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Re: [WIP] Caching constant stable expressions per execution

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Marti Raudsepp <marti(at)juffo(dot)org>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [WIP] Caching constant stable expressions per execution
Date: 2011-09-10 22:51:31
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Lists: pgsql-hackers
Marti Raudsepp <marti(at)juffo(dot)org> writes:
> This is a proof of concept patch for recognizing stable function calls
> with constant arguments and only calling them once per execution. I'm
> posting it to the list to gather feedback whether this is a dead end
> or not.

Hmm.  This is an interesting hack, but I'm not sure it's more than a

Usually what people have meant when they ask for "function caching"
is that multiple executions of a given stable function with the same
argument values be folded to just one execution.  In the general case
that would require some sort of hash table in which we memoize sets of
argument values and the corresponding result.  The reason that I've
generally discouraged this line of thought is that I see no way that
such a hash table could be managed or searched with sufficient
efficiency to make it a win.

What you've done here is to solve the problem for the special case of
a single (textual) instance of the function called with
plan-time-constant arguments.  While that's probably a common case,
I'm not sure it's common enough to justify extra planner and executor

The patch as given has a bunch of implementation issues, but I think
it's close enough for crude performance testing, and your numbers do
show a potential performance benefit.  The question that I think is
unresolved is whether the set of cases covered is wide enough to be
useful in practice.  I have no data on that ...

			regards, tom lane

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