From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
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
In-reply-to: <20190710212609.5mrl7hltbozoxpq7@development>
References: <8736jdhbhc.fsf@ansel.ydns.eu> <15097.1562792274@sss.pgh.pa.us>
 <20190710212609.5mrl7hltbozoxpq7@development>
Comments: In-reply-to Tomas Vondra <tomas.vondra@2ndquadrant.com>
	message dated "Wed, 10 Jul 2019 23:26:09 +0200"
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Date: Wed, 10 Jul 2019 17:45:24 -0400
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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.

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

			regards, tom lane