IF OBJECT_ID('tempdb..#mytesttable') IS NOT NULL
DROP TABLE #mytesttable
SELECT id, name
FROM
INTO #mytesttable mytable
Question: is it good to check temp table existence (e.g: OBJECT_ID('tempdb..#mytesttable') ) when I first time create this temp table inside a procedure?
What will be the best practice in terms of performance?
It is good practice to check if table exists or not. and it doesn't have any performance impact. anyhow, temp tables automatically drops once procedure execution completes in sql server. sql server appends unique num with temp table name. so if same procedure executes more than once at same time also , it will not cause any issue. it depends on session. all procs executions have different session.
Yes, it is a good practice. I am always doing this at the beginning of the routine. In SQL Server 2016 + objects can DIE (Drop If Exists), so you can simplify the call:
DROP TABLE IF EXISTS #mytesttable;
Also, even at the end of the routine you temporary objects are destroyed and even the SQL Engine gives unique names to temporary tables, there is still another behavior to consider.
If you are naming your temporary table with same name, when nested procedure calls are involved (one stored procedure calls another) it is possible to get an error or corrupt your data. This is due to the fact that temporary table are visible in the current execution scope - so, #table in one procedure will be visible in the second called procedure (so you can modify its data or drop it).
Related
Image #1: I am creating a stored procedure for inserting records into #t1.
In the same session I am executing the Localtemp1 stored procedure for any number of times, and it works fine:
Image #2: again executing the stored procedure in another session & works fine as well:
Image #3: now creating stored procedure for inserting records into ##tt. For the first execution of globaltemp1 stored procedure, it works well:
Image #4: but when I executed it a second time, it is showing errors (does not exist in DB):
Image #5: then I closed the session where globaltemp stored procedure was created, and in a new session, I executed the stored procedure, and it works well for the first time:
Image #6: but when I execute it a second time, again it is showing errors (does not exist in DB):
What I know is scope of local temp & global temp, but in stored procedures, they were completely different
Can someone tell me
Execution of localtemp1 stored procedure for many times gives output but while executing globaltemp1 sp for the first time gives output and second time results in an error
As far as I know, after execution of stored procedure temptable gets dropped. Then why localtemp1 stored procedure is getting executed across all sessions and many number of times?
Why globaltemp1 stored procedure is executing once and for second time showing an error?
Final one, Globaltemp stored procedure shows output in another session for the first time only when created session was closed
I mean
56 ----> globaltemp sp was created
57 ----> to get o/p i need to close 56
58 ----> to get o?p i need to close 57 ( WHY ??? )
I am a beginner at SQL and please, someone make me understand because if I don't find logic & correct reason I could not dive into another topic.
The concept of temp table is to hold records temporarily. It's some kind of an array where you can store multiple records using the same variable.
When you create a Temp Table, actually it is being created in the tempdb of the corresponding server. Even if you are naming it as just #temp, the name on which it was created on the tempdb will be having some additional parameters like your database name from which the table was created and your session id etc.
I just created the following temp table in my master database
and this is how it was named in the tempdb
still, in my database, I can access it using the name #temp. But The limitation of such temp table is that they are local and can be accessed only from that session, So if I try to access this #temp from any other Query Window (Session) even on the same database, I won't be able to access it. That's where we use Global temp tables. So If I add one more # to the table name then it becomes global temp table which can be accessed across the sessions. It is still created on the Tempdb but like this
Whenever you close the query window/session both Local and Global temp tables are automatically dropped.
So in the case of stored procedures, the starting and ending time of the sp is treated as one session. So once the sp execution is completed all the temp tables created inside that sp is dropped. So you can not use one temp table that was created by one SP in another one.
Hope this helps
Local temporary tables are visible only in the current session, and global temporary tables are visible to all sessions. Global temporary tables are automatically dropped when the session that created the table ends and all other tasks have stopped referencing them. The association between a task and a table is maintained only for the life of a single Transact-SQL statement. This means that a global temporary table is dropped at the completion of the last Transact-SQL statement that was actively referencing the table when the creating session ended.
If you have absolutely have to have a global temp table available your best solution would be to create a permanent table and then drop the table at the end of the stored procedure. You can check for it's existence before creating it:
IF OBJECT_ID('dbo.yourtablenamehere', 'U') IS NOT NULL
DROP TABLE dbo.yourtablenamehere;
The differences between a temp table and a permanent table are really not that much different, mostly that the temp table drops automatically. If you are using this in an application that is calling this procedure, it might be best to have the application load the temp table into array and do the comparisons for you since it can maintain the array while executing and re-executing stored procs.
There is a good reason to write to a temp table instead of creating a table then dropping it... access rights. If you are creating tables to be used by people who are only granted read only privileges, they will not be able to create the table.
I have a temporary table in the stored procedure which is causing the time out for the query as it is doing a complex calculation. I want to drop it after it is used. It was created like
DECLARE #SecondTable TABLE
Now I cannot drop it using
drop #SecondTable
in fact I have to use
drop #SecondTable
Does somebody know why?
I'm by no means a SQL guru, but why is the drop even necessary?
If it's a table variable, it will no longer exist once the stored proc exits.
I'm actually surprised that DROP #SecondTable doesn't error out on you; since you're dropping a temporary table there; not a table variable.
EDIT
So based on your comment, my updates are below:
1.) If you're using a table variable (#SecondTable); then no drop is necessary. SQL Server will take care of this for you.
2.) It sounds like your timeout is caused by the calculations using the table, not the dropping of the table itself. In this case; I'd probably recommend using a temporary table instead of a table variable; since a temporary table will let you add indexes and such to improve performance; while a table variable will not. If this still isn't sufficient; you might need to increase the timeout duration on the query.
3.) In SQL; a table variable (#SecondTable) and temporary table (#SecondTable) are two completely different things. I'd refer to the MSDN documentation for Table Variables and Temporary Tables
here's the situation:
I have an SSRS report that uses an SP as a Dataset. The SP creates a Temp Table, inserts a bunch of data into it, and select's it back out for SSRS to report. Pretty straight forward.
Question:
If multiple users run the report with different parameters selected, will the temp table created by the SP collide in the tempdb and potentially not return the data set expected?
Most likely not. If the temp table is defined as #temp or #temp, then you're safe, as those kind of temp tables can only be accessed by the creating connection, and will only last for the duration of the execution of the stored procedure. If, however, you're using ##temp tables (two "pound" signs), while those tables also only last for as long as the creating stored procedure runs, they are exposed to and accessible by all connections to that SQL instance.
Odds are good that you're not using ##tables, so you are probably safe.
A temp table with a single # is a local temporary table and its scope is limited to the session that created it, so collisions should not be a problem.
I am making use of temporary tables #tempTable in my stored procedure - that I make use to run my ASP.net Reports (Reporting services)
I am doing something like
eg. Code
SELECT * INTO #tempTable FROM Contacts WHERE ContactID < 10
Then I use something like
SELECT o.* FROM #tempTable t INNER JOIN Orders o ON t.ContactID =o.ContactID
to return values to my reports aka results for the stored procedure
I do not get rid of my #tempTable
i.e. I don't do
DROP TABLE #tempTable
I have read that the scope of temporary table is only for the stored procedure - so is doing the above necessary - if I dont do the above what problems will I get into in the future
First, local temporary tables created within a procedure are dropped once the procedure finishes. From the BOL on Create Table:
A local temporary table created in a stored procedure is dropped automatically when the stored procedure is finished. The table can be referenced by any nested stored procedures executed by the stored procedure that created the table. The table cannot be referenced by the process that called the stored procedure that created the table.
If your data access code is properly opening a connection, calling a stored procedure and then closing the connection, the temp table is created in the procedure is effectively destroyed.
I say "effectively" to bring up another point. I would not recommend dropping the temp table at the end of your procedure although I would add a check just before I created the temp table and drop it if exists (e.g. if object_id('tempdb..#Foo') is not null). The argument against dropping the temp table at the end is that by calling the Drop statement, you are forcing SQL Server to expend resources to destroy the table then and there while you wait for your procedure to end. If instead, you let it go out of scope, your procedure ends immediately and you let SQL Server destroy the table at a time of its own choosing.
The #Temp table is limited scope to YOUR SESSION and lifespan of the batch, meaning nobody else can see your temp table and anyone else can create their own #Temp table with the same name. Once your session or batch ends, SQL Server will clean up the temp table.
On another note the ##Temp table behaves like a normal table. Everyone can see it, and there cannot be more than 1 ##Temp table with the same name. SQL Server will clean these ##Temp tables when the server restarts.
It's considered good coding practice to explicitly drop every temporary table you create. If you are executing scripts through SQL Server Management Studio/Query Analyzer the temp tables are kept until you explicitly drop them or until you close the session.
In general, you're probably not going to have problems by not dropping temporary tables. The local temp table has session scope, or SP scope in your case. It will drop automatically when the session is closed or the SP completes.
However, you do increase the risk of problems by regularly following this practice. For example, if you don't use an SP, but submit your SELECT statement from ASP .net and leave the SQL Server connection open, the temp table will continue to exist. Continued use of the connection and other temp tables will result in tempdb growth over time.
I also support the other comments regarding the use of temp tables in this case. If you create a solution without temp tables, you'll probably have a faster report and avoid the DROP temp table command too.
I've read around the subject of temporary tables and scope and all the answers i've seen don't seem to talk about one of my concerns.
I understand that a local temporary table's scope is only valid withing the lifetime of a stored procedure or child stored procedures. However what is the situation with regard to concurency. i.e. if i have a stored procedure that creates a temporary table which is called from two different processes but from the same user/connection string, will that temporary table be shared between the two calls to that one stored procedure or will it be a case of each call to the stored procedure creates an unique temporary table instance.
I would assume that the temporary table belongs to the scope of the call to the stored procdure but i want to be sure before i go down a path with this.
Local temporary tables (start with #) are limited to your session; other sessions, even from the same user/connection string, can't see them. The rules for the lifetime depend on whether the local temporary table was created in a stored procedure:
A local temporary table that is created in a stored procedure is dropped when the procedure ends; other stored procedures, or the calling process, can't see them.
Other local temporary tables are dropped when the session ends.
Global temporary tables (start with ##) are shared between sessions. They are dropped when:
The session that created them ends
AND no other session is referring to them
This command can be handy to see which temporary tables exist:
select TABLE_NAME from tempdb.information_schema.tables
And this is handy to drop temporary tables if you're not sure they exist:
if object_id('tempdb..#SoTest') is not null drop table #SoTest
See this MSDN article for more information.
The temporary table will be accesible to the instance of the procedure that creates it
The following script
Exec ('Select 1 as col Into #Temp Select * From #Temp')
Exec ('Select 2 as col Into #Temp Select * From #Temp')
Returns
Col
1
Col
2
Not
Col
1
2
Or an error because the table already exists.
The temporary table will also be accesible by any 'child' procedures that the initial procedure runs as well.
You might also think about using table variables. They have a very well-defined scope, and they are sometimes faster than their temporary table counterparts. The only problem with table variables is that they cannot be indexed, so some performance could be lost despite their nature. Check here for some more information on the subject.