question on TRUNCATE vs VACUUM FULL - Mailing list pgsql-performance

Hi folks,

We are running Postgres 8.2.5.

I have 3 tables, call them A, B, and C

Table A houses info on all emails that have ever been created for the purpose of being delivered to our end customers.

Big table.  About 23 million rows.

  Table B, the ‘holding’ table is populated with Table A key information via an after trigger whenever Table A is updated or inserted to.

  Table C, the ‘work’ table is populated by function D from table B.  It is configured exactly like table B.

  PLPGSQL Function D inserts a predefined number of rows from table B to table C. For purposes of discussion, say 500.  

  Function D, after it does its thing, then deletes the 500 rows it processed from table B, and ALL 500 rows from table C.

This entire process, after a sleep period of 10 seconds, repeats itself all day.

After each fifth iteration of function D, we perform a VACUUM FULL on both tables B and C.

   Takes less than 5 seconds.

In terms of transaction processing:

  Table A is processed by many transactions (some read, some update),

  Table B is processed by

-         any transaction updating or inserting to Table A via the after trigger (insert, update)

-         Function D (insert, update, delete)

  Table C is processed ONLY by function D (insert, update, delete).  Nothing else touches it;

    PG_LOCKS table verifies that that this table is totally free of any transaction

        Between iterations of function D.

So my question is this:  Shouldn’t VACUUM FULL clean Table C and reclaim all its space?

It doesn’t.  It usually reports the same number of pages before and after the Vacuum.

We have to resort to TRUNCATE to clean and reclaim this table, which

Must be empty at the beginning of function D.

Any insights appreciated. Thanks,