Re: Mutating (SQL) execution plan!?...is that possible...stranges t thing I have every seen

Steve.

Not only does Oracle re-parse the statement when it finds there is a predicate on a column with a histogram - it does bind variable substitution first and THEN 'de-substitutes'. So the parse costs go up (rather than 'just' staying the same) if you have redundant histograms in place.

I have never seen what Oracle calls an "unsafe" statement (i.e. <, >, between, those types of conditions in the where clause) be reparsed unless the histogram condition was met - have you ?

Even though the claim is lurking somewhere in an official Oracle document (was it an OW white-paper), I suspect it is complete rubbish.
(at least up to 9i - it occurs to me that I haven't
repeated that set of tests on 10g yet).

Regards

Jonathan Lewis

http://www.jlcomp.demon.co.uk/faq/ind_faq.html The Co-operative Oracle Users' FAQ

http://www.jlcomp.demon.co.uk/seminar.html Public Appearances - schedule updated March 9th 2005

• Original Message ----- From: "Dirschel, Steve" <Steve.Dirschel_at_bestbuy.com> To: <breitliw_at_centrexcc.com>; <john.kanagaraj_at_hds.com> Cc: <oracle-l_at_freelists.org> Sent: Thursday, March 10, 2005 7:05 PM Subject: RE: Mutating (SQL) execution plan!?...is that possible...stranges t thing I have every seen

I don't know I'd recommend they go to SIMILAR from FORCE if they're using a lot of histograms. SIMILAR causes Oracle to reparse sql statements if they're considered "unsafe" (i.e. <, >, between, those types of conditions in the where clause).

After much pain we found Oracle has a feature where if SIMILAR is used and histograms exist on WHERE clause columns Oracle will consider that statement=20
"unsafe" and reparse (version_count increases for the statement). Even if there is a histogram on a PK column and the WHERE clause has WHERE pk_column =3D value Oracle still reparses it...(every time it executes, not every time the plan changes). If they're running FORCE now and they change to SIMILAR I'd expect them to all of a sudden experience
(assuming a relatively active system) latching related to the parsing
and possible shared pool issues (4031). It took us 3+ months of working with Oracle to determine SIMILAR + HISTOGRAMS =3D REPARSING. But once = it
was determined that was the case Oracle came back saying the behavior was expected

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