Hi,
we came across a database where a table had a toasted table,
keeping huge amounts of disk space allocated. However,
the table's current definition didn't explain why there was
a toasted table. Then upon some experiments, it struck me.
There _was_ a toasted field but as the schema was modified,
the fields was dropped, leaving only inline stored fields.
VACUUM [FULL] [ANALYZE] didn't cleaned up the space
that was used by the toasted table. My tests were done on 8.3.3.
As every statements that reference a table puts a lock on the
pg_class record, ALTER TABLE cannot progress until all locks
are gone, i.e. the transactions referencing the table finished.
It's true vice-versa, ALTER TABLE blocks every transactions
that may reference the table. Judging from that, the toasted table
cleanup may be part of ALTER TABLE DROP COLUMN.
Best regards,
Zoltán Böszörményi
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Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
http://www.postgresql.at/