Postgres 13

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Help with an interesting Postgres question: Why isn't an Index Only Scan used on a partition accessed via the parent table?.

CREATE TEMP TABLE tbl AS
SELECT (random() * 10000)::int AS lots
     , (random() * 4)::int     AS few
FROM generate_series (1, 100000);  -- 100k rows

DELETE FROM tbl WHERE random() > 0.9;  -- create some dead tuples, more "real-life"

SELECT 100000

DELETE 9975

VACUUM ANALYZE tbl;

VACUUM

SELECT count(distinct lots)   -- ~ 10000
     , count(distinct few)    --   5
FROM   tbl;

SELECT *
FROM   tbl
WHERE  lots = 2345
AND    few = 2;

count
count

10001
5

SELECT 1

lots
few

2345
2

SELECT 1

CREATE INDEX tbl_lf_idx ON tbl(lots, few);

CREATE INDEX

EXPLAIN (ANALYZE, TIMING OFF)
SELECT * FROM tbl WHERE lots = 2345 AND few = 2;

QUERY PLAN

Index Only Scan using tbl_lf_idx on tbl  (cost=0.29..4.33 rows=2 width=8) (actual rows=1 loops=1)

  Index Cond: ((lots = 2345) AND (few = 2))

  Heap Fetches: 0

Planning Time: 0.221 ms

Execution Time: 0.094 ms

EXPLAIN

-- repeat
EXPLAIN (ANALYZE, TIMING OFF)
SELECT * FROM tbl WHERE lots = 2345 AND few = 2;

QUERY PLAN

Index Only Scan using tbl_lf_idx on tbl  (cost=0.29..4.33 rows=2 width=8) (actual rows=1 loops=1)

  Index Cond: ((lots = 2345) AND (few = 2))

  Heap Fetches: 0

Planning Time: 0.086 ms

Execution Time: 0.032 ms

EXPLAIN

DROP INDEX tbl_lf_idx;
CREATE INDEX tbl_fl_idx ON tbl(few, lots);

DROP INDEX

CREATE INDEX

EXPLAIN (ANALYZE, TIMING OFF)
SELECT * FROM tbl WHERE lots = 2345 AND few = 2;

QUERY PLAN

Index Only Scan using tbl_fl_idx on tbl  (cost=0.29..4.33 rows=2 width=8) (actual rows=1 loops=1)

  Index Cond: ((few = 2) AND (lots = 2345))

  Heap Fetches: 0

Planning Time: 0.220 ms

Execution Time: 0.075 ms

EXPLAIN

-- repeat
EXPLAIN (ANALYZE, TIMING OFF)
SELECT * FROM tbl WHERE lots = 2345 AND few = 2;

QUERY PLAN

Index Only Scan using tbl_fl_idx on tbl  (cost=0.29..4.33 rows=2 width=8) (actual rows=1 loops=1)

  Index Cond: ((few = 2) AND (lots = 2345))

  Heap Fetches: 0

Planning Time: 0.050 ms

Execution Time: 0.023 ms

EXPLAIN