Re: When is parallelism not used by optimizer - but defined?

Hello Bernhard,
not quite sure if I get your question right - are you talking about reasons why you get a serial plan even if you specified some kind of DOP or are you talking about reasons why the runtime engine decreases DOP or execute it in serial?

For the latter you want to check out qerpxSetupAndAllocSlaves() and its reasons (e.g. check here for an example: https://gist.github.com/xtender/a48edf801516a3a1c9e328edaedffe2c) ... and for the former it is a plain cost based decision (e.g. check here: https://tanelpoder.com/2013/03/20/alter-session-force-parallel-query-doesnt-really-force-anything/ or here: https://jonathanlewis.wordpress.com/2007/06/17/hints-again/)

Best Regards
Stefan Koehler

Independent Oracle performance consultant and researcher Website: www.soocs.de
Twitter: _at_OracleSK

> Bernhard de Cock Buning <bdcbuning_at_grid-it.nl> hat am 14.11.2023 19:01 CET geschrieben:
>
> Hello all,
>
> One of the possibilities is to execute workloads in parallel, But now I am looking for input when parallelism will not work.
> Especially in the relation to the oracle optimizer, when does the optimiser to decide when not to perform a statement in parallel although parallelisme is defined.
>
> I would like to make a "list" of when not and use this in performance cases.
> All input is useful,
>
> ThanksKind regards,
> Bernhard

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