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From: "[Quipsy] Markus Karg" <karg@quipsy.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
CC: "pgsql-bugs@lists.postgresql.org" <pgsql-bugs@lists.postgresql.org>
Subject: AW: WHERE column = X AND column = Y will always be zero matching rows
Thread-Topic: WHERE column = X AND column = Y will always be zero matching
 rows
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Date: Fri, 4 Aug 2023 06:12:55 +0000
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Tom,

thanks a lot for your kind explanation, this was very interesting to read!

In fact, I have missed the "never executed" marks.

BTW, I did not "complain". I just noticed something and was unclear if this=
 is a bug or not, and I clearly told so in the opening of my posting. It wa=
s definitively not my intention to bother or annoy anybody, actually. Sorry=
 to have troubled so many in this mailing list.

-Markus


-----Urspr=FCngliche Nachricht-----
Von: Tom Lane <tgl@sss.pgh.pa.us>=20
Gesendet: Donnerstag, 3. August 2023 19:59
An: [Quipsy] Markus Karg <karg@quipsy.de>
Cc: pgsql-bugs@lists.postgresql.org
Betreff: Re: WHERE column =3D X AND column =3D Y will always be zero matchi=
ng rows

"[Quipsy] Markus Karg" <karg@quipsy.de> writes:
> (In the following X and Y are literals; X <> Y) I noticed is that...
> EXPLAIN SELECT column FROM Table WHERE column =3D X AND column =3D Y=20
> ...says that PostgreSQL actually wants to perform a Scan!

You should learn to read plans before complaining about them.

What you actually get is something like

regression=3D# explain analyze select * from tenk1 where hundred =3D 42 and=
 hundred =3D 99;
                                             QUERY PLAN                    =
                         =20
---------------------------------------------------------------------------=
--------------------------
 Result  (cost=3D5.04..224.95 rows=3D1 width=3D244) (actual time=3D0.001..0=
.002 rows=3D0 loops=3D1)
   One-Time Filter: false
   ->  Bitmap Heap Scan on tenk1  (cost=3D5.04..224.95 rows=3D1 width=3D244=
) (never executed)
         Recheck Cond: (hundred =3D 42)
         ->  Bitmap Index Scan on tenk1_hundred  (cost=3D0.00..5.04 rows=3D=
100 width=3D0) (never executed)
               Index Cond: (hundred =3D 42)  Planning Time: 0.148 ms  Execu=
tion Time: 0.046 ms
(8 rows)

See the "never executed" markings?  The one-time filter prevents anything b=
elow the Result from actually doing anything.