Re: Can the V7.3 EXPLAIN ANALYZE be trusted? - Mailing list pgsql-performance

You're probably right about my being overly optimistic about the load
imposed by EXPLAIN ANALYZE.  It was just that in my previous experience
with it, I'd never seen such a large runtime discrepancy before.  I even
allowed for a "caching effect" by making sure the server was all but
quiescent, and then running the three queries as quickly after one another
as I could.

The server itself is an IBM x345 with dual Xeon 3ghz CPU's (hyperthreading
turned off) and 2.5gb of RAM.  O/S is RHEL3 Update 4.  Disks are a
ServeRAID of some flavor, I'm not sure what.

Thanks for the heads-up about the performance of IN in 7.3.  We're looking
to migrate to 8.0 or 8.0.1 when they become GA, but some of our databases
are in excess of 200gb-300gb, and we need to make sure we have a good
migration plan in place (space to store the dump out of the 7.3 db) before
we start.
___________________________________________________________________________________

Steven Rosenstein
IT Architect/Developer | IBM Virtual Server Administration
Voice/FAX: 845-689-2064 | Cell: 646-345-6978 | Tieline: 930-6001
Text Messaging: 6463456978 @ mobile.mycingular.com
Email: srosenst @ us.ibm.com

"Learn from the mistakes of others because you can't live long enough to
make them all yourself." -- Eleanor Roosevelt



             Tom Lane
             <tgl@sss.pgh.pa.u
             s>                                                         To
                                       Steven Rosenstein/New
             02/06/2005 05:46          York/IBM@IBMUS
             PM                                                         cc
                                       pgsql-performance@postgresql.org
                                                                   Subject
                                       Re: [PERFORM] Can the V7.3 EXPLAIN
                                       ANALYZE be trusted?










> From: pgsql-performance-owner@postgresql.org
[mailto:pgsql-performance-owner@postgresql.org]On Behalf Of Steven
Rosenstein
> >> I don't think EXPLAIN ANALYZE puts that much overhead on a query.

I think you're being overly optimistic.  The explain shows that the
Materialize subnode is being entered upwards of 32 million times:

   ->  Materialize  (cost=505.06..511.38 rows=632 width=4) (actual
time=0.00..0.02 rows=43 loops=752066)

43 * 752066 = 32338838.  The instrumentation overhead is basically two
gettimeofday() kernel calls per node entry.  Doing the math shows that
your machine is able to do gettimeofday() in about half a microsecond,
which isn't stellar but it's not all that slow for a kernel call.
(What's the platform here, anyway?)  Nonetheless it's a couple of times
larger than the actual time needed to pull a row from a materialized
array ...

The real answer to your question is "IN (subselect) sucks before PG 7.4;
get a newer release".

                                     regards, tom lane