Re: Slowing UPDATEs inside a transaction

On Thu, Mar 3, 2011 at 9:13 AM, Matt Burke <mattblists(at)icritical(dot)com> wrote:
> Hi. I've only been using PostgreSQL properly for a week or so, so I
> apologise if this has been covered numerous times, however Google is
> producing nothing of use.
>
> I'm trying to import a large amount of legacy data (billions of
> denormalised rows) into a pg database with a completely different schema,
> de-duplicating bits of it on-the-fly while maintaining a reference count.
> The procedures to do this have proven easy to write, however the speed is
> not pretty. I've spent some time breaking down the cause and it's come down
> to a simple UPDATE as evidenced below:

PostgreSQL uses MVCC, which means that transactions see a snapshot of
the database at existed at a certain point in time, usually the
beginning of the currently query. Old row versions have to be kept
around until they're no longer of interest to any still-running
transaction. Sadly, our ability to detect which row versions are
still of interest is imperfect, so we sometimes keep row versions that
are technically not required. Unfortunately, repeated updates by the
same transaction to the same database row are one of the cases that we
don't handle very well - all the old row versions will be kept until
the transaction commits. I suspect if you look at the problem case
you'll find that the table and index are getting bigger with every set
of updates, whereas when you do the updates in separate transactions
the size grows for a while and then levels off.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company