From: Tom Lane <tgl@sss.pgh.pa.us>
To: Karl Czajkowski <karlcz@isi.edu>
Cc: pgsql-performance@postgresql.org
Subject: Re: optimizing immutable vs. stable function calls?
Date: Wed, 18 Jan 2017 17:54:09 -0500
Message-ID: <14964.1484780049@sss.pgh.pa.us> (raw)
In-Reply-To: <20170118223633.GC23081@moraine.isi.edu>
References: <20170118223633.GC23081@moraine.isi.edu>
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Karl Czajkowski <karlcz@isi.edu> writes:
> The query planner does not seem to
> recognize that it can eliminate redundant calls to a STABLE function.
No, it doesn't.
> In my case, the function call does not take any arguments and is thus
> trivially independent of row data, and appears in a WHERE clause being
> compared to constants. Why wouldn't the optimizer treat this case the
> same as IMMUTABLE?
"The same as IMMUTABLE" would be to reduce the function to a constant at
plan time, which would be the wrong thing. It would be valid to execute
it only once at query start, but there's no built-in mechanism for that.
But you could force it by putting it in a sub-SELECT, that is if you
don't like the performance of
SELECT ... slow_stable_function() ...
try this:
SELECT ... (SELECT slow_stable_function()) ...
That works because it's an uncorrelated sub-query, which gets evaluated
just once per run. But the overhead associated with that mechanism is
high enough that forcing it automatically for every stable function would
be a loser. I'd recommend doing it only where it *really* matters.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, karlcz@isi.edu
Subject: Re: optimizing immutable vs. stable function calls?
In-Reply-To: <14964.1484780049@sss.pgh.pa.us>
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