RE: constraints
- From: "Powell, Mark" <mark.powell2@xxxxxx>
- To: "oracle-l (oracle-l@xxxxxxxxxxxxx)" <oracle-l@xxxxxxxxxxxxx>
- Date: Thu, 14 Aug 2014 15:56:55 +0000
I believe the ‘less useful to the optimizer’ part is that to be deferrable the
PK and UK constraints must be supported by non-unique indexes rather than
unique indexes so some CBO optimizations that rely on uniqueness are lost.
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On
Behalf Of Kim Berg Hansen
Sent: Thursday, August 14, 2014 8:38 AM
To: gints.plivna@xxxxxxxxx
Cc: Brian.Zelli@xxxxxxxxxxxxxxx; oracle-l (oracle-l@xxxxxxxxxxxxx)
Subject: Re: constraints
Another thing to be aware of, is that deferrable constraints are less useful to
the optimizer. If a constraint is deferrable, the optimizer cannot do
optimizations that depend on the constraints being valid, as the constraints
might not be valid until commit.
Of course there are valid usecases for deferrable constraints, but start by
asking the developer if his logic really needs to violate the constraints
during the transaction or if he can do the work with a different logic that
would allow the constraints to stay. I think in many cases the developer can
rewrite the logic so deferring constraints are not needed. But if that is not
possible, then yes, deferrable constraints can be useful, though their presence
might cause you to have to hint some queries that the optimizer otherwise would
have optimized perfectly ;-)
Regards
Kim Berg Hansen
http://dspsd.blogspot.com
kibeha@xxxxxxxxx<mailto:kibeha@xxxxxxxxx>
@kibeha
On Thu, Aug 14, 2014 at 2:24 PM,
<gints.plivna@xxxxxxxxx<mailto:gints.plivna@xxxxxxxxx>> wrote:
We have used deferred constraints a few times instead of immediate constraints.
But only because the logic could be violated during the transaction. The main
thing is to remember that in case of deferred constraint violation all
transaction will be rolled back, not only last statement. Sometimes it matters,
especially in case the transaction is big or logic complex...
Gints
On 2014. gada 14. aug., at 14:50, "Zelli, Brian"
<Brian.Zelli@xxxxxxxxxxxxxxx<mailto:Brian.Zelli@xxxxxxxxxxxxxxx>> wrote:
I have a developer who wants me to disable or defer constraints while he does
updates. I said no to disable since many of the tables have 4 or 5 constraints.
But what about deferring? Pros/cons? I have never used it. What would be the
harm? This is in a Dev environment.
Brian
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