From: Tom Lane <tgl@sss.pgh.pa.us>
To: Marc Cousin <cousinmarc@gmail.com>
cc: Alexander Korotkov <a.korotkov@postgrespro.ru>,
        Julien Rouhaud <rjuju123@gmail.com>,
        Tomas Vondra <tomas.vondra@2ndquadrant.com>,
        Nikita Glukhov <n.gluhov@postgrespro.ru>,
        PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Avoid full GIN index scan when possible
In-reply-to: <eca8612b-e289-e2fc-c5df-84be46f75ec1@gmail.com>
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Comments: In-reply-to Marc Cousin <cousinmarc@gmail.com>
	message dated "Mon, 01 Jul 2019 17:56:17 +0200"
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Date: Tue, 30 Jul 2019 13:27:48 -0400
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Marc Cousin <cousinmarc@gmail.com> writes:
> By the way, while preparing this, I noticed that it seems that during th=
is kind of index scan, the interrupt signal is masked
> for a very long time. Control-C takes a very long while to cancel the qu=
ery. But it's an entirely different problem :)

Yeah, that seems like an independent problem/patch, but it's not obvious
where to fix --- can you provide a self-contained test case?

Meanwhile, I looked at the v3 patch, and it seems like it might not be
too far from committable.  I think we should *not* let this get bogged
down in questions of whether EXPLAIN can report which index quals were
used or ignored.  That's a problem that's existed for decades in the
btree code, with more or less zero user complaints.

I do think v3 needs more attention to comments, for instance this
hunk is clearly falsifying the adjacent comment:

@ -141,7 +141,8 @@ ginFillScanKey(GinScanOpaque so, OffsetNumber attnum,
 	uint32		i;
 =

 	/* Non-default search modes add one "hidden" entry to each key */
-	if (searchMode !=3D GIN_SEARCH_MODE_DEFAULT)
+	if (searchMode !=3D GIN_SEARCH_MODE_DEFAULT &&
+		(searchMode !=3D GIN_SEARCH_MODE_ALL || nQueryValues))
 		nQueryValues++;
 	key->nentries =3D nQueryValues;
 	key->nuserentries =3D nUserQueryValues;

Also, I agree with Julien that this

+			so->forcedRecheck =3D key->triConsistentFn(key) !=3D GIN_TRUE;

probably needs to be

+			so->forcedRecheck |=3D key->triConsistentFn(key) !=3D GIN_TRUE;


			regards, tom lane