Date: Wed, 10 Jul 2019 23:59:04 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andreas Seltenreich <seltenreich@gmx.de>,
	pgsql-hackers@lists.postgresql.org,
	Tomas Vondra <tomas.vondra@postgresql.org>
Subject: Re: [sqlsmith] Crash in mcv_get_match_bitmap
Message-ID: <20190710215904.ncaqlswrylliisv3@development>
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On Wed, Jul 10, 2019 at 05:45:24PM -0400, Tom Lane wrote:
>Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
>> Yeah, that's a bug. Will fix (not sure how yet).
>
>You could do worse than replace this:
>
>        ok = (NumRelids(clause) == 1) &&
>            (is_pseudo_constant_clause(lsecond(expr->args)) ||
>             (varonleft = false,
>              is_pseudo_constant_clause(linitial(expr->args))));
>
>with something like
>
>	if (IsA(linitial(expr->args), Var) &&
>	    IsA(lsecond(expr->args), Const))
>	   ok = true, varonleft = true;
>	else if (IsA(linitial(expr->args), Const) &&
>	         IsA(lsecond(expr->args), Var))
>	   ok = true, varonleft = false;
>
>Or possibly get rid of varonleft as such, and merge extraction of the
>"var" and "cst" variables into this test.
>

OK, thanks for the suggestion.

I probably also need to look at the "is compatible" test in
extended_stats.c which also looks at the clauses. It may not crash as it
does not attempt to extract the const values etc. but it likely needs to
be in sync with this part.

>BTW, I bet passing a unary-argument OpExpr also makes this code
>unhappy.

Whooops :-(


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services