Dear Gurus,
Please feel free to show me to the archives if my question has already been
answered.
Last week I fumbled with CPU_TUPLE_COST and revealed that 4 out of 4 tested
queries improved by 10-60% if I changed it from 0.01 (default) to 0.40
(ugh). Setting it higher did not bring any improvement.
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QUESTION1: is there a (theoretical or practical) relation between this one
and the other cpu costs? Should I also increase those values by the same
rate and find a balance that way?
As far as I can guess, there should be a linear relation, i.e.
cpu_tuple_cost:cpu_index_tuple_cost:cpu_operator_cost should be a constant
ratio, but then again, I suspect there is a cause that they have separate
entries in the config file ;)
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The queries were, or contained, something like:
SELECT s.qty FROM a, s WHERE a.id = s.a_id AND a.b_id = 1234;
where
* "a" and "s" are in 1:N relation,
* "b" and "a" are in 1:N relation,
* a.id is pkey in "a" and b.id is pkey in "b".
These queries usually return up to 6-10% of the tuples in s (about 16k of
220k) and the planner chose seq scans on s. Disabling seq scan and some
other things finally brought up a plan containing index scans that improved
two queries. (I tested the other two after I found out the solution of
these, to see if they improve or get worse)
Also noticed that the largest gain was from the smallest change on
cpu_tuple_cost: the query with the largest improvement (to 32% of orig time)
chose the better plan from 0.03, but the other one improved (to 79%) only if
set cpu_tuple_cost to 0.40 or higher.
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QUESTION2: am I right setting cpu_tuple_cost, or may there be another cause
of poor plan selection? Also tried lowering random_page_cost, but even 1.0
didn't yield any improvement.
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CONFIGURATION: PostgreSQL 7.3.4, IBM Xeon 2x2.4GHz HT, 5x36GB 10krpm HW
RAID-5.
We found out quite early that random page cost is quite low (now we have it
at 1.5-- maybe it's still to high) and it's true that tasks that require raw
cpu power aren't very much faster than PIII-800. Unfortunately I can't test
the same hw on 7.4 yet, since it's a production server.
TIA,
G.
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