Postgres 15

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CREATE TABLE tab (
  col text[]
);

INSERT INTO tab
SELECT ARRAY(
   SELECT array_to_string(ARRAY(
      SELECT substr('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', trunc(random() * 62)::integer + 1, 1)
      FROM generate_series(1, slen)), '')
   FROM (SELECT trunc(random() * 60)::integer + 10 AS slen FROM generate_series(1,tlen)) x) AS col
FROM (SELECT id, trunc(random() * 5)::integer + 1 AS tlen FROM generate_series(1,1000) AS id) x;  -- keeping it minimal

CREATE TABLE

INSERT 0 1000

-- helper func
CREATE OR REPLACE FUNCTION arr_to_upper_string(_arr text[], _sep text = ' ') 
  RETURNS text
  LANGUAGE sql IMMUTABLE STRICT PARALLEL SAFE
BEGIN ATOMIC
SELECT upper(array_to_string($1, $2));
END;

COMMENT ON FUNCTION arr_to_upper_string(text[], text) IS 'array_to_string() is not immutable, as it takes "anyarray" input, and some types have no immutable conversion to text.
For text[] input it is, in fact immutable. We need an immutable fucntion for indexes. Hence the wrapper.
upper(text) is immutable anyway.
But feeding other array types to this function (and casting implicitly or explicitly) may not be immutable!'

CREATE FUNCTION

COMMENT

CREATE INDEX tab_col_tsv_gin_idx ON tab USING gin (to_tsvector('simple', arr_to_upper_string(col)));

CREATE INDEX

-- index is used
EXPLAIN ANALYZE
SELECT * FROM tab
WHERE to_tsvector('simple', arr_to_upper_string(col)) @@ to_tsquery('simple', 'ORUNC:*');

QUERY PLAN

Bitmap Heap Scan on tab  (cost=20.15..54.83 rows=20 width=32) (actual time=0.021..0.022 rows=0 loops=1)

  Recheck Cond: (to_tsvector('simple'::regconfig, arr_to_upper_string(col, ' '::text)) @@ '''orunc'':*'::tsquery)

  ->  Bitmap Index Scan on tab_col_tsv_gin_idx  (cost=0.00..20.15 rows=20 width=0) (actual time=0.017..0.018 rows=0 loops=1)

        Index Cond: (to_tsvector('simple'::regconfig, arr_to_upper_string(col, ' '::text)) @@ '''orunc'':*'::tsquery)

Planning Time: 0.840 ms

Execution Time: 0.065 ms

EXPLAIN