Re: Stable function being evaluated more than once in a single query - Mailing list pgsql-performance

From Tom Lane
Subject Re: Stable function being evaluated more than once in a single query
Date
Msg-id 16007.1137198448@sss.pgh.pa.us
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In response to Re: Stable function being evaluated more than once in a single query  ("Jim C. Nasby")
Responses Re: Stable function being evaluated more than once in a single query  ("Jim C. Nasby")
List pgsql-performance
"Jim C. Nasby" <> writes:
> Is the issue that the optimizer won't combine two function calls (ie:
> SELECT foo(..) ... WHERE foo(..)), or is it that sometimes it won't make
> the optimization (maybe depending on the query plan, for example)?

What the STABLE category actually does is give the planner permission to
use the function within an indexscan qualification, eg,
    WHERE indexed_column = f(42)
Since an indexscan involves evaluating the comparison expression just
once and using its value to search the index, this would be incorrect
if the expression's value might change from row to row.  (For VOLATILE
functions, we assume that the correct behavior is the naive SQL
semantics of actually computing the WHERE clause at each candidate row.)

There is no function cache and no checking for duplicate expressions.
I think we do check for duplicate aggregate expressions, but not
anything else.

            regards, tom lane

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