I wrote a new patch to add a lot more tests around hash-based plans.
This is intended to apply separately from the other patch, and the other
patch would then "flip" some of the test cases.
On 2020-11-13 20:51, Tom Lane wrote:
> * The new test in with.sql claims to be testing row hashing, but
> it's not very apparent that any such thing actually happens. Maybe
> EXPLAIN the query, as well as execute it, to confirm that a
> hash-based plan is used.
The recursive union requires hashing, but this is not visible in the
plan. You only get an error if there is no hashing support for a type.
I have added a test for this.
For the non-recursive union, I have added more tests that show this in
the plans.
> * Is it worth devising a test case in which hashing is not possible
> because one of the columns isn't hashable? I have mixed feelings
> about this because the set of suitable column types may decrease
> to empty over time, making it hard to maintain the test case.
I used the money type for now. If someone adds hash support for that,
we'll change it. I don't think this will change too rapidly, though.
--
Peter Eisentraut
2ndQuadrant, an EDB company
https://www.2ndquadrant.com/
