CREATE EXTENSION pg_freespacemap;
CREATE TABLE freespace_tab (c1 int) WITH (autovacuum_enabled = off);
CREATE INDEX freespace_brin ON freespace_tab USING brin (c1);
CREATE INDEX freespace_btree ON freespace_tab USING btree (c1);
CREATE INDEX freespace_hash ON freespace_tab USING hash (c1);
-- report all the sizes of the FSMs for all the relation blocks.
WITH rel AS (SELECT oid::regclass AS id FROM pg_class WHERE relname ~ 'freespace')
  SELECT rel.id, fsm.blkno, (fsm.avail > 0) AS is_avail
    FROM rel, LATERAL pg_freespace(rel.id) AS fsm
    ORDER BY 1, 2;
       id        | blkno | is_avail 
-----------------+-------+----------
 freespace_brin  |     0 | f
 freespace_brin  |     1 | f
 freespace_brin  |     2 | t
 freespace_btree |     0 | f
 freespace_hash  |     0 | f
 freespace_hash  |     1 | f
 freespace_hash  |     2 | f
 freespace_hash  |     3 | f
 freespace_hash  |     4 | f
 freespace_hash  |     5 | f
 freespace_hash  |     6 | f
 freespace_hash  |     7 | f
 freespace_hash  |     8 | f
 freespace_hash  |     9 | f
(14 rows)

INSERT INTO freespace_tab VALUES (1);
VACUUM freespace_tab;
WITH rel AS (SELECT oid::regclass AS id FROM pg_class WHERE relname ~ 'freespace')
  SELECT rel.id, fsm.blkno, (fsm.avail > 0) AS is_avail
    FROM rel, LATERAL pg_freespace(rel.id) AS fsm
    ORDER BY 1, 2;
       id        | blkno | is_avail 
-----------------+-------+----------
 freespace_tab   |     0 | t
 freespace_brin  |     0 | f
 freespace_brin  |     1 | f
 freespace_brin  |     2 | t
 freespace_btree |     0 | f
 freespace_btree |     1 | f
 freespace_hash  |     0 | f
 freespace_hash  |     1 | f
 freespace_hash  |     2 | f
 freespace_hash  |     3 | f
 freespace_hash  |     4 | f
 freespace_hash  |     5 | f
 freespace_hash  |     6 | f
 freespace_hash  |     7 | f
 freespace_hash  |     8 | f
 freespace_hash  |     9 | f
(16 rows)

DELETE FROM freespace_tab;
VACUUM freespace_tab;
WITH rel AS (SELECT oid::regclass AS id FROM pg_class WHERE relname ~ 'freespace')
  SELECT rel.id, fsm.blkno, (fsm.avail > 0) AS is_avail
    FROM rel, LATERAL pg_freespace(rel.id) AS fsm
    ORDER BY 1, 2;
       id        | blkno | is_avail 
-----------------+-------+----------
 freespace_brin  |     0 | f
 freespace_brin  |     1 | f
 freespace_brin  |     2 | t
 freespace_btree |     0 | f
 freespace_btree |     1 | f
 freespace_hash  |     0 | f
 freespace_hash  |     1 | f
 freespace_hash  |     2 | f
 freespace_hash  |     3 | f
 freespace_hash  |     4 | f
 freespace_hash  |     5 | f
 freespace_hash  |     6 | f
 freespace_hash  |     7 | f
 freespace_hash  |     8 | f
 freespace_hash  |     9 | f
(15 rows)

-- failures with incorrect block number
SELECT * FROM pg_freespace('freespace_tab', -1);
ERROR:  invalid block number
SELECT * FROM pg_freespace('freespace_tab', 4294967295);
ERROR:  invalid block number
DROP TABLE freespace_tab;