Re: Query runtime is slow in view
Date: Wed, 3 Feb 2021 09:53:36 +0100
Message-ID: <87e43ebc-6052-5115-2fc8-ed84adf82f8c_at_bluewin.ch>
Quite true. An Index is the quick fix. Rewrite is better because it
allows the optimizer for more options.
Thanks for pointing at the higher perspective, which also shows that
indexing is not everything and maybe developers skills are still needed
when automatic indexing is in place.
Generally you just have to consider that a column value select is an
implicit outer join which makes a rewrite cumbersome in some cases.
Am 03.02.2021 um 09:40 schrieb Noveljic Nenad:
>
> >As the activity column in the sql Monitor suggests, time is almost
> exclusively spent in this column value subquery:
>
> Scalar subquery (SSQ) in the select clause is generally a problem with
> large driving row sources. Unlike SQL Server, Oracle optimizer doesn’t
> unnest them, even in cases when that would be feasible.
>
> Many times, I got much better performance by manually rewriting such
> queries by replacing SSQ with either semi or outer join. It could be a
> better solution also in your case.
>
> Examples are listed here:
>
> https://nenadnoveljic.com/blog/correlated-subqueries-in-the-select-clause/
> <https://nenadnoveljic.com/blog/correlated-subqueries-in-the-select-clause/>
>
> https://nenadnoveljic.com/blog/correlated-subqueries-in-the-select-clause-2/
> <https://nenadnoveljic.com/blog/correlated-subqueries-in-the-select-clause-2/>
>
> Best regards,
>
> Nenad
>
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-- http://www.freelists.org/webpage/oracle-lReceived on Wed Feb 03 2021 - 09:53:36 CET