Thanks for pointing this out. I understand the issue with relying on F_EQJOINSEL to determine the equality-join context, especially since PostgreSQL has multiple join selectivity estimators.
Is this issue still open? If so, I’d be happy to take a look at implementing the explicit is_eqjoin propagation you suggested and add the necessary regression tests.
This is checking against one specific selectivity function, but eqjoinsel() is not the only join-selectivity estimation - it's just the most common one (used by = operators). pg_proc.dat alone registers over a dozen others as JOIN estimators. We need a different mechanism. The only way I see is to stop interfering the context and pass it in explicitly give try_coalesce_eq() a bool is_eqjoin parameter.
For example, consider this scenario:
``` CREATE TABLE a (x1 int, x2 int, y int); CREATE TABLE b (w int);
INSERT INTO a (x1, x2, y) SELECT CASE WHEN i % 3 = 0 THEN NULL ELSE i % 1000 END, CASE WHEN i % 3 = 0 THEN i % 500 ELSE NULL END, i % 200 FROM generate_series(1, 100000) i;
INSERT INTO b (w) SELECT i % 1000 FROM generate_series(1, 100000) i;
CREATE INDEX a_coalesce_x1x2_idx ON a (COALESCE(x1, x2));
ANALYZE a, b;
EXPLAIN ANALYZE SELECT * FROM a JOIN b ON COALESCE(COALESCE(a.x1, a.x2), a.y) = b.w; QUERY PLAN ----------------------------------------------------------------------------------------------------------------------- Hash Join (cost=2693.00..688019.33 rows=66488333 width=16) (actual time=11.824..343.641 rows=10000000.00 loops=1) Hash Cond: (COALESCE(COALESCE(a.x1, a.x2), a.y) = b.w) Buffers: shared read=886 -> Seq Scan on a (cost=0.00..1443.00 rows=100000 width=12) (actual time=0.355..2.294 rows=100000.00 loops=1) Buffers: shared read=443 -> Hash (cost=1443.00..1443.00 rows=100000 width=4) (actual time=10.691..10.693 rows=100000.00 loops=1) Buckets: 131072 Batches: 1 Memory Usage: 4540kB Buffers: shared read=443 -> Seq Scan on b (cost=0.00..1443.00 rows=100000 width=4) (actual time=0.243..3.641 rows=100000.00 loops=1) Buffers: shared read=443 Planning: Buffers: shared hit=139 read=33 Planning Time: 1.712 ms Execution Time: 431.061 ms (14 rows) ```
Estimated rows are 6 times bigger than actual ones.
