I spent some time over the weekend trying out the different modes of
parallel query (seq scan, aggregate, join) in combination with PostGIS
and have written up the results here:


The TL:DR; is basically

* With some adjustments to function COST both parallel sequence scan
and parallel aggregation deliver very good parallel performance
* The cost adjustments for sequence scan and aggregate scan are not
consistent in magnitude.
* Parallel join does not seem to work for PostGIS indexes yet, but
perhaps there is some magic to learn from PostgreSQL core on that.

The two findings at the end are ones that need input from parallel
query masters...

We recognize we'll have to adjust costs to that our particular use
case (very CPU-intensive calculation per function) is planned better,
but it seems like different query modes are interpreting costs in
order-of-magnitude different ways in building plans.

Parallel join would be a huge win, so some help/pointers on figuring
out why it's not coming into play when our gist operators are in
effect would be helpful.

Happy Easter to you all,

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