From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
cc: Andrew Dunstan <andrew@dunslane.net>, Peter Geoghegan <pg@heroku.com>,
 Jim Nasby <Jim.Nasby@bluetreble.com>,
 Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: Less than ideal error reporting in pg_stat_statements
In-reply-to: <20151005154032.GF8531@alvherre.pgsql>
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Comments: In-reply-to Alvaro Herrera <alvherre@2ndquadrant.com>
 message dated "Mon, 05 Oct 2015 12:40:32 -0300"
Date: Mon, 05 Oct 2015 11:50:30 -0400
Message-ID: <31122.1444060230@sss.pgh.pa.us>
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Sender: pgsql-hackers-owner@postgresql.org

Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> Andrew Dunstan wrote:
>> FWIW, (a) and (b) but not (c) is probably the right description for my
>> client who has been seeing problems here.

> I think the fact that long IN lists are fingerprinted differently
> according to the number of elements in the list makes the scenario
> rather very likely -- not particularly narrow.

That's certainly something worth looking at, but I think it's probably
more complicated than that.  If you just write "WHERE x IN (1,2,3,4)",
that gets folded to a ScalarArrayOp with a single array constant, which
the existing code would deal with just fine.  We need to identify
situations where that's not the case but yet we shouldn't distinguish.

In any case, that's just a marginal tweak for one class of query.
I suspect the right fix for the core problem is the one Peter mentioned
in passing earlier, namely make it possible to do garbage collection
without having to slurp the entire file into memory at once.  It'd be
slower, without a doubt, but we could continue to use the existing code
path unless the file gets really large.

			regards, tom lane


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