On Friday, July 4, 2003, at 07:07 AM, Brian Tarbox wrote:
> We had about 40 tables in the db, with joined queries on about 8-12
> tables.
>
A while ago a tested a moderately complex schema on MySQL, Pg, and
Oracle. I usually heavily normalize schemas and then define views as a
denormalized API, which sends MySQL to the book of toys already. The
views more often than not would join anywhere from 6-12 tables, using
plain (as opposed to compound) foreign keys to primary key straight
joins.
I noticed that Pg was more than an order of magnitude slower for joins
> 8 tables than Oracle. I won't claim that none of this can have been
due to lack of tuning. My point is the following though. After I dug in
it turned out that of the 4 secs Pg needed to execute the query it
spent 3.9 secs in the planner. The execution plan Pg came up with was
pretty good - it just needed an extraordinary amount of time to arrive
at it, spoiling its own results.
Asking this list I then learned how to tweak GEQO such that it would
pick up the planning and do it faster than it would otherwise. I was
able to get the planner time down to a quarter - still a multitude of
the actual execution time.
I was told on this list that query planning suffers from combinatorial
explosion very quickly - and I completely buy that. It's just - Oracle
planned the same query in a fraction of a second, using the cost-based
optimizer, on a slower machine. I've seen it plan 15-table joins in
much less than a second, and I have no idea how it would do that. In
addition, once you've prepared a query in Oracle, the execution plan is
pre-compiled.
If I were a CS student I'd offer myself to the hall of humiliation and
set out to write a fast query planner for Pg ...
-hilmar
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Hilmar Lapp email: lapp at gnf.org
GNF, San Diego, Ca. 92121 phone: +1-858-812-1757
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