Re: question about seq scan and index scan
От | Shridhar Daithankar |
---|---|
Тема | Re: question about seq scan and index scan |
Дата | |
Msg-id | 3DBD9931.28849.2EEB3B@localhost обсуждение исходный текст |
Ответ на | question about seq scan and index scan (Sébastien PALLEAU <spalleau@elma.fr>) |
Список | pgsql-general |
On 25 Oct 2002 at 18:18, Sébastien PALLEAU wrote: > > hi, > I tried to execute the following request on a postgresql 7.2.3 system : > > select count(*) from table tablea a, tableb b, tablec c where a.id=b.id and > a.id=c.id and a.mynumber > 5000; > tablea, tableb and tablec have primaryon id that identify a unique member. > > tablea contains 471413 records tableb contains 471413 records tablec contains > 471413 records > > a.id is tablea primary key > > > > b.id is tableb primary key c.id is tablec primary key > > > > An explain on the request provides the following results. > seq scan on tableb.id (I dont agree) > seq scan on tablec.id (i dont agree) > seq scan on tablea.id (seems normal but why executed last ?) > > and the most strange is that for the following request : > explain select count(*) from table tablea a, tableb b, tablec c where a.id=b.id > and a.id=c.id and a.mynumber > 20000; > provides : > index scan using numberpoints_tablea_key > index scan using tableb_pkey > index scan using tablec_pkey > > why doesn't postgres uses indexes in the first case ? umm. What happens if you phrase like a.mynumber > 20000 and a.id=b.id and a.id=c.id and BTW what's the difference between two queries? I failed to spot any.. Bye Shridhar -- Cynic, n.: One who looks through rose-colored glasses with a jaundiced eye.
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