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Re: I/O on select count(*)

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Greg Smith <gsmith(at)gregsmith(dot)com>
Cc: Matthew Wakeling <matthew(at)flymine(dot)org>, pgsql-performance(at)postgresql(dot)org
Subject: Re: I/O on select count(*)
Date: 2008-05-16 20:15:28
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
Greg Smith wrote:

> After reading the code and that documentation a bit, the part I'm still  
> not sure about is whether the CLOG entry is created when the XID is  
> assigned and then kept current as the state changes, or whether that 
> isn't even in CLOG until the transaction is committed.  It seems like the 
> latter, but there's some ambiguity in the wording and too many code paths 
> for me to map right now.

pg_clog is allocated in pages of 8kB apiece(*).  On allocation, pages are
zeroed, which is the bit pattern for "transaction in progress".  So when
a transaction starts, it only needs to ensure that the pg_clog page that
corresponds to it is allocated, but it need not write anything to it.

(*) Each transaction needs 2 bits, so on a 8 kB page there is space for
4 transactions/byte * 8 pages * 1kB/page = 32k transactions.

Alvaro Herrera                      
The PostgreSQL Company - Command Prompt, Inc.

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