Re: How to compare two different SQL executions plans (question is more deeper then seams from first look)?

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From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: Wed, 30 Jun 2004 09:19:24 +0100
Message-ID: <007d01c45e7a$f787a6e0$7102a8c0@Primary>

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)

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_at_rsiz.com>

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_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org]On Behalf Of J.Velikanovs_at_alise.lv

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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Received on Wed Jun 30 2004 - 03:16:13 CDT