Postgres 12

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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 TABLE table1("id"  VARCHAR(3),"qty" int);
INSERT INTO table1 VALUES('A1',   25),('A2',  32)
2 rows affected

CREATE TABLE table2 AS
WITH sub_isbuild AS (
  SELECT CONCAT("id",'_',generate_series(1, "qty")) as sub_id
  FROM table1
)
 SELECT sub_id FROM sub_isbuild;
57 rows affected

SELECT * FROM table2

sub_id

A1_1

A1_2

A1_3

A1_4

A1_5

A1_6

A1_7

A1_8

A1_9

A1_10

A1_11

A1_12

A1_13

A1_14

A1_15

A1_16

A1_17

A1_18

A1_19

A1_20

A1_21

A1_22

A1_23

A1_24

A1_25

A2_1

A2_2

A2_3

A2_4

A2_5

A2_6

A2_7

A2_8

A2_9

A2_10

A2_11

A2_12

A2_13

A2_14

A2_15

A2_16

A2_17

A2_18

A2_19

A2_20

A2_21

A2_22

A2_23

A2_24

A2_25

A2_26

A2_27

A2_28

A2_29

A2_30

A2_31

A2_32