Did you place this as a database idea? (I might have even voted for it
and not remember..)
Am 03.02.2021 um 10:00 schrieb Noveljic Nenad:
>which makes a rewrite cumbersome in some cases.
A reason more to implement SSQ unnesting as an optimizer feature.
Best regards,
Nenad
*From:*Lothar Flatz <l.flatz@xxxxxxxxxx>
*Sent:* Mittwoch, 3. Februar 2021 09:54
*To:* Noveljic Nenad <nenad.noveljic@xxxxxxxxxxxx>; Laurentiu Oprea <laurentiu.oprea06@xxxxxxxxx>; eramitsaroha@xxxxxxxxx
*Cc:* ORACLE-L (oracle-l@xxxxxxxxxxxxx) <oracle-l@xxxxxxxxxxxxx>
*Subject:* Re: Query runtime is slow in view
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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