Re: Why index access is not happening in parallel

Seeing another query with a similar pattern, though the query is different and its simple "Select query". But in this case too even in one place the query is executed with parallel-4 hint and another with parallel-8 hint in the same database and same objects. But the throughput seems more or less the same. So it seems here also the index access is not happening in parallel too.

Below is the sql monitor details :-

https://gist.github.com/oracle9999/ff2073c222398416c8095d109c233765

Not sure if it's a silly one, but I am wondering if my understanding is wrong , when and how the "index access" can happen in parallel fashion? Appreciate your guidance.

Regards
Pap

On Sat, Jun 24, 2023 at 8:13 PM Pap <oracle.developer35_at_gmail.com> wrote:

> Hello Listers,
> This Oracle database version 19C and its exadata. We have a customer query
> in which one of the executions is happening in parallel(2) and another
> execution is happening in parallel(4) in two different places. The query is
> exactly the same , just that the place in which it's getting used with
> PARALLEL(4) it's also happening with the APPEND hint.
>
> People here are expecting the query which runs using parallel(4) with
> Append should be faster as compared to the other one (i.e. with just
> parallel(2)). But it's not happening that way. Want to understand why?
>
> And also in this query the step no which is coming as top contributor in
> ASH is table access RFFT using index access path - RFFT_IX7. This is a big
> table partitioned on column C_KEY. Other tables are small tables. So just
> wondering if the higher parallel threads were not helping the query because
> the index access is happening using BATCHED index range scan (i.e. TABLE
> ACCESS BY LOCAL INDEX ROWID BATCHED). or are we misinterpreting the
> execution plan here and something in the query is not making the index scan
> to happen in parallel?
>
> Below is the sql monitor details of the two plans. Below are two sql
> monitors, in one case it has processed 2million vs other 3million , however
> if we see the processing speed i.e. ~67 seconds for 3million vs 38 seconds
> for 2million. The throughput seems to be more or less the same only. The
> Parallel-8+ Append seems to be comparatively slower though, which is odd.
>
> https://gist.github.com/oracle9999/618251c1e48b315dc70c73e157443773
>
> Regards
> Pap
>

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