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?.
 create table mytable
(
id       serial     not null primary key,
text_id                      varchar(20)              not null,
customer_unit_id              text
);

INSERT INTO mytable (text_id,customer_unit_id)
SELECT 'K-2021-'||i,i FROM generate_series (1,100000) j(i);

CREATE TABLE

INSERT 0 100000

CREATE INDEX IF NOT EXISTS trgrm_ts_vec_idx ON mytable USING gin (to_tsvector('english', text_id || ' ' || customer_unit_id));

EXPLAIN (ANALYSE,COSTS OFF)
SELECT id, text_id
FROM mytable
WHERE 
 to_tsvector('english', text_id || ' ' || customer_unit_id) @@ 
 to_tsquery('english', 'k-2021-8 | 176');

CREATE INDEX

QUERY PLAN

Bitmap Heap Scan on mytable (actual time=0.416..0.475 rows=2 loops=1)

  Recheck Cond: (to_tsvector('english'::regconfig, (((text_id)::text || ' '::text) || customer_unit_id)) @@ '''k'' & ''-2021'' & ''-8'' | ''176'''::tsquery)

  Heap Blocks: exact=2

  ->  Bitmap Index Scan on trgrm_ts_vec_idx (actual time=0.255..0.255 rows=2 loops=1)

        Index Cond: (to_tsvector('english'::regconfig, (((text_id)::text || ' '::text) || customer_unit_id)) @@ '''k'' & ''-2021'' & ''-8'' | ''176'''::tsquery)

Planning Time: 11.494 ms

Execution Time: 0.543 ms

EXPLAIN

ALTER TABLE mytable ADD COLUMN ts tsvector;
UPDATE mytable SET 
  ts =  to_tsvector('english', text_id || ' ' || customer_unit_id);

CREATE INDEX trgrm_ts_vec_idx2 ON mytable USING gin (ts);

EXPLAIN (ANALYSE,COSTS OFF)
SELECT id, text_id
FROM mytable
WHERE 
 ts @@ to_tsquery('english', 'k-2021-8 | 176')

ALTER TABLE

UPDATE 100000

CREATE INDEX

QUERY PLAN

Bitmap Heap Scan on mytable (actual time=0.185..0.203 rows=2 loops=1)

  Recheck Cond: (ts @@ '''k'' & ''-2021'' & ''-8'' | ''176'''::tsquery)

  Heap Blocks: exact=2

  ->  Bitmap Index Scan on trgrm_ts_vec_idx2 (actual time=0.155..0.155 rows=2 loops=1)

        Index Cond: (ts @@ '''k'' & ''-2021'' & ''-8'' | ''176'''::tsquery)

Planning Time: 0.352 ms

Execution Time: 0.231 ms

EXPLAIN