Postgres 16

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CREATE TABLE tbl(id int, fixtures json);
INSERT INTO tbl VALUES
  (1, '{"fixture_1":"fixture1", "fixture_2":"fixture2", "fixture_123":"fixture123"}')
, (2, '{"fixture_1":"fixture1","fixture_123":"fixture123"}')  -- dupes
, (3, '{"fixture_6":"fixture6", "fixture_66":"fixture66"}')
;

CREATE TABLE

INSERT 0 3

-- all IDs (incl. dupes)
SELECT split_part(json_object_keys(fixtures), '_', 2)::int AS id
FROM   tbl
ORDER  BY id;

id

1

1

2

6

66

123

123

SELECT 7

-- distinct IDs
SELECT DISTINCT split_part(json_object_keys(fixtures), '_', 2)::int AS id
FROM   tbl
ORDER  BY id;

id

1

2

6

66

123

SELECT 5

-- all IDs per row (no dupes by definition - if your key names are consitent)
SELECT t.id, k.ids
FROM   tbl t
LEFT   JOIN LATERAL (
  SELECT ARRAY (SELECT split_part(json_object_keys(t.fixtures), '_', 2)::int)
  ) k(ids) ON true;

id
ids

1
{1,2,123}

2
{1,123}

3
{6,66}

SELECT 3