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Re: GIN fast insert

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Jeff Davis <pgsql(at)j-davis(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Teodor Sigaev <teodor(at)sigaev(dot)ru>, Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: GIN fast insert
Date: 2009-02-24 19:56:25
Message-ID: 27040.1235505385@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Jeff Davis <pgsql(at)j-davis(dot)com> writes:
> On Tue, 2009-02-24 at 00:18 -0500, Robert Haas wrote: 
>> It only took me about 5 minutes to come up with a test case against CVS
>> HEAD where disabling index scans resulted in a significant dropoff in
>> performance.  Here it is:

> On the other hand, Teodor showed a typical use case and a very
> substantial performance gain:

Yeah.  Whatever we do here is a tradeoff (and whether Robert likes it
or not, reliability and code maintainability weigh heavily in the
tradeoff).

> I wonder how many people really use GIN with non-bitmap scans for some
> benefit? And even if the benefit exists, does the planner have a way to
> identify those cases reliably, or does it have to be done manually?

A relevant point there is that most of the estimator functions for
GIN-amenable operators are just smoke and mirrors; so if the planner
is making a good choice between indexscan and bitmapscan at all, it's
mostly luck.  This might get better someday, but not in 8.4.

			regards, tom lane

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