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Avoiding Bottlenecks with Temporary Tables



Were you aware that the act of populating a temporary table can cause

system-wide bottlenecks on your server?  Problems can occur both with SQL

Server 6.5 and 7 in different ways, and in this article I will discuss how best

to avoid them.

Bottlenecks in version 6.5

Many people use a select...into query to create a temporary table,

something like this:

    select *

    into #tempTable

    from source

While this works, it creates locks against the temporary database for the

duration of the select statement (quite a while if you are trawling through a

lot of data in the source table, and longer still if the select...into is at the start of a

longer-running explicit transaction) While this lock is in place, no other user

can create temporary tables. The actual location of the bottleneck is a lock on

tempdb system tables. In later versions of SQL Server the locking model has

changed and the problem is avoided.

If a number of concurrent processes are trying to load temporary tables in

this way, particularly if large amounts of data are involved, then a bottleneck

is inevitable. 

The trick to freeing up tempdb is to ensure that the "create temporary

table" part of the operation is committed as quickly as possible. To do

this, recode the above statement it the following format: 

    create table #temp(



    insert #temp

    select *

    from sourceTable

In this manner we create our temporary table and free the sysobjects or

schema lock as quickly as possible.

Short cut to a solution

If you want to avoid coding the insert...into statement, or if you are writing a generic piece of

code and will not know the exact table definition until run-time, you can revert

to this trick:

    select *

    into #temp

    from sourceTable

    where 1 = 0

    insert #temp

    select *

    from sourceTable

Obviously where 1 = 0 is never true. No matter how much data is in sourceTable

SQL Server's optimiser is usually smart enough to realise that because one is

never equal to zero, it's not worth trawling through the source table. (If I can

I always

check the query execution plan just to make sure, but I have never caught it out yet)

Even though  SQL Server will not trawl through the source table,

the #temp table  will still be defined, based on the format of the

data in the select statement, but it will

contain no rows. You can then run the insert...select statement secure in

the knowledge that you are not blocking other processes access to the tempdb.

Bottlenecks in version 6.5 and 7

For the most part, the problem described above does not apply to version 7,

but there is is one instance where you can still unintentionally create these

bottlenecks under either version.

The problem arises when you use the INSERT...EXEC statement to load a temporary

table, and the stored procedure itself creates temporary tables, you end up with

blocking locks in tempdb similar to those described above. The prescribed

workarounds are either "don't do it in the first place", which is

inconvenient if you do not want to mess with legacy code or code you do not

control, or otherwise  to execute the stored procedure as a remote stored

procedure, (i.e. "INSERT #temp EXEC server.database.owner.proc") which

again is not ideal in all circumstances.

Further reading

Check out this Technet

article for more information on tempdb locking problems in SQL 6.5, and see this

article for both versions 6.5 and 7

About the author

Neil Boyle is an independent SQL Server consultant working out of London,

England. Neil's free SQL Server guide is available on-line at http://www.impetus-sql.co.uk


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