count(*) optimization

From: huaxin zhang <uwcssa(at)gmail(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Subject: count(*) optimization
Date: 2005-09-06 19:21:16
Message-ID: f2f5625105090612215aafe162@mail.gmail.com
Views: Raw Message | Whole Thread | Download mbox | Resend email
Thread:
Lists: pgsql-hackers

not sure where to put this.

I run two queries:

1. select count(*) from table where indexed_column<10;
2. select * from table where indexed_column<10;

the indexed column is not clustered at all. I saw from the trace that
both query runs
through index scans on that index and takes the same amount of buffer
hits and disk read. However, shouldn't the optimizer notice that the
first query only needs to look at the indexes
and possibly reduce the amount of buffer/disk visits?

thanks

In response to

Responses

Browse pgsql-hackers by date

  From Date Subject
Next Message Jonah H. Harris 2005-09-06 19:44:13 Re: count(*) optimization
Previous Message Jim C. Nasby 2005-09-06 18:56:24 Re: Simple tester for MVCC in PostgreSQL