Mark,
Have you hit a case where the total elapsed time of multiple executions of
a statement has been significantly effected because the optimizer decided
to keep going? I haven’t since version 11.2 at least. I’ve seen cases where
parse time was huge but they have always been actual bugs rather than just
the optimizer optimizing till it can’t optimize any more.
I’m sure your concerns were valid back when everything was designed with
the RBO in mind but I don’t think they’re very well placed now.
Thanks,
Andrew
On Mon, 16 Sep 2019 at 16:52, Mark W. Farnham <mwf@xxxxxxxx> wrote:
nods. what is still missing is if cost < epsilon after even one plan (or at
any point), stop planning. The 8.1 psuedo fix was a complicated hidden
concession to the better strategy that pretended to address it.
The algorithm below is compatible and not a horrible idea, but it is not
the
absolute bail-out for cheap queries.
Picture a gps guidance system doing laps in a circular driveway at the
destination trying to get just a little bit closer when you are already
there.
But a simple bail-out was somehow unsatisfying to someone somewhere in the
decision loop.
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Jonathan Lewis
Sent: Sunday, September 15, 2019 1:35 PM
To: 'Mladen Gogala'; krishsingh.111@xxxxxxxxx; 'Chris Taylor';
mwf@xxxxxxxx
Cc: 'ORACLE-L'
Subject: Re: Best course to understand why a bad plan is chosen by
optimizer
Mark,
I think there's been a limiter for a long time (since 8.1 at least) which
says somthing like:
stop optimizing when best cost so far < 0.3 * join orders tested so far *
number of "non-single-row" tables.
("single-row" tables are tables that are guaranteed by the combination of
constraints and predicates to return no more than one row, "non-single-row"
is all the rest.)
Regards
Jonathan Lewis
________________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> on
behalf of Mark W. Farnham <mwf@xxxxxxxx>
Sent: 15 September 2019 17:20
To: 'Mladen Gogala'; krishsingh.111@xxxxxxxxx; 'Chris Taylor'
Cc: 'ORACLE-L'
Subject: RE: Best course to understand why a bad plan is chosen by
optimizer
Clarifying because the context of adaptation has been removed: any other
parameter combinations [regarding plan feedback and adaptation].
Ironically feedback adaptation is part of the attempt to
"make_things_go_faster!"
Graham Wood and I separately proposed (before the release of 7, if memory
serves) "don't keep optimizing if the cost of further optimization exceeds
the cost of the query." This unfortunately got translated into a knob to
limit permutations attempted instead of "the cost is below X, stop trying
to
improve it" and hilarity ensued.
That, and "cost the rule based plan, then try to beat it" as a strategy to
avoid retrograde plans moving from RULE to COST would have saved a whole
lot
of grief* since 1989. Unfortunately a lot of code (probably nearly all of
which was later discarded) costing the row sources NOT in the order of what
RULE would have done was already designed so the "that's not compatible
with
how we do cost" was the reply instead of "holy cow, that is a great idea."
I
believe Graham and I both independently suggested that as well.
Together those two bits would have been the parameter setting
"make_things_go_slower=FALSE."
*grief: also known, as per Moans Nogood, the wealth generation mechanism
for
a lot of consultants and consulting companies.
From: Mladen Gogala [mailto:gogala.mladen@xxxxxxxxx]
Sent: Sunday, September 15, 2019 11:20 AM
To: Mark W. Farnham; krishsingh.111@xxxxxxxxx; 'Chris Taylor'
Cc: 'ORACLE-L'
Subject: Re: Best course to understand why a bad plan is chosen by
optimizer
On 9/15/19 10:55 AM, Mark W. Farnham wrote:
What we *should* have is a session and system level single parameter:
no_adapt=TRUE|FALSE that either completely disables or allows following any
other parameter combinations.
What we should have is the parameter proposed by Jonathan Lewis when Oracle
9i was the current version: "make_things_go_faster". Unfortunately, we
still
don't have that desperately needed parameter.
--
Mladen Gogala
Database Consultant
Tel: (347) 321-1217
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