Re: Help tuning postgres - Mailing list pgsql-performance

On Thu, Oct 13, 2005 at 10:15:03AM +0200, Csaba Nagy wrote:
>
> OK, this sounds interesting, but I don't understand: why would an update
> "chase down a lot of dead tuples" ? Should I read up on some docs, cause
> I obviously don't know enough about how updates work on postgres...

Right.  Here's the issue:

MVCC does not replace rows when you update.  Instead, it marks the
old row as expired, and sets the new values.  The old row is still
there, and it's available for other transactions who need to see it.
As the docs say (see
<http://www.postgresql.org/docs/8.0/interactive/transaction-iso.html>),
"In effect, a SELECT query sees a snapshot of the database as of the
instant that that query begins to run."  And that can be true because
the original data is still there, although marked as expired for
subsequent transactions.

UPDATE works the same was as SELECT in terms of searching for rows
(so does any command that searches for data).

Now, when you select data, you actually have to traverse all the
existing versions of the tuple in order to get the one that's live
for you.  This is normally not a problem: VACUUM goes around and
cleans out old, expired data that is not live for _anyone_.  It does
this by looking for the oldest transaction that is open.  (As far as
I understand it, this is actually the oldest transaction in the
entire back end; but I've never understood why that should the the
case, and I'm too incompetent/dumb to understand the code, so I may
be wrong on this point.)  If you have very long-running transactions,
then, you can end up with a lot of versions of dead tuples on the
table, and so reading the few records you want can turn out actually
to be a very expensive operation, even though it ought to be cheap.

You can see this by using the VERBOSE option to VACUUM:

test=# VACUUM VERBOSE eval1 ;
INFO:  vacuuming "public.eval1"
INFO:  "eval1": found 0 removable, 0 nonremovable row versions in 0
pages
DETAIL:  0 dead row versions cannot be removed yet.
There were 0 unused item pointers.
0 pages are entirely empty.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO:  vacuuming "pg_toast.pg_toast_18831"
INFO:  index "pg_toast_18831_index" now contains 0 row versions in 1
pages
DETAIL:  0 index pages have been deleted, 0 are currently reusable.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO:  "pg_toast_18831": found 0 removable, 0 nonremovable row
versions in 0 pages
DETAIL:  0 dead row versions cannot be removed yet.
There were 0 unused item pointers.
0 pages are entirely empty.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
VACUUM

Note those "removable" and "nonremovable" row versions.  It's the
unremovable ones that can hurt.  WARNING: doing VACUUM on a big table
on a disk that's already pegged is going to cause you performance
pain, because it scans the whole table.  In some cases, though, you
have no choice: if the winds are already out of your sails, and
you're effectively stopped, anything that might get you moving again
is an improvement.

> And how would the analyze help in finding this out ? I thought it would
> only show me additionally the actual timings, not more detail in what
> was done...

Yes, it shows the actual timings, and the actual number of rows.  But
if the estimates that the planner makes are wildly different than the
actual results, then you know your statistics are wrong, and that the
planner is going about things the wrong way.  ANALYSE is a big help.
There's also a verbose option to it, but it's usually less useful in
production situations.

A

--
Andrew Sullivan  | ajs@crankycanuck.ca
It is above all style through which power defers to reason.
        --J. Robert Oppenheimer