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Re: [PATCHES] Proposed patch: synchronized_scanning GUC variable

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Neil Conway <neilc(at)samurai(dot)com>, Gregory Stark <stark(at)enterprisedb(dot)com>, "Jonah H(dot) Harris" <jonah(dot)harris(at)gmail(dot)com>, pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: [PATCHES] Proposed patch: synchronized_scanning GUC variable
Date: 2008-01-28 21:21:44
Message-ID: 7380.1201555304@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> Rather than having a boolean GUC, we should have a number and make the
> parameter "synchronised_scan_threshold".

This would open up a can of worms I'd prefer not to touch, having to do
with whether the buffer-access-strategy behavior should track that or
not.  As the note in heapam.c says,

     * If the table is large relative to NBuffers, use a bulk-read access
     * strategy and enable synchronized scanning (see syncscan.c).  Although
     * the thresholds for these features could be different, we make them the
     * same so that there are only two behaviors to tune rather than four.

It's a bit late in the cycle to be revisiting that choice.  Now we do
already have three behaviors to worry about (BAS on and syncscan off)
but throwing in a randomly settable knob will take it back to four,
and we have no idea how that fourth case will behave.  The other tack we
could take (having the one GUC variable control both thresholds) is
not good since it will result in pg_dump trashing the buffer cache.

			regards, tom lane

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