From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Robert Treat <xzilla(at)users(dot)sourceforge(dot)net> |
Cc: | pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: sort operation leads planner to different number of rows? |
Date: | 2006-01-11 21:39:25 |
Message-ID: | 28515.1137015565@sss.pgh.pa.us |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-hackers |
Robert Treat <xzilla(at)users(dot)sourceforge(dot)net> writes:
> Sort (cost=616.64..620.56 rows=1568 width=12) (actual time=46.579..54.641 rows=6407 loops=1)
> Sort Key: latest_download.host_id
> -> Subquery Scan latest_download (cost=498.14..533.42 rows=1568 width=12) (actual time=43.657..45.594 rows=472 loops=1)
> I am wondering why it would end up with a different number of rows after
> the sort operation.
The planner's estimate didn't change: 1568 at both steps. The "actual"
is the number of rows actually pulled from the node at runtime, and the
discrepancy here occurs because this is the inner side of a mergejoin.
mergejoin has to rescan duplicate inner rows to join them to duplicate
outer rows. It looks like you have a pretty fair number of
duplicates...
regards, tom lane
From | Date | Subject | |
---|---|---|---|
Next Message | Bruce Momjian | 2006-01-11 22:53:00 | Re: [HACKERS] Inconsistent syntax in GRANT |
Previous Message | Robert Treat | 2006-01-11 21:31:20 | sort operation leads planner to different number of rows? |