-- what an operator sees: sift.version(), sift.index_info(), the extents and dead documents of sift.segments()
CREATE EXTENSION pg_sift;
SELECT sift.version() = extversion AS library_matches_extension FROM pg_extension WHERE extname = 'pg_sift';
 library_matches_extension 
---------------------------
 t
(1 row)

CREATE TABLE oi (id int, body text);
INSERT INTO oi SELECT i, 'alpha beta ' || i FROM generate_series(1, 3000) i;
CREATE INDEX oi_idx ON oi USING sift (body);
SELECT relation_blocks = high_water AS blocks_in_use, free_blocks, reusable_blocks, free_extents, segments, max_segments,
	ndocs, maintenance_busy
FROM sift.index_info('oi_idx');
 blocks_in_use | free_blocks | reusable_blocks | free_extents | segments | max_segments | ndocs | maintenance_busy 
---------------+-------------+-----------------+--------------+----------+--------------+-------+------------------
 t             |           0 |               0 |            0 |        1 |          128 |  3000 | f
(1 row)

SELECT kind, ndocs, extents, dead_docs FROM sift.segments('oi_idx');
   kind    | ndocs | extents | dead_docs 
-----------+-------+---------+-----------
 immutable |  3000 |       1 |         0
(1 row)

-- VACUUM marks the deleted documents dead; merging the segment drops them and frees its blocks
DELETE FROM oi WHERE id <= 300;
VACUUM oi;
SELECT kind, ndocs, extents, dead_docs FROM sift.segments('oi_idx');
   kind    | ndocs | extents | dead_docs 
-----------+-------+---------+-----------
 immutable |  3000 |       1 |       300
(1 row)

SELECT sift.merge('oi_idx', true);
 merge 
-------
     1
(1 row)

SELECT kind, ndocs, dead_docs FROM sift.segments('oi_idx');
   kind    | ndocs | dead_docs 
-----------+-------+-----------
 immutable |  2700 |         0
(1 row)

SELECT free_blocks > 0 AS blocks_freed, reusable_blocks <= free_blocks AS reusable_within_free, ndocs
FROM sift.index_info('oi_idx');
 blocks_freed | reusable_within_free | ndocs 
--------------+----------------------+-------
 t            | t                    |  2700
(1 row)

-- a write buffer has no extents and no dead documents to report
INSERT INTO oi VALUES (5000, 'gamma');
SELECT kind, extents IS NULL AS no_extents, dead_docs IS NULL AS no_dead FROM sift.segments('oi_idx') ORDER BY gen;
   kind    | no_extents | no_dead 
-----------+------------+---------
 immutable | f          | f
 buffer    | t          | t
(2 rows)

DROP TABLE oi;
DROP EXTENSION pg_sift;