Re: SELECT DISTINCT chooses parallel seqscan instead of indexscan on huge table with 1000 partitions - Mailing list pgsql-general

From Dimitrios Apostolou
Subject Re: SELECT DISTINCT chooses parallel seqscan instead of indexscan on huge table with 1000 partitions
Date
Msg-id 410018fd-1f9b-41b7-6257-89844b984564@gmx.net
Whole thread Raw
In response to Re: SELECT DISTINCT chooses parallel seqscan instead of indexscan on huge table with 1000 partitions  (David Rowley <dgrowleyml@gmail.com>)
Responses Re: SELECT DISTINCT chooses parallel seqscan instead of indexscan on huge table with 1000 partitions
List pgsql-general

On Tue, 14 May 2024, David Rowley wrote:

> That assumes the Append won't ever use > 1 worker per subnode, but
> that's not the case for your plan as the subnodes are "Parallel".
> That means all the workers could be working on the same subnode which
> could result in one group being split between 2 or more workers.

Didn't think of that, makes sense!

> Parallel Append can also run in a way that the Append child nodes will
> only get 1 worker each.

How can I tell which case it is, from the EXPLAIN output (for example
the output at [1]) ?

[1] https://www.postgresql.org/message-id/69077f15-4125-2d63-733f-21ce6eac4f01%40gmx.net

Dimitris




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