Re: I/O on select count(*)

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                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.