Parallel safety of contrib extensions

Hi all,

There was a thread about this back in 2016[1], but I've just been bitten by it and wondered if any (more) extensions, particularly btree_gist, will have their operators/functions verified and marked as parallel-safe?

In our case, we're interested in the float8 <-> float8 operator. Although this is defined in btree_gist, we also have plain btree indexes that use this operator in functional columns. We're currently running v11.7.

An example:

CREATE EXTENSION IF NOT EXISTS btree_gist;

CREATE TABLE distance_test AS
SELECT id, mod(id, 10) as m10, mod(id, 17) as m17, random() as r
FROM generate_series(1, 1000000) as ids(id);

ANALYZE distance_test;

CREATE INDEX idx_distance_test ON distance_test USING btree (m10, m17, (r <-> 0.5));

SET force_parallel_mode = on;
SET parallel_setup_cost = 0;
SET parallel_tuple_cost = 0;

EXPLAIN ANALYZE
(
SELECT DISTINCT ON (m10, m17) m10, m17
FROM distance_test
WHERE m10 = 5
ORDER BY m10, m17
)
UNION ALL
(
SELECT DISTINCT ON (m10, m17) m10, m17
FROM distance_test
WHERE m10 = 3
ORDER BY m10, m17
)

This gives a nice parallel plan - the two subqueries are farmed out to workers, each doing a Unique of an Index Only Scan, and the results are Parallel-Appended.

However, if the two "ORDER BY" clauses have the functional column "r <-> 0.5" added then parallelism is no longer allowed - the plan becomes a plain Append of two Subquery Scans.

As an aside, the values of "r <-> 0.5" have been precomputed - which is why an Index Only Scan can take place - so the supposed parallel-unsafe operator isn't even being invoked here!

But, either way, the loss of parallelism in my real-world case wasn't easy to track down, so it would be great if some more of the contrib-but-core-really extensions, such as btree_gist, could have their functions and operators marked parallel safe.

And, yes, if I'd used "abs(r - 0.5)" here then parallelism would not have been lost, but we do use gist indexes elsewhere and we don't want to have to switch between this and "<->" depending on which index we think a particular query needs, or worry about subtle differences between them when handling - say - NaNs.

I can also force parallelism to return if I manually mark the "float8_dist" function as parallel-safe:
UPDATE pg_proc SET proparallel = 's' WHERE oid = 'float8_dist'::regproc

Looking at the source code[2] I can't see why this would cause issues, but I'm not an expert - any comments on why this might be an incredibly bad idea would be most welcome (e.g. will the setting survive a major upgrade of the db, and an upgrade of the btree_gist extension? Is it in fact subtly NOT parallel safe?)

Best wishes,
Steven.

[1] https://www.postgresql.org/message-id/flat/573E3509.9040309%40proxel.se
[2] https://github.com/postgres/postgres/blob/master/contrib/btree_gist/btree_float8.c#L102

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