Re: Strange coding in mvdistinct.c

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
Cc: pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: Strange coding in mvdistinct.c
Date: 2019-04-15 22:12:24
Message-ID: 30400.1555366344@sss.pgh.pa.us
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Oh, and as I continue to grep, I found this in dependencies.c:

dependencies = (MVDependencies *) repalloc(dependencies,
offsetof(MVDependencies, deps)
+ dependencies->ndeps * sizeof(MVDependency));

I'm pretty sure this is an actual bug: the calculation should be

offsetof(MVDependencies, deps)
+ dependencies->ndeps * sizeof(MVDependency *));

because deps is an array of MVDependency* not MVDependency.

This would lead to an overallocation not underallocation, and it's
probably pretty harmless because ndeps can't get too large (I hope;
if it could, this would have O(N^2) performance problems). Still,
you oughta fix it.

(There's a similar calculation later in the file that gets it right.)

regards, tom lane

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