Re: POC, WIP: OR-clause support for indexes

I gave this patch a quick spin and noticed a strange query plan.

CREATE TABLE test (a int, b int, c int);
CREATE INDEX ON test USING gin (a, b, c);
INSERT INTO test SELECT i % 7, i % 9, i % 11 FROM generate_series(1,
1000000) i;
EXPLAIN ANALYZE SELECT * FROM test WHERE (a = 3 OR b = 5) AND c = 2;

QUERY PLAN

----------------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on test (cost=829.45..4892.10 rows=21819 width=12)
(actual time=66.494..76.234 rows=21645 loops=1)
Recheck Cond: ((((a = 3) AND (c = 2)) OR ((b = 5) AND (c = 2))) AND
(c = 2))
Heap Blocks: exact=5406
-> Bitmap Index Scan on test_a_b_c_idx (cost=0.00..824.00
rows=2100 width=0) (actual time=65.272..65.272 rows=21645 loops=1)
Index Cond: ((((a = 3) AND (c = 2)) OR ((b = 5) AND (c = 2)))
AND (c = 2))
Planning time: 0.200 ms
Execution time: 77.206 ms
(7 rows)

Shouldn't the index condition just be "((a = 3) AND (c = 2)) OR ((b = 5)
AND (c = 2))"?

Also when applying and reading the patch I noticed some minor
issues/nitpick.

- I get whitespace warnings from git apply when I apply the patches.
- You have any insconstent style for casts: I think "(Node*)clause"
should be "(Node *) clause".
- Same with pointers. "List* quals" should be "List *quals"
- I am personally not a fan of seeing the "isorderby == false &&
index->rd_amroutine->amcanorclause" clause twice. Feels like a risk for
diverging code paths. But it could be that there is no clean alternative.

Andreas