Re: Size of Postgres Transaction Logs

Lincoln Yeoh <lyeoh(at)pop(dot)jaring(dot)my> writes:
> So what happens typically is:
> 1) open DB connection.
> 2) wait for http request.
> 3) rollback (to implicitly begin a new transaction so that 'now' isn't
> years ago :) ).
> 4) do DB stuff
> 5) rollback/commit.

> Would we get massive WAL growth if the long running transactions are
> inactive (stuck at step 2) - not doing anything, but other transactions are
> active?

Not if they haven't done anything yet. "BEGIN" doesn't really start a
transaction, it's only your first database access that gets the
transaction engine running. Furthermore, if I recall the WAL stuff
correctly, what really counts for WAL is your first *write* access.
A read-only xact doesn't exist as far as WAL is concerned.

Nonetheless, I'm pretty concerned about this log-growth issue. See
current pghackers thread about whether we really need WAL to provide
UNDO support. AFAICS, we could truncate the WAL log at each checkpoint
if we didn't need UNDO.

regards, tom lane