As you can see, PostgreSQL needs to do a sequencial scan to count because its MVCC nature and indices don't have transaction information. It's a known drawback inherent to the way PostgreSQL works and which gives very good results in other areas. It's been talked about adding some kind of approximated count which wouldn't need a full table scan but I don't think there's anything there right now. A Dijous 22 Març 2007 11:53, Andreas Tille va escriure: > Hi, > > I just try to find out why a simple count(*) might last that long. > At first I tried explain, which rather quickly knows how many rows > to check, but the final count is two orders of magnitude slower. > > My MS_SQL server using colleague can't believe that. > > $ psql InfluenzaWeb -c 'explain SELECT count(*) from agiraw ;' > QUERY PLAN > ----------------------------------------------------------------------- > Aggregate (cost=196969.77..196969.77 rows=1 width=0) > -> Seq Scan on agiraw (cost=0.00..185197.41 rows=4708941 width=0) > (2 rows) > > real 0m0.066s > user 0m0.024s > sys 0m0.008s > > $ psql InfluenzaWeb -c 'SELECT count(*) from agiraw ;' > count > --------- > 4708941 > (1 row) > > real 0m4.474s > user 0m0.036s > sys 0m0.004s > > > Any explanation? > > Kind regards > > Andreas. -- Albert Cervera Areny Dept. Informàtica Sedifa, S.L. Av. Can Bordoll, 149 08202 - Sabadell (Barcelona) Tel. 93 715 51 11 Fax. 93 715 51 12 ==================================================================== ........................ AVISO LEGAL ............................ La presente comunicación y sus anexos tiene como destinatario la persona a la que va dirigida, por lo que si usted lo recibe por error debe notificarlo al remitente y eliminarlo de su sistema, no pudiendo utilizarlo, total o parcialmente, para ningún fin. Su contenido puede tener información confidencial o protegida legalmente y únicamente expresa la opinión del remitente. El uso del correo electrónico vía Internet no permite asegurar ni la confidencialidad de los mensajes ni su correcta recepción. En el caso de que el destinatario no consintiera la utilización del correo electrónico, deberá ponerlo en nuestro conocimiento inmediatamente. ==================================================================== ........................... DISCLAIMER ............................. This message and its attachments are intended exclusively for the named addressee. If you receive this message in error, please immediately delete it from your system and notify the sender. You may not use this message or any part of it for any purpose. The message may contain information that is confidential or protected by law, and any opinions expressed are those of the individual sender. Internet e-mail guarantees neither the confidentiality nor the proper receipt of the message sent. If the addressee of this message does not consent to the use of internet e-mail, please inform us inmmediately. ====================================================================
