Overview
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