attempt to use the Oracle Flexible Architecture standard or another partitioning scheme to ensure proper separation of SYSTEM, ROLLBACK, REDO LOG, DATA, TEMPORARY and INDEX segments.
3. You see multiple fragments in the SYSTEM tablespace, what should you check first?
Level: Low
Expected answer: Ensure that users don?t have the SYSTEM tablespace as their TEMPORARY or DEFAULT tablespace assignment by checking the DBA_USERS view.
4. What are some indications that you need to increase the SHARED_POOL_SIZE parameter?
Level: Intermediate
Expected answer: Poor data dictionary or library cache hit ratios, getting error ORA-04031. Another indication is steadily decreasing performance with all other tuning parameters the same.
5. What is the general guideline for sizing db_block_size and db_multi_block_read for an application that does many full table scans?
Level: High
Expected answer: Oracle almost always reads in 64k chunks. The two should have a product equal to 64 or a multiple of 64.
6. What is the fastest query method for a table?
Level: Intermediate Expected answer: Fetch by rowid
7. Explain the use of TKPROF? What initialization parameter should be turned on to get full TKPROF output?
Level: High
Expected answer: The tkprof tool is a tuning tool used to determine cpu and execution times for SQL statements. You use it by first setting timed_statistics to true in the initialization file and then turning on tracing for either the entire database via the sql_trace parameter or for the session using the ALTER SESSION command. Once the trace file is generated you run the tkprof tool against the trace file and then look at the output from the tkprof tool. This can also be used to generate explain plan output.
8. When looking at v$sysstat you see that sorts (disk) is high. Is this bad or good? If bad -How do you correct it?
Level: Intermediate
Expected answer: If you get excessive disk sorts this is bad. This indicates you need to tune the sort area parameters in the initialization files. The major sort are parameter is the SORT_AREA_SIZe parameter.
9. When should you increase copy latches? What parameters control copy latches? Level: high Expected answer: When you get excessive contention for the copy latches as shown by the "redo copy" latch hit ratio. You can increase copy latches via the initialization parameter LOG_SIMULTANEOUS_COPIES to twice the number of CPUs on your system.
10. Where can you get a list of all initialization parameters for your instance? How about an indication if they are default settings or have been changed?
Level: Low
Expected answer: You can look in the init.ora file for an indication of manually set parameters. For all parameters, their value and whether or not the current value is the default value, look in the v$parameter view.
11. Describe hit ratio as it pertains to the database buffers. What is the difference between instantaneous and cumulative hit ratio and which should be used for tuning?
Level: Intermediate
Expected answer: The hit ratio is a measure of how many times the database was able to read a value from the buffers verses how many times it had to re-read a data value from the disks. A value greater than 80-90% is good, less could indicate problems. If you simply take the ratio of existing parameters this will be a cumulative value since the database started. If you do a comparison between pairs of readings based on some arbitrary time span, this is the instantaneous ratio for that time span. Generally speaking an instantaneous reading gives more valuable data since it will tell you what your instance is doing for the time it was generated over.
12. Discuss row chaining, how does it happen? How can you reduce it? How do you correct it?
Level: high
Expected answer: Row chaining occurs when a VARCHAR2 value is updated and the length of the new value is longer than the old value and won?t fit in the remaining block space. This results in the row chaining to another block. It can be reduced by setting the storage parameters on the table to appropriate values. It can be corrected by export and import of the effected table.
13. When looking at the estat events report you see that you are getting busy buffer waits. Is this bad? How can you find what is causing it?
Level: high
Expected answer: Buffer busy waits could indicate contention in redo, rollback or data blocks. You need to check the v$waitstat view to see what areas are causing the problem. The value of the "count" column tells where the problem is, the "class" column tells you with what. UNDO is rollback segments, DATA is data base buffers.
14. If you see contention for library caches how can you fix it?
Level: Intermediate
Expected answer: Increase the size of the shared pool.
15. If you see statistics that deal with "undo" what are they really talking about?
Level: Intermediate
Expected answer: Rollback segments and associated structures.
16. If a tablespace has a default pctincrease of zero what will this cause (in relationship to the smon process)?
Level: High
Expected answer: The SMON process won?t automatically coalesce its free space fragments.
17. If a tablespace shows excessive fragmentation what are some methods to defragment the tablespace? (7.1,7.2 and 7.3 only)
Level: High
Expected answer: In Oracle 7.0 to 7.2 The use of the 'alter session set events 'immediate trace name coalesce level ts#';? command is the easiest way to defragment contiguous free space fragmentation. The ts# parameter corresponds to the ts# value found in the ts$ SYS table. In version 7.3 the ?alter tablespace coalesce;? is best. If the free space isn?t contiguous then export, drop and import of the tablespace contents may be the only way to reclaim non-contiguous free space.
18. How can you tell if a tablespace has excessive fragmentation?
Level: Intermediate
If a select against the dba_free_space table shows that the count of a tablespaces extents is greater than the count of its data files, then it is fragmented.
19. You see the following on a status report: redo log space requests 23 redo log space wait time 0 Is this something to worry about? What if redo log space wait time is high? How can you fix this?
Level: Intermediate
Expected answer: Since the wait time is zero, no. If the wait time was high it might indicate a need for more or larger redo logs.
20. What can cause a high value for recursive calls? How can this be fixed?
Level: High
Expected answer: A high value for recursive calls is cause by improper cursor usage, excessive dynamic space management actions, and or excessive statement re-parses. You need to determine the cause and correct it By either relinking applications to hold cursors, use proper space management techniques (proper storage and sizing) or ensure repeat queries are placed in packages for proper reuse.
21. If you see a pin hit ratio of less than 0.8 in the estat library cache report is this a problem? If so, how do you fix it?
Level: Intermediate
Expected answer: This indicate that the shared pool may be too small. Increase the shared pool size.
22. If you see the value for reloads is high in the estat library cache report is this a matter for concern?
Level: Intermediate
Expected answer: Yes, you should strive for zero reloads if possible. If you see excessive reloads then increase the size of the shared pool.
23. You look at the dba_rollback_segs view and see that there is a large number of shrinks and they are of relatively small size, is this a problem? How can it be fixed if it is a problem?
Level: High
Expected answer: A large number of small shrinks indicates a need to increase the size of the rollback segment extents. Ideally you should have no shrinks or a small number of large shrinks. To fix this just increase the size of the extents and adjust optimal accordingly.
24. You look at the dba_rollback_segs view and see that you have a large number of wraps is this a problem?
Level: High
Expected answer: A large number of wraps indicates that your extent size for your rollback segments are probably too small. Increase the size of your extents to reduce the number of wraps. You can look at the average transaction size in the same view to get the information on transaction size.
25. In a system with an average of 40 concurrent users you get the following from a query on rollback extents: ROLLBACK CUR EXTENTS
--------------------------
R01 11 R02 8 R03 12 R04 9 SYSTEM 4 You have room for each to grow by 20 more extents each. Is there a problem? Should you take any action?
Level: Intermediate
Expected answer: No there is not a problem. You have 40 extents showing and an average of 40 concurrent users. Since there is plenty of room to grow no action is needed.
26. You see multiple extents in the temporary tablespace. Is this a problem?
Level: Intermediate
Expected answer: As long as they are all the same size this isn?t a problem. In fact, it can even improve performance since Oracle won?t have to create a new extent when a user needs one.
[Edit section] Installation/Configuration
1. Define OFA.
Level: Low
Expected answer: OFA stands for Optimal Flexible Architecture. It is a method of placing directories and files in an Oracle system so that you get the maximum flexibility for future tuning and file placement.
2. How do you set up your tablespace on installation? Level: Low
Expected answer: The answer here should show an understanding of separation of redo and rollback, data and indexes and isolation os SYSTEM tables from other tables. An example would be to specify that at least 7 disks should be used for an Oracle installation so that you can place SYSTEM tablespace on one, redo logs on two (mirrored redo logs) the TEMPORARY tablespace on another, ROLLBACK tablespace on another and still have two for DATA and INDEXES. They should indicate how they will handle archive logs and exports as well. As long as they have a logical plan for combining or further separation more or less disks can be specified.
3. What should be done prior to installing Oracle (for the OS and the disks)?
Level: Low
Expected Answer: adjust kernel parameters or OS tuning parameters in accordance with installation guide. Be sure enough contiguous disk space is available.
4. You have installed Oracle and you are now setting up the actual instance. You have been waiting an hour for the initialization script to finish, what should you check first to determine if there is a problem?
Level: Intermediate to high
Expected Answer: Check to make sure that the archiver isn?t stuck. If archive logging is turned on during install a large number of logs will be created. This can fill up your archive log destination causing Oracle to stop to wait for more space.
5. When configuring SQLNET on the server what files must be set up?
Level: Intermediate
Expected answer: INITIALIZATION file, TNSNAMES.ORA file, SQLNET.ORA file
6. When configuring SQLNET on the client what files need to be set up?
Level: Intermediate
Expected answer: SQLNET.ORA, TNSNAMES.ORA
7. What must be installed with ODBC on the client in order for it to work with Oracle?
Level: Intermediate
Expected answer: SQLNET and PROTOCOL (for example: TCPIP adapter) layers of the transport programs.
8. You have just started a new instance with a large SGA on a busy existing server. Performance is terrible, what should you check for?
Level: Intermediate
Expected answer: The first thing to check with a large SGA is that it isn?t being swapped out.
9. What OS user should be used for the first part of an Oracle installation (on UNIX)?
Level: low
Expected answer: You must use root first.
10. When should the default values for Oracle initialization parameters be used as is?
Level: Low
Expected answer: Never
11. How many control files should you have? Where should they be located?
Level: Low
Expected answer: At least 2 on separate disk spindles. Be sure they say on separate disks, not just file systems.
12. How many redo logs should you have and how should they be configured for maximum recoverability?
Level: Intermediate
Expected answer: You should have at least three groups of two redo logs with the two logs each on a separate disk spindle (mirrored by Oracle). The redo logs should not be on raw devices on UNIX if it can be avoided.
13. You have a simple application with no "hot" tables (i.e. uniform IO and access requirements). How many disks should you have assuming standard layout for SYSTEM, USER, TEMP and ROLLBACK tablespaces?
Expected answer: At least 7, see disk configuration answer above.
[Edit section] Data Modeler
1. Describe third normal form?
Level: Low
Expected answer: Something like: In third normal form all attributes in an entity are related to the primary key and only to the primary key
2. Is the following statement true or false: "All relational databases must be in third normal form" Why or why not? Level: Intermediate Expected answer: False. While 3NF is good for logical design most databases, if they have more than just a few tables, will not perform well using full 3NF. Usually some entities will be denormalized in the logical to physical transfer process.
3. What is an ERD? Level: Low Expected answer: An ERD is an Entity-Relationship-Diagram. It is used to show the entities and relationships for a database logical model.
4. Why are recursive relationships bad? How do you resolve them? Level: Intermediate A recursive relationship (one where a table relates to itself) is bad when it is a hard relationship (i.e. neither side is a "may" both are "must") as this can result in it not being possible to put in a top or perhaps a bottom of the table (for example in the EMPLOYEE table you couldn?t put in the PRESIDENT of the company because he has no boss, or the junior janitor because he has no subordinates). These type of relationships are usually resolved by adding a small intersection entity.
5. What does a hard one-to-one relationship mean (one where the relationship on both ends is "must")? Level: Low to intermediate Expected answer: This means the two entities should probably be made into one entity.
6. How should a many-to-many relationship be handled? Level: Intermediate Expected answer: By adding an intersection entity table
7. What is an artificial (derived) primary key? When should an artificial (or derived) primary key be used? Level: Intermediate Expected answer: A derived key comes from a sequence. Usually it is used when a concatenated key becomes too cumbersome to use as a foreign key.
8. When should you consider denormalization? Level: Intermediate Expected answer: Whenever performance analysis indicates it would be beneficial to do so without compromising data integrity.
[Edit section] UNIX
1. How can you determine the space left in a file system?
Level: Low Expected answer: There are several commands to do this: du, df, or bdf
2. How can you determine the number of SQLNET users logged in to the UNIX system? Level: Intermediate Expected answer: SQLNET users will show up with a process unique name that begins with oracle, if you do a ps -ef|grep oracle|wc -l you can get a count of the number of users.
3. What command is used to type files to the screen? Level: Low Expected answer: cat, more, pg
4. What command is used to remove a file? Level: Low Expected answer: rm
5. Can you remove an open file under UNIX? Level: Low Expected answer: yes
6. How do you create a decision tree in a shell script? Level: intermediate Expected answer: depending on shell, usually a case-esac or an if-endif or fi structure
7. What is the purpose of the grep command? Level: Low Expected answer: grep is a string search command that parses the specified string from the specified file or files
8. The system has a program that always includes the word nocomp in its name, how can you determine the number of processes that are using this program? Level: intermediate Expected answer: ps -ef|grep *nocomp*|wc -l
9. What is an inode? Level: Intermediate Expected answer: an inode is a file status indicator. It is stored in both disk and memory and tracts file status. There is one inode for each file on the system.
10. The system administrator tells you that the system hasn?t been rebooted in 6 months, should he be proud of this? Level: High Expected answer: Maybe. Some UNIX systems don?t clean up well after themselves. Inode problems and dead user processes can accumulate causing possible performance and corruption problems. Most UNIX systems should have a scheduled periodic reboot so file systems can be checked and cleaned and dead or zombie processes cleared out.
11. What is redirection and how is it used? Level: Intermediate Expected answer: redirection is the process by which input or output to or from a process is redirected to another process. This can be done using the pipe symbol "|", the greater than symbol ">" or the "tee" command. This is one of the strengths of UNIX allowing the output from one command to be redirected directly into the input of another command.
12. How can you find dead processes? Level: Intermediate Expected answer: ps -ef|grep zombie -- or -- who -d depending on the system.
13. How can you find all the processes on your system? Level: Low Expected answer: Use the ps command
14. How can you find your id on a system? Level: Low Expected answer: Use the "who am i" command.
15. What is the finger command? Level: Low Expected answer: The finger command uses data in the passwd file to give information on system users.
16. What is the easiest method to create a file on UNIX? Level: Low Expected answer: Use the touch command
17. What does >> do? Level: Intermediate Expected answer: The ">>" redirection symbol appends the output from the command specified into the file specified. The file must already have been created.
18. If you aren?t sure what command does a particular UNIX function what is the best way to determine the command? Expected answer: The UNIX man -k command will search the man pages for the value specified. Review the results from the command to find the command of interest.
[Edit section] Oracle Troubleshooting
1. How can you determine if an Oracle instance is up from the operating system level? Level: Low Expected answer: There are several base Oracle processes that will be running on multi-user operating systems, these will be smon, pmon, dbwr and lgwr. Any answer that has them using their operating system process showing feature to check for these is acceptable. For example, on UNIX a ps -ef|grep pmon will show what instances are up.
2. Users from the PC clients are getting messages indicating : Level: Low ORA-06114: (Cnct err, can't get err txt. See Servr Msgs & Codes Manual) What could the problem be? Expected answer: The instance name is probably incorrect in their connection string.
3. Users from the PC clients are getting the following error stack: Level: Low ERROR: ORA-01034: ORACLE not available ORA-07318: smsget: open error when opening sgadef.dbf file. HP-UX Error: 2: No such file or directory What is the probable cause? Expected answer: The Oracle instance is shutdown that they are trying to access, restart the instance.
4. How can you determine if the SQLNET process is running for SQLNET V1? How about V2? Level: Low Expected answer: For SQLNET V1 check for the existence of the orasrv process. You can use the command "tcpctl status" to get a full status of the V1 TCPIP server, other protocols have similar command formats. For SQLNET V2 check for the presence of the LISTENER process(s) or you can issue the command "lsnrctl status".
5. What file will give you Oracle instance status information? Where is it located? Level: Low Expected answer: The alert.ora log. It is located in the directory specified by the background_dump_dest parameter in the v$parameter table.
6. Users aren?t being allowed on the system. The following message is received: Level: Intermediate ORA-00257 archiver is stuck. Connect internal only, until freed What is the problem? Expected answer: The archive destination is probably full, backup the archive logs and remove them and the archiver will re-start.
7. Where would you look to find out if a redo log was corrupted assuming you are using Oracle mirrored redo logs? Level: Intermediate Expected answer: There is no message that comes to the SQLDBA or SRVMGR programs during startup in this situation, you must check the alert.log file for this information.
8. You attempt to add a datafile and get: Level: Intermediate ORA-01118: cannot add anymore datafiles: limit of 40 exceeded What is the problem and how can you fix it? Expected answer: When the database was created the db_files parameter in the initialization file was set to 40. You can shutdown and reset this to a higher value, up to the value of MAX_DATAFILES as specified at database creation. If the MAX_DATAFILES is set to low, you will have to rebuild the control file to increase it before proceeding.
9. You look at your fragmentation report and see that smon hasn?t coalesced any of you tablespaces, even though you know several have large chunks of contiguous free extents. What is the problem? Level: High Expected answer: Check the dba_tablespaces view for the value of pct_increase for the tablespaces. If pct_increase is zero, smon will not coalesce their free space.
10. Your users get the following error: Level: Intermediate ORA-00055 maximum number of DML locks exceeded What is the problem and how do you fix it? Expected answer: The number of DML Locks is set by the initialization parameter DML_LOCKS. If this value is set to low (which it is by default) you will get this error. Increase the value of DML_LOCKS. If you are sure that this is just a temporary problem, you can have them wait and then try again later and the error should clear.
11. You get a call from you backup DBA while you are on vacation. He has corrupted all of the control files while playing with the ALTER DATABASE BACKUP CONTROLFILE command. What do you do?
Level: High Expected answer: As long as all datafiles are safe and he was successful with the BACKUP controlfile command you can do the following: CONNECT INTERNAL STARTUP MOUNT (Take any read-only tablespaces offline before next step ALTER DATABASE DATAFILE .... OFFLINE;) RECOVER DATABASE USING BACKUP CONTROLFILE ALTER DATABASE OPEN RESETLOGS; (bring read-only tablespaces back online) Shutdown and backup the system, then restart If they have a recent output file from the ALTER DATABASE BACKUP CONTROL FILE TO TRACE; command, they can use that to recover as well. If no backup of the control file is available then the following will be required: CONNECT INTERNAL STARTUP NOMOUNT CREATE CONTROL FILE .....; However, they will need to know all of the datafiles, logfiles, and settings for MAXLOGFILES, MAXLOGMEMBERS, MAXLOGHISTORY, MAXDATAFILES for the database to use the command.
8/03/2007
Are you using the standard port number 1433 for communicating with SQL Server 2005? Have you considered setting up SQL Server to listen on a different port number than 1433? Well I did. In this article, I will discuss how I did it and the issues I came across when I setup up SQL Server 2005 to use a non-standard port number.
Issues with Using default Port Numbers
When you install SQL Server, the default instance is setup to listen on port 1433. This is the default port number and it is well known. Because of this, you commonly see unwanted attacks against port 1433 from hackers. So to minimize the potential threat of a hacker gaining access through this default port number you should consider using a non-standard port number other than 1433.
When a named instance is installed, by default it dynamically determines the port number to use when it starts up. Therefore, each time a named instance is started it has the potential to be listening on a different port number. This can be problematic, especially if you need traffic to come through a firewall. Since the port number might change dynamically, you can’t restrict traffic to your SQL Server for a specific port number with your firewall rules. Therefore, in order to restrict traffic to your SQL Server box you will need to configure SQL Server to listen on specific ports.
How to Configure SQL Server to Listen on Specific Port
Since the default port number is vulnerable to hacker attaches and the named instances dynamically set the port number, you should consider specifying each of your instances of SQL Server to listen on a specific port. Having SQL Server using a specific port number provides a more secure environment. So how do you configure SQL Server 2005 to listen on a different port? To accomplish this, SQL Server provides the SQL Server Configuration Manager tool.
To start Configuration Manager left click on the “Start” button, navigate to “All Programs”, then to “Microsoft SQL Server 2005”, then to “Configuration Tools”, and then finally left click on the “SQL Server Configuration Manager” drop down. Once you have SQL Server Configuration Manager up and running expand the “SQL Server 2005 Network Configuration” folder. When you do this, you should see a screen similar to the one below:
Here you can see I have 4 different instances of SQL Server running. One is the default instance, named “MSSQLSERVER”, one is an Express edition named “SQLEXPRESS” and then I have two named instances, named “SERVER1” and “SERVER2”.
To change the port assignment of one of these instances I would left click on the instance, which would then display the different available protocols. Below is a screen shot where I left clicked on instance “SERVER2”:
Here you can see I have two protocols enabled, “Shared Memory” and “TCP/IP”. To change the port assignment, right click on the “TCP/IP” protocol in the right pane, and select the “Properties” option. When you do this, the following screen shot should be displayed:
From this screen, you can see that the “Listen All” setting is “Yes”. This means that this instance will listen on all IP addresses. Listening on all IP addresses is the default when you install an instance of SQL Server. If you only want SQL Server to listen on a specific IP address then you would need to change the “Listen All” setting to “No”.
To identify a specific port that you want SQL Server to listen on you need to left click on the “IP Addresses” tab on the above screen. When you do this, a screen similar to below will be displayed:
As you can see IP1 and IP2 are disabled (“Enabled” set to “No”), and the IPALL “TCP Dynamic Ports” is set to “1317”. Since SERVER2 is a named instance the IP address is set dynamically when the instance is started, port “1317” just happens to be the port number my named instances is currently using. If I was to stop this instance and restart it there is the potential that when this instance comes back up it might be using a different port, because the port number is set dynamically. When the “TCP Dynamic Ports” setting is set to “0” it indicates that the Database Engine is listening on dynamic ports.
To ensure that a named instances uses a consistence port number, or you want to change the default port number for the default instance all you need to do is identify a specific port number that your SQL Server instance should be listen on, like so:
Here I have specified that I want to use port “8484” as the port number for my “SERVER2” named instance. I did this by placing this port number on the “TCP Port” option.
Connecting Clients When using Specific Part Assignments
Connecting a client to a SQL Server instance using a non-standard, depending on your installation setup, can be problematic, especially if you are not running the SQL Server Browser service. There are three different ways to connect a client to an instance that is using a specific non-standard port number.
The first method is to run the SQL Server Browser service. When this service is running, it communicates with the client informing the client what port number is being used for the instance in which a connection is being requested. This eliminates the client from having to know the port number when connecting to an instance of SQL Server. If you want a more secure environment then you will not run the SQL Server Browser service. By not running the SQL Server Browser service, the client will need to identify the correct port number in the connection string in order to connect to SQL Server.
The next way to connect a client is to create a SQL Server Alias on the client machine. An alias can be created on the client machine by running the SQL Server Configuration Manager tool. Under the SQL Server Native Client folder, right click on the “Aliases” item and select the “New Aliases” item. When doing this the “Alias – New” dialog box will be displayed. In the new alias dialog box, you can create an alias where you can specify the instance name and the port number it is using. Below is a screen shot of how I would create an alias for my “SERVER2” named instance to identify that I want to connect using port 8484.
Note I have identified an “Alias Name” of “SERVER2”, which is using a “Port No” of “8484”, and a “Server” value of “SERVER2”. Building this alias associated the alias name “SERVER2” with server “SERVER2” and port number “8484”. Once I’ve specified this information I can either click on “OK”, or “Apply” and then “OK” to create my alias.
The last method is to programmatically specify the port number in the connection string. I’m not sure of all the different methods to accomplish this since I’m not a connection string guru, but one way is to specify the port number along with the server name in the connection string. This is done by placing “,
Data Source=SERVER2,8484;Initial Catalog=AdventureWorks
Note I’ve added a “,8484” right after the name of my server in the “Data Source” parameter of the connection string.
Issues with Using Non-Standard Port
I’ve found no real technology issues associated with using a Non-Standard Port number, or specifying a specific port number for a named instance. The biggest problem I’ve encountered is the fact that programmers keep coming to me saying, “I can’t connect to SQL Server”. Then when I work with them to identify the connectivity issue, we find that they have forgotten to specify the port number association along with the server name when they try to connect. Therefore, if you plan to use non-standard port numbers you need to make sure you clearly explain how the connection strings will need to have a port number in it to ensure a connection will be made successfully.
Conclusion
As you can see there isn’t that much work to setup your SQL Server instances to use a specific port number. The biggest issue is making sure the connection strings are coded correctly. When setting a port number make sure you find a high port number that is not being used. To make your environment more secure I would suggest you turn off the SQL Server Browser services. By doing this all clients will be required to specify the port number when connecting to SQL Server.