Re: How to compare two different SQL executions plans (question is more deeper then seams from first look)?
- From: "Jonathan Lewis" <jonathan@xxxxxxxxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Wed, 30 Jun 2004 09:19:24 +0100
The example is demonstrating the relevant of the question
that you quoted, but from the opposite direction to normal.
The two SQL statements repeated below
are 'iso-functional' - in fact the performance
tuning guide tells us that the optimizer may
choose to convert from the IN form to the
EXISTS form before optimizing.
The demonstration is showing, though, that
for a constant amount of CPU (ca. 45 seconds)
a) One version can be executed 12 times
whilst the other can only be executed 4 times
and
b) The statement that can be executed more times
for the same amount of CPU is the one that would
be highlighted by many tools as the more expensive
statement. (viz. any tool that does a 'top 10' display
by LIO - which is one of the commonest performance
monitoring strategies around).
Regards
Jonathan Lewis
http://www.jlcomp.demon.co.uk
http://www.jlcomp.demon.co.uk/faq/ind_faq.html
The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/seminar.html
Optimising Oracle Seminar - schedule updated May 1st
----- Original Message -----
From: "Mark W. Farnham" <mwf@xxxxxxxx>
Are you asking "If, given two iso-functional SQL-s operating on the same
dataset, is it usually true that the one which causes fewer LIOs is overall
cheaper in systems resources?" then I think I'm not sure what your examples
have to do with the question.
In the first case, example one gets 800,000 rows in 3.3 seconds from the
full table scan and example two gets 2.4 million rows in 7.5 seconds. Are
these the same datasets? A baseline select count(m.v) from main_tab m might
be illustrative. It appears that this full table scan is required (unless
there are resources inapparent in what is given, and hash won't be used in
RULE.)
In the second case, you're requiring an extraneous (and apparently
purposeless) sort operation.
Any iso-functional SQL can be tweaked to add extraneous use of CPU. If you
take two comparable queries and impose extra all in memory CPU use on one of
them, you indeed mask the usefulness of LIO as a thumbrule. So take the one
with comparable LIO that uses less CPU.
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx]On Behalf Of J.Velikanovs@xxxxxxxx
TESTCASE 1 (output from 10046)
====================================================
SELECT /*+ RULE */ count(m.v) from main_tab m
where
m.n in
(select f.n from
filter_tab f where f.v='a')
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ----------
----------
Parse 2 0.00 0.00 0 0 0 0
Execute 4 0.00 0.00 0 0 0 0
Fetch 4 44.48 46.17 562 1201856 0 4
------- ------ -------- ---------- ---------- ---------- ----------
----------
total 10 44.48 46.17 562 1201856 0 4
Rows Row Source Operation
------- ---------------------------------------------------
2 SORT AGGREGATE (cr=801234 r=562 w=0 time=31244261 us)
800000 NESTED LOOPS (cr=801234 r=562 w=0 time=28962918 us)
800000 TABLE ACCESS FULL MAIN_TAB (cr=1230 r=562 w=0 time=3309977 us)
800000 TABLE ACCESS BY INDEX ROWID FILTER_TAB (cr=800004 r=0 w=0
time=16177875 us)
800000 INDEX UNIQUE SCAN FILTER_TAB_I1 (cr=4 r=0 w=0 time=5226711
us)(object id 9699)
SELECT /*+ RULE */ count(m.v) from main_tab m
where
exists
(select v from
filter_tab f where f.n=m.n and f.v='a')
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ----------
----------
Parse 2 0.00 0.00 0 0 0 0
Execute 12 0.01 0.00 0 0 0 0
Fetch 12 42.38 42.97 1686 5616 0 12
------- ------ -------- ---------- ---------- ---------- ----------
----------
total 26 42.39 42.98 1686 5616 0 12
Rows Row Source Operation
------- ---------------------------------------------------
6 SORT AGGREGATE (cr=3738 r=1686 w=0 time=28675198 us)
2400000 FILTER (cr=3738 r=1686 w=0 time=22306438 us)
2400000 TABLE ACCESS FULL MAIN_TAB (cr=3690 r=1686 w=0 time=7478255 us)
24 TABLE ACCESS BY INDEX ROWID FILTER_TAB (cr=48 r=0 w=0 time=524
us)
24 INDEX UNIQUE SCAN FILTER_TAB_I1 (cr=24 r=0 w=0 time=242
us)(object id 9699)
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