From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
cc: exclusion@gmail.com, pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #17334: Assert failed inside computeDistance() on gist index
 scanning
In-reply-to: <7c191fd2-9103-ae81-8f4f-22f39edd3c90@enterprisedb.com>
References: <17334-135f485c21739caa@postgresql.org>
 <7c191fd2-9103-ae81-8f4f-22f39edd3c90@enterprisedb.com>
Comments: In-reply-to Tomas Vondra <tomas.vondra@enterprisedb.com>
	message dated "Sun, 12 Dec 2021 23:59:43 +0100"
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Date: Sun, 12 Dec 2021 18:37:21 -0500
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Tomas Vondra <tomas.vondra@enterprisedb.com> writes:
> FWIW I can reproduce this on master too. The failure happens because of =

> NaN value in the index:

Man, what a pain those are.

> I'm not sure if the issue is in allowing the NaN to be added to the =

> index, or not handling it correctly during the index scan.

Surely omitting the entry from the index would lead to incorrect
answers.  Without any index, we get

regression=3D# CREATE TABLE point_tbl(f1 point);
CREATE TABLE
regression=3D# INSERT INTO point_tbl SELECT ('0,0') FROM generate_series(1=
, 2);
INSERT 0 2
regression=3D# INSERT INTO point_tbl VALUES ('0,NaN');
INSERT 0 1
regression=3D# SELECT f1, f1 <-> point '(0,0)' AS dist FROM point_tbl ORDE=
R BY dist;
   f1    | dist =

---------+------
 (0,0)   |    0
 (0,0)   |    0
 (0,NaN) |  NaN
(3 rows)

You can argue about where the NaN distance should sort, but
not about whether the row should appear at all.

> It's interesting btree_gist does not have issues (for NaN in float8 =

> columns). It seems not to store NaN in the index,  It seems to replace =

> them with tiny values, at least according to pageinspect.

Yipes, that's even worse, if true.

			regards, tom lane