Investigating the reason for a very big TOAST table size

From: Liron Shiri
Sent: Sunday, August 26, 2012 3:47 PM
To: 'pgsql-performance(at)postgresql(dot)org'
Subject: Investigating the reason for a very big TOAST table size
Importance: High

Hi,

We have a table which its TOAST table size is 66 GB, and we believe should be smaller.
The table size is 472 kb. And the table has 4 columns that only one of them should be toasted.

The table has only 8 dead tuples, so apparently this is not the problem.

This table contains a column with bytea type data (kept as TOAST). We tried to check what is the size of the toasted data in each row by using the following query (the data_blob is the bytea column):

SELECT nid, octet_length(data_blob) FROM my_table ORDER BY octet_length(data_blob) DESC;

This result contain 1782 rows. The sizes I get from each row are between 35428 to 42084.

1782 * 38000 = 67716000 byte = 64.579 MB .

What can be the reason for a table size of 66 GB? What else should I check?

Thanks in advance,
Liron