RE: Best course to understand why a bad plan is chosen by optimizer
Date: Sun, 15 Sep 2019 12:20:56 -0400
Message-ID: <05ca01d56be1$8e1d5bc0$aa581340$_at_rsiz.com>
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.”
From: Mladen Gogala [mailto:gogala.mladen_at_gmail.com]
Sent: Sunday, September 15, 2019 11:20 AM
To: Mark W. Farnham; krishsingh.111_at_gmail.com; '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 -- http://www.freelists.org/webpage/oracle-lReceived on Sun Sep 15 2019 - 18:20:56 CEST