Tom Lane wrote:
Peter Wilson <[EMAIL PROTECTED]> writes:
Tom Lane wrote:
Oh? Could you provide a test case for this? I can certainly believe
that the planner might choose a bad plan if it has no statistics, but
it shouldn't take a long time to do it.
On investigation the problems occurs on 'EXPLAIN ANALYZE' - which is
what pgadminIII does when you press the explain button.
Ah. Well, this is an ideal example of why you need statistics ---
without 'em, the planner is more or less flying blind about the number
of matching rows. The original plan had
-> Index Scan using ca_pk on contact_att subb (cost=0.00..6.01
rows=1 width=8) (actual time=0.207..234.423 rows=3 loops=2791)
Index Cond: ((instance = '0'::bpchar) AND
((client_id)::text = 'gadget'::text))
Filter: ((contact_id)::numeric = 3854.000000)
while your "after a vacuum" (I suppose really a vacuum analyze) plan has
-> Index Scan using ca_pk on contact_att subb
(cost=0.00..1433.95 rows=78 width=8) (actual time=0.367..259.617 rows=3 loops=1)
Index Cond: ((instance = '0'::bpchar) AND
((client_id)::text = 'gadget'::text))
Filter: ((contact_id)::numeric = 3854.000000)
This is the identical scan plan ... but now that the planner realizes
it's going to be pretty expensive, it arranges the join in a way that
requires only one scan of contact_att and not 2791 of 'em.
The key point here is that the index condition on instance/client_id
is not selective --- it'll pull out a lot of rows. All but 3 of 'em are
then discarded by the contact_id condition, but the damage in terms
of runtime was already done. With stats, the planner can realize this
--- without stats, it has no chance.
Looking at your table definition, I suppose you were expecting the
contact_id condition to be used with the index, but since contact_id is
bigint, comparing it to a numeric-type constant is not considered indexable.
You want to lose the ".000000" in the query.
regards, tom lane
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Thanks for that Tom - especially the bit about removing the .00000 from
the numbers. I'm pretty new to some of this database stuff - even newer at
trying to optimise queries and 'think like the planner'. Never occurred to
me the number format would have that effect.
Removing the zeroes actaully knocked a few ms of the execution times in
real-life querries :-)
Just out of interest - is there an opportunity for the planner to realise
the sub-select is basically invariant for the outer-query and execute once,
regardless of stats. Seems like the loop-invariant optimisation in a 'C'
compiler. If you have to do something once v. doing it 2791 times then
I'd plop for the once!
Thanks again Tom, much appreciated for that little nugget
Pete
--
Peter Wilson. YellowHawk Ltd, http://www.yellowhawk.co.uk
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