PostgreSQL – How to Remove Numbers Before First Comma

This is a gaps-and-islands problem. Assuming there are no gaps or duplicates in the same id_set set:

WITH partitioned AS (
  SELECT
    *,
    number - ROW_NUMBER() OVER (PARTITION BY id_set) AS grp
  FROM atable
  WHERE status = 'FREE'
),
counted AS (
  SELECT
    *,
    COUNT(*) OVER (PARTITION BY id_set, grp) AS cnt
  FROM partitioned
)
SELECT
  id_set,
  number
FROM counted
WHERE cnt >= 3
;

Here's a SQL Fiddle demo^* link for this query: http://sqlfiddle.com/#!1/a2633/1.

UPDATE

To return only one set, you could add in one more round of ranking:

WITH partitioned AS (
  SELECT
    *,
    number - ROW_NUMBER() OVER (PARTITION BY id_set) AS grp
  FROM atable
  WHERE status = 'FREE'
),
counted AS (
  SELECT
    *,
    COUNT(*) OVER (PARTITION BY id_set, grp) AS cnt
  FROM partitioned
),
ranked AS (
  SELECT
    *,
    RANK() OVER (ORDER BY id_set, grp) AS rnk
  FROM counted
  WHERE cnt >= 3
)
SELECT
  id_set,
  number
FROM ranked
WHERE rnk = 1
;

Here's a demo for this one too: http://sqlfiddle.com/#!1/a2633/2.

If you ever need to make it one set per id_set, change the RANK() call like this:

RANK() OVER (PARTITION BY id_set ORDER BY grp) AS rnk

Additionally, you could make the query return the smallest matching set (i.e. first try to return the first set of exactly three consecutive numbers if it exists, otherwise four, five etc.), like this:

RANK() OVER (ORDER BY cnt, id_set, grp) AS rnk

or like this (one per id_set):

RANK() OVER (PARTITION BY id_set ORDER BY cnt, grp) AS rnk

_{* The SQL Fiddle demos linked in this answer use the 9.1.8 instance as the 9.2.1 one doesn't appear to be working at the moment.}

IN queries with huge sets are notoriously slow. It's often faster to use a JOIN instead:

SELECT nodes
FROM   planet_osm_ways
JOIN   (
   SELECT ltrim(member, 'w')::bigint AS id
   FROM  (
      SELECT unnest(members) AS member
      FROM   planet_osm_rels
      WHERE  (tags_hstore @> '"type"=>"boundary", "admin_level"=>"2", ...')
      ) u
   WHERE member LIKE 'w%'
   ) x USING (id);

But that's not the most important problem here. I wonder why the GIN index planet_osm_rels_tags_hstore_idx is not being used. Are you selecting large enough parts of the table planet_osm_rels to justify a sequential scan?

Oh, and id is type bigint. So cast to bigint instead of int for less friction.

If you can extract "way IDs" and save them redundantly in a separate column way_ids bigint[] in your table, your query would become quite a bit simpler and faster, with one less subquery level:

SELECT nodes
FROM   planet_osm_ways
JOIN   (
   SELECT unnest(way_ids) AS id
   FROM   planet_osm_rels
   WHERE  (tags_hstore @> '"type"=>"boundary", "admin_level"=>"2", ...')
   ) u USING (id);