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Re: Confusion and Questions about blocks read

From: Markus Schaber <schabi(at)logix-tt(dot)com>
To: "Pgsql-Performance ((E-mail))" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Confusion and Questions about blocks read
Date: 2006-09-23 12:19:42
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
Hi, Tom,

Tom Lane wrote:
> "Alex Turner" <armtuk(at)gmail(dot)com> writes:
>> Home come the query statistics showed that 229066 blocks where read given
>> that all the blocks in all the tables put together only total 122968?
> You forgot to count the indexes.  Also, the use of indexscans in the
> mergejoins probably causes multiple re-reads of some table blocks,
> depending on just what the physical ordering of the rows is.

As far as I understand, Index Bitmap Scans improve this behaviour, by
ensuring that every table block is read only once.

Btw, would it be feasible to enhance normal index scans by looking at
all rows in the current table block whether they meet the query
criteria, fetch them all, and blacklist the block for further revisiting
during the same index scan?

I think that, for non-sorted cases, this could improve index scans a
little, but I don't know whether it's worth the effort, given that
bitmap indidex scans exist.


Markus Schaber | Logical Tracking&Tracing International AG
Dipl. Inf.     | Software Development GIS

Fight against software patents in Europe!

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