Message-ID: <4BC4BBD4.40706@setonhome.org>
Date: Tue, 13 Apr 2010 14:45:40 -0400
From: Lewis Kapell <lkapell@setonhome.org>
Organization: Seton Home Study School
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To: pgsql-admin@postgresql.org
Subject: TOAST behavior in 8.3 and 8.4
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I have a table with a text column, wherein most values range between 800 
and 1700 bytes.  The data should be highly compressible, however these 
values appear to be too small to trigger the TOAST mechanism - I 
understand the default value of TOAST_TUPLE_THRESHOLD is about 2kb.

This table is by far the largest in our database (1006450 rows; 
pg_relation_size() gives 1580 MB), and I am keen to get the data 
compressed.  We compile from source and are currently running 8.3.10. 
Based on some discussions I found in the archives, I thought it might be 
worthwhile to lower TOAST_TUPLE_THRESHOLD and rebuild.

However, I checked to see whether the behavior of TOAST had been changed 
in 8.4, since I am hoping to upgrade soon.  I found the following 
comment in the release notes:

"Consider TOAST compression on values as short as 32 bytes (previously 
256 bytes)"

I don't understand what that '256 bytes' refers to.  That is a far cry 
from 2kb.  I would be grateful if anyone can fill in the evident gap in 
my knowledge here.

-- 

Thank you,

Lewis Kapell
Computer Operations
Seton Home Study School