Re: COALESCE patch

While reviewing try_coalesce_eq() I noticed this

+ bool is_eqjoin = (!fcinfo->flinfo != NULL && fcinfo->flinfo->fn_oid ==
F_EQJOINSEL)

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.

--
Best regards,
Ilia Evdokimov,
Tantor Labs LLC,
https://tantorlabs.com/