Wednesday, July 14, 2010

Table Variables and Query Optimizer

In yesterday's post I mentioned that I replaced the temp tables in a stored procedure with table variables. This worked perfectly functionally, but doomed the performance by orders of magnitude. The problem was that the query optimizer was choosing MERGE JOINs instead of HASH JOINs for the table variables. Forcing the hash joins with query hints fixed the problem.

[Ed. note 7/26]Here's another interesting discussion about this. Note the post that reads "And also, because table variables do not have statistics, the query optimizer will often take dubious choices because it believes the row count to be 1 all the time - leading to inappropriate join methods etc..."

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