Received: from malur.postgresql.org ([217.196.149.56]) by arkaria.postgresql.org with esmtp (Exim 4.80) (envelope-from ) id 1WgzUp-0002KC-Nk for pgsql-sql@arkaria.postgresql.org; Sun, 04 May 2014 16:42:35 +0000 Received: from localhost ([127.0.0.1] helo=postgresql.org) by malur.postgresql.org with smtp (Exim 4.80) (envelope-from ) id 1WgzUo-00026X-LM for pgsql-sql@arkaria.postgresql.org; Sun, 04 May 2014 16:42:34 +0000 Received: from makus.postgresql.org ([2001:4800:7903:4::125]) by malur.postgresql.org with esmtp (Exim 4.80) (envelope-from ) id 1WgzUn-00026Q-Ez for pgsql-sql@postgresql.org; Sun, 04 May 2014 16:42:33 +0000 Received: from prod2.officenet.no ([195.159.87.246]) by makus.postgresql.org with esmtp (Exim 4.80) (envelope-from ) id 1WgzUe-0000PM-Mz for pgsql-sql@postgresql.org; Sun, 04 May 2014 16:42:32 +0000 Received: from localhost ([127.0.0.1] helo=prod2) by prod2.officenet.no with esmtp (Exim 4.76) (envelope-from ) id 1WgzSZ-00013G-TF; Sun, 04 May 2014 18:40:15 +0200 Date: Sun, 4 May 2014 18:40:15 +0200 (CEST) From: Andreas Joseph Krogh To: =?UTF-8?Q?Brice_Andr=C3=A9?= Cc: pgsql-sql Message-ID: In-Reply-To: Subject: Re: Optimize query for listing un-read messages MIME-Version: 1.0 X-Mailer: OfficeNet Mail 1.9.0-SNAPSHOT X-Pg-Spam-Score: -0.1 (/) Content-Type: multipart/related; boundary="----=_Part_181_1552405097.1399221615284" List-Archive: List-Help: List-ID: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: X-Mailing-List: pgsql-sql Precedence: bulk Sender: pgsql-sql-owner@postgresql.org ------=_Part_180_642535859.1399221615284 Content-Type: multipart/related; boundary="----=_Part_181_1552405097.1399221615284" ------=_Part_181_1552405097.1399221615284 Content-Type: multipart/alternative; boundary="----=_Part_182_1201624097.1399221615303" ------=_Part_182_1201624097.1399221615303 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable P=C3=A5 s=C3=B8ndag 04. mai 2014 kl. 14:06:35, skrev Brice Andr=C3=A9 >: Dear Andreas, =C2=A0 For me, putting both "LEFT OUTER JOIN" and "NOT EXISTS" is a bad id= ea. =C2=A0 As the "LEFT OUTER JOIN" will put fields of non-existing right tabl= e to=20 null, I would simply rewrite it : SELECT ... FROM message m =C2=A0=C2=A0=C2=A0 LEFT OUTER JOIN message_property prop ON prop.message_i= d =3D m.id=20 AND prop.person_id =3D 1 WHERE prop.is_read =3D TRUE =C2=A0 I would also ensure that an efficient index is used for the outer j= oin. I=20 would probably try at least a multi-column index on (message_id, person_id)= for=20 the property table. I would also maybe give a try to an index on (message_i= d,=20 person_id, is_read), just to see if it improves performances. =C2=A0 The pr= oblem is=20 that your suggested query doesn't return the desired results as it effectiv= ely=20 is an INNER JOIN because you have "WHERE prop.is_read=3DTRUE", defeating th= e=20 whole purpose of a LEFT OUTER JOIN. =C2=A0 -- Andreas Jospeh Krogh CTO / Pa= rtner -=20 Visena AS Mobile: +47 909 56 963 andreas@visena.com =20 www.visena.com =C2=A0 ------=_Part_182_1201624097.1399221615303 Content-Type: text/html;charset=UTF-8 Content-Transfer-Encoding: quoted-printable
P=C3=A5 s=C3=B8ndag 04. mai 2014 kl. 14:06:35, skrev Brice Andr=C3=A9 = <brice@famil= le-andre.be>:
Dear Andreas,
=C2=A0
For me, putting both "LEFT OUTER JOIN" and "NOT EXISTS"= is a bad idea.
=C2=A0
As the "LEFT OUTER JOIN" will put fields of non-existing right ta= ble to null, I would simply rewrite it :
SELECT ... FROM message m
=C2=A0=C2=A0=C2=A0 LEFT OUTER JOIN message_property prop ON prop.message_id= =3D m.id AND prop.person_id = =3D 1
WHERE prop.is_read =3D TRUE
=C2=A0
I would also ensure that an efficient index is used for the outer join. I w= ould probably try at least a multi-column index on (message_id, person_id) = for the property table. I would also maybe give a try to an index on (messa= ge_id, person_id, is_read), just to see if it improves performances.
=C2=A0
The problem is that your suggested query doesn't return the desired re= sults as it effectively is an INNER JOIN because you have "WHERE prop.= is_read=3DTRUE", defeating the whole purpose of a LEFT OUTER JOIN.
=C2=A0
--
Andrea= s Jospeh Krogh
CTO / Partner<= /span> - Visena AS
Mobile: +47 90= 9 56 963
3D""
=C2=A0
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