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Re: SELECT DISTINCT chooses parallel seqscan instead of indexscan on huge table with 1000 partitions

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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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