count(*) optimization - Mailing list pgsql-hackers

From huaxin zhang
Subject count(*) optimization
Date
Msg-id f2f5625105090612215aafe162@mail.gmail.com
Whole thread Raw
In response to Re: Simple tester for MVCC in PostgreSQL  (Matt Miller <mattm@epx.com>)
Responses Re: count(*) optimization
Re: count(*) optimization
List 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


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