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 (Apologies if my company's mail servers append an annoying disclaimer here...) ** Cantab Capital Partners LLP is now named GAM Systematic LLP. Please note that our email addresses have changed from @cantabcapital.com to @gam.com.** This email was sent by and on behalf of GAM Investments. 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