Postgres 11

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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?.
-- random bitstrings of all legal lengths for bigint
CREATE TABLE t AS
SELECT i, ltrim(i::bit(64)::text, '0')::varbit AS b
FROM   generate_series (1,100000) g
, LATERAL (SELECT (random() * 2^(g%63))::int8) AS x(i);
100000 rows affected

CREATE OR REPLACE FUNCTION varbit2bigint2(b varbit)
  RETURNS bigint AS
$func$
SELECT OVERLAY(bit(64) '0' PLACING b FROM 65 - bit_length(b))::bigint
$func$  LANGUAGE SQL IMMUTABLE STRICT;
CREATE OR REPLACE FUNCTION varbit2bigint1(b varbit)
  RETURNS bigint AS
$func$
  SELECT CASE bit_length($1)
   WHEN  1 THEN $1::bit(1)::bigint
   WHEN  2 THEN $1::bit(2)::bigint
   WHEN  3 THEN $1::bit(3)::bigint
   WHEN  4 THEN $1::bit(4)::bigint
   WHEN  5 THEN $1::bit(5)::bigint
   WHEN  6 THEN $1::bit(6)::bigint
   WHEN  7 THEN $1::bit(7)::bigint
   WHEN  8 THEN $1::bit(8)::bigint
   WHEN  9 THEN $1::bit(9)::bigint
   WHEN 10 THEN $1::bit(10)::bigint
   WHEN 11 THEN $1::bit(11)::bigint
   WHEN 12 THEN $1::bit(12)::bigint
   WHEN 13 THEN $1::bit(13)::bigint
   WHEN 14 THEN $1::bit(14)::bigint
   WHEN 15 THEN $1::bit(15)::bigint
   WHEN 16 THEN $1::bit(16)::bigint
   WHEN 17 THEN $1::bit(17)::bigint
   WHEN 18 THEN $1::bit(18)::bigint
   WHEN 19 THEN $1::bit(19)::bigint
   WHEN 20 THEN $1::bit(20)::bigint
   WHEN 21 THEN $1::bit(21)::bigint
   WHEN 22 THEN $1::bit(22)::bigint
   WHEN 23 THEN $1::bit(23)::bigint
   WHEN 24 THEN $1::bit(24)::bigint
   WHEN 25 THEN $1::bit(25)::bigint
   WHEN 26 THEN $1::bit(26)::bigint
   WHEN 27 THEN $1::bit(27)::bigint
   WHEN 28 THEN $1::bit(28)::bigint
   WHEN 29 THEN $1::bit(29)::bigint
   WHEN 30 THEN $1::bit(30)::bigint
   WHEN 31 THEN $1::bit(31)::bigint
   WHEN 32 THEN $1::bit(32)::bigint
   WHEN 33 THEN $1::bit(33)::bigint
   WHEN 34 THEN $1::bit(34)::bigint
   WHEN 35 THEN $1::bit(35)::bigint
   WHEN 36 THEN $1::bit(36)::bigint
   WHEN 37 THEN $1::bit(37)::bigint
   WHEN 38 THEN $1::bit(38)::bigint
   WHEN 39 THEN $1::bit(39)::bigint
   WHEN 40 THEN $1::bit(40)::bigint
   WHEN 41 THEN $1::bit(41)::bigint
   WHEN 42 THEN $1::bit(42)::bigint
   WHEN 43 THEN $1::bit(43)::bigint
   WHEN 44 THEN $1::bit(44)::bigint
   WHEN 45 THEN $1::bit(45)::bigint
   WHEN 46 THEN $1::bit(46)::bigint
   WHEN 47 THEN $1::bit(47)::bigint
   WHEN 48 THEN $1::bit(48)::bigint
   WHEN 49 THEN $1::bit(49)::bigint
   WHEN 50 THEN $1::bit(50)::bigint
   WHEN 51 THEN $1::bit(51)::bigint
   WHEN 52 THEN $1::bit(52)::bigint
   WHEN 53 THEN $1::bit(53)::bigint
   WHEN 54 THEN $1::bit(54)::bigint
   WHEN 55 THEN $1::bit(55)::bigint
   WHEN 56 THEN $1::bit(56)::bigint
   WHEN 57 THEN $1::bit(57)::bigint
   WHEN 58 THEN $1::bit(58)::bigint
   WHEN 59 THEN $1::bit(59)::bigint
   WHEN 60 THEN $1::bit(60)::bigint
   WHEN 61 THEN $1::bit(61)::bigint
   WHEN 62 THEN $1::bit(62)::bigint
   WHEN 63 THEN $1::bit(63)::bigint
   WHEN 64 THEN $1::bit(64)::bigint
   ELSE NULL::bigint
  END
$func$  LANGUAGE SQL IMMUTABLE STRICT;
CREATE OR REPLACE FUNCTION varbit2bigint3(b varbit)
  RETURNS bigint AS
$func$
SELECT lpad(b::text, 64, '0')::bit(64)::bigint
$func$  LANGUAGE SQL IMMUTABLE STRICT;
CREATE OR REPLACE FUNCTION varbit2bigint4(b varbit)
  RETURNS bigint AS
$func$
SELECT (bit(64) '0' || b << bit_length(b))::bit(64)::bigint
$func$  LANGUAGE SQL IMMUTABLE STRICT;
SELECT *
,  varbit2bigint1(b) AS f1
,  varbit2bigint2(b) AS f2
,  varbit2bigint3(b) AS f3
,  varbit2bigint3(b) AS f4
FROM t
LIMIT 10;


i
b
f1
f2
f3
f4


0

null
0
0
0

2
10
2
2
2
2

5
101
5
5
5
5

9
1001
9
9
9
9

2
10
2
2
2
2

40
101000
40
40
40
40

115
1110011
115
115
115
115

146
10010010
146
146
146
146

251
11111011
251
251
251
251

651
1010001011
651
651
651
651


EXPLAIN (ANALYZE, COSTS OFF, TIMING OFF)
SELECT varbit2bigint2(b)        -- Execution time: 33.687 ms
FROM t;


QUERY PLAN


Seq Scan on t (actual rows=100000 loops=1)

Planning Time: 0.073 ms

Execution Time: 37.052 ms


EXPLAIN (ANALYZE, COSTS OFF, TIMING OFF)
SELECT varbit2bigint1(b)        -- Execution time: 6260.294 ms  -- !!
FROM t;


QUERY PLAN


Seq Scan on t (actual rows=100000 loops=1)

Planning Time: 0.449 ms

Execution Time: 6216.246 ms


EXPLAIN (ANALYZE, COSTS OFF, TIMING OFF)
SELECT varbit2bigint3(b)        -- Execution time: 90.351 ms
FROM t;


QUERY PLAN


Seq Scan on t (actual rows=100000 loops=1)

Planning Time: 0.076 ms

Execution Time: 89.847 ms


EXPLAIN (ANALYZE, COSTS OFF, TIMING OFF)
SELECT varbit2bigint3(b)        -- Execution time: 90.076 ms
FROM t;


QUERY PLAN


Seq Scan on t (actual rows=100000 loops=1)

Planning Time: 0.068 ms

Execution Time: 90.165 ms
i	b	f1	f2	f3	f4
0		null	0	0	0
2	10	2	2	2	2
5	101	5	5	5	5
9	1001	9	9	9	9
2	10	2	2	2	2
40	101000	40	40	40	40
115	1110011	115	115	115	115
146	10010010	146	146	146	146
251	11111011	251	251	251	251
651	1010001011	651	651	651	651