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Re: generalizing the planner knobs

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Jonah H(dot) Harris" <jonah(dot)harris(at)gmail(dot)com>
Cc: Neil Conway <neilc(at)samurai(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: generalizing the planner knobs
Date: 2005-12-01 18:20:24
Message-ID: 15364.1133461224@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
"Jonah H. Harris" <jonah(dot)harris(at)gmail(dot)com> writes:
> In the last couple weeks I too have been thinking about planner hints.
> Assuming I have read your post correctly, the issue I see with this idea is
> that, in most cases, there won't be much of a difference between adding an
> arbitrary cost value to each type of node and disabling it completely.
> Also, by fiddling with an arbitrary cost the user may introduce a lot of
> variation into the planner which may actually result in worse query plans.

Which is pretty much exactly the problem with "planner hints", too.
I've resisted that suggestion in the past and will continue to do so,
because hints are accidents waiting to happen.  Even if the hint is right
today for your current Postgres version and current data distribution,
it's likely not to be right further down the road --- but once the hint
is embedded in your application, how often are you going to revisit it?
As an example, a hint forcing the planner to use an indexscan with a
particular index might have been a great idea in PG 8.0 and a lousy idea
in 8.1, because it would prevent substitution of a possibly-far-better
bitmap indexscan.

The enable_foo switches are debug aids, not something you are expected
to fool with for production purposes, and the same would be true of
Neil's suggested multipliers.  While I don't feel any strong need for
variable multipliers, they'd be a small enough incremental amount of
work that the suggestion doesn't require a lot of supporting argument.
Adding a planner hint facility would be several orders of magnitude
more work, and it would be taking the system in a design direction that
I think is fundamentally misguided.

			regards, tom lane

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