RE: constraints

From: Powell, Mark <mark.powell2_at_hp.com>
Date: Thu, 14 Aug 2014 15:56:55 +0000
Message-ID: <1E24812FBE5611419EFAFC488D7CCDD126EE207A_at_G6W2491.americas.hpqcorp.net>

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_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Kim Berg Hansen Sent: Thursday, August 14, 2014 8:38 AM To: gints.plivna_at_gmail.com
Cc: Brian.Zelli_at_RoswellPark.org; oracle-l (oracle-l_at_freelists.org) 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_at_gmail.com<mailto:kibeha_at_gmail.com> _at_kibeha

On Thu, Aug 14, 2014 at 2:24 PM, <gints.plivna_at_gmail.com<mailto:gints.plivna_at_gmail.com>> 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_at_RoswellPark.org<mailto:Brian.Zelli_at_RoswellPark.org>> 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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Received on Thu Aug 14 2014 - 17:56:55 CEST