postgresql - postgres 9.2 table size with pg_total_relation_size

Question

I process a table with ~10^7 rows the following way: take last N rows, update them in some way, and delete, then vacuum table. In the end I make a query for pg_total_relation_size. Loop repeats until the table is over. Each iteration last for several seconds. There are no any other queries for this table except mentioned above. The problem is that I get the same results for table size. It changes about once a several hours.

So the question is -- does postgres store somewhere the table size or does it calculate it every time the function is invoked? I.e., does my table size really stays the same in spite of its processing?

Answer 1

Your table really does stay the same size on disk despite the DELETEs and VACUUMing you're doing. As per the documentation on VACUUM, ordinary VACUUM only releases space back to the OS if it can do so by truncating free space from the end of the file without rearranging live rows.

The space is still "free" in that PostgreSQL can re-use it for other new rows. It is much, much faster to re-use space that PostgreSQL hasn't given back to the OS than it is to extend a relation with new space, so this is often preferable.

The other reason Pg doesn't just give this space back is that it can only give space back to the OS when it's a contiguous chunk with no visible rows until the end of the file. This doesn't happen much so in practice Pg needs to move some rows around to compact the table and allow it to free space at the end, kind of like a defrag on a file system. This is an inefficient and slow process that can counter-intuitively make the table slower to access instead of faster, so it's not always a good idea.

If you have a relation that's mostly but not entirely empty it can be worth doing a VACUUM FULL (Pg 9.0 and above) or CLUSTER (all versions) to free the space. If you expect to refill the table this is usually counter-productive; it's actually better to leave it as-is.

(For what I mean by terms like "live" and "visible" see the documentation on MVCC which will help you understand Pg's table organisation.)

Personally I'd skip the manual VACUUM in your case. Turn autovacuum up if you need to. If you really need to you could consider partitioning your table, processing it partition by partition and TRUNCATE each partition when you're done processing it.