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Re: pg_freespacemap question

Alvaro Herrera <alvherre@commandprompt.com>

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From: Alvaro Herrera <alvherre@commandprompt.com>
To: Tatsuo Ishii <ishii@sraoss.co.jp>
Cc: markir@paradise.net.nz, peter_e@gmx.net, pgsql-hackers@postgresql.org, tgl@sss.pgh.pa.us
Date: 2006-03-07T23:49:24Z
Lists: pgsql-hackers 
Tatsuo Ishii wrote:

> Just for curiousity, why FSM gathers info for indexes? I thought FSM
> is only good for tables.

It's part of the implementation of the page-recycling algorithm for
btrees Tom did for 7.4.  When a btree page is empty after a vacuum, it's
entered in the free space map.  When a page is split, the new page is
taken from the FSM (or the relation is extended if there isn't any.)

That's why the bytes-free number is zero: when a btree page makes it
into the FSM, we are sure it's completely empty.

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support