Re: Resolving Index Bloat

Thanks, Tom.

The database was dumped/restored on Monday, 12 October and the autovac settings applied right after. The query showing the bloat was issued on Monday, 19 October so a time period of 1 week elapsed since baseline. Data types in the format <index_name> - <data_type> as follows:

billingitemrating_tariff_idx - integer
billingitemrating_itemdescription_idx - integer
billingitemrating_pkey1 - bigint
billingitemrating_psi_idx - integer
billingitemrating_bpid_idx - integer
importitem_pkey - integer
importitem_status_ignored_idx - multicolumn (integer, text)
importitem_subscriptionid_idx - text
idx_importitem_importitemgroup - integer
idx_importitem_importitemgroup_status - multicolumn (integer, integer)
idx_importitem_status - integer
billingitemrating_biid_idx - integer
billingitemrating_ebid_idx - integer

I'll have a look at pgstattuple

Sam

-----Original Message-----
From: Tom Lane [mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us]
Sent: Tuesday, 20 November 2012 10:50 AM
To: Samuel Stearns
Cc: pgsql-admin(at)postgresql(dot)org
Subject: Re: [ADMIN] Resolving Index Bloat

Samuel Stearns <SStearns(at)internode(dot)com(dot)au> writes:
> We have a problem with index bloat on a couple of our tables even though we have applied more aggressive autovac/analyze settings in the schema:

Hard to tell much about this without knowing the baseline condition or what's happened since the baseline. It looks like your tables have grown circa 2X (eg billingitemrating), but is that due to new data or heavy update activity?

If the baseline condition is freshly-built-or-REINDEXed indexes, a fair amount of "bloat" is to be expected. The traditional rule of thumb about btree indexes is that the steady-state load factor is about two-thirds full. By default, PG builds indexes tightly packed --- so just allowing the index to reach steady state will incur 50% "bloat"
on average. It's usually counterproductive to try to maintain a fill factor better than that, unless the table receives only minimal insert/update traffic. (Indeed, usually the better policy for a heavy-update table is to create the indexes with 66% fillfactor to begin
with.)

Your indexes on billingitemrating seem to have expanded a bit more than what would be expected from the combination of these factors, but I'm not sure they're enormously out of line. You could delve a bit deeper by using contrib/pgstattuple to measure the actual dead space in both the tables and the indexes. Also, it'd be useful to know the data types of the columns being indexed.

regards, tom lane