From: Tom Lane <tgl@sss.pgh.pa.us>
To: Peter Geoghegan <pg@bowt.ie>
cc: pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Add deduplication to nbtree.
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References: <E1j73sn-0003FQ-KO@gemulon.postgresql.org>
Comments: In-reply-to Peter Geoghegan <pg@bowt.ie>
	message dated "Wed, 26 Feb 2020 21:06:17 +0000"
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Date: Sun, 01 Mar 2020 13:24:55 -0500
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Peter Geoghegan <pg@bowt.ie> writes:
> Add deduplication to nbtree.

Coverity isn't very happy with the coding in _bt_update_posting():

*** CID 1460433:  Memory - corruptions  (ARRAY_VS_SINGLETON)
/srv/coverity/git/pgsql-git/postgresql/src/backend/access/nbtree/nbtdedup.=
c: 723 in _bt_update_posting()
717     	{
718     		if (d < vacposting->ndeletedtids && vacposting->deletetids[d] =3D=
=3D i)
719     		{
720     			d++;
721     			continue;
722     		}
>>>     CID 1460433:  Memory - corruptions  (ARRAY_VS_SINGLETON)
>>>     Using "htids" as an array.  This might corrupt or misinterpret adj=
acent memory locations.
723     		htids[ui++] =3D *BTreeTupleGetPostingN(origtuple, i);
724     	}
725     	Assert(ui =3D=3D nhtids);
726     	Assert(d =3D=3D vacposting->ndeletedtids);
727     	Assert(nhtids =3D=3D 1 || _bt_posting_valid(itup));

I can see its point: asserting after the fact that you didn't clobber
memory isn't a terribly safe coding method, especially in a production
build where you won't even have the asserts.  Not sure if there's a
better way though.

			regards, tom lane