-- vacuum_segments: the directory entries VACUUM merges an index down to (default 2), changed with ALTER INDEX and no REINDEX.
CREATE EXTENSION pg_sift;
SET enable_seqscan = off;
CREATE TABLE vs (id serial, body text) WITH (autovacuum_enabled = off);
INSERT INTO vs (body) SELECT 'k' || (i % 7) || ' g' || (i % 1000) || ' common word' || (i % 50) FROM generate_series(1, 30000) i;
-- built without the option: the default
CREATE INDEX vs_idx ON vs USING sift (body) WITH (build_segments = 60);
SELECT count(*) AS built FROM sift.segments('vs_idx');
 built 
-------
    60
(1 row)

SELECT count(*) AS hits FROM vs WHERE body @@> 'common';
 hits  
-------
 30000
(1 row)

VACUUM vs;
SELECT count(*) AS after_default FROM sift.segments('vs_idx');
 after_default 
---------------
             2
(1 row)

SELECT count(*) AS hits FROM vs WHERE body @@> 'common';
 hits  
-------
 30000
(1 row)

-- a higher value: less merging, more segments
DROP INDEX vs_idx;
CREATE INDEX vs_idx ON vs USING sift (body) WITH (build_segments = 60);
ALTER INDEX vs_idx SET (vacuum_segments = 4);
SELECT sift.verify('vs_idx');
 verify 
--------
 
(1 row)

VACUUM vs;
SELECT count(*) AS after_4 FROM sift.segments('vs_idx');
 after_4 
---------
       4
(1 row)

SELECT count(*) AS hits FROM vs WHERE body @@> 'common';
 hits  
-------
 30000
(1 row)

SELECT sift.verify('vs_idx', true);
 verify 
--------
 
(1 row)

-- lowered to 1, VACUUM merges further; raised above the segments it has: a no-op
ALTER INDEX vs_idx SET (vacuum_segments = 1);
VACUUM vs;
SELECT count(*) AS after_1 FROM sift.segments('vs_idx');
 after_1 
---------
       1
(1 row)

CREATE TEMP TABLE before AS SELECT gen FROM sift.segments('vs_idx');
ALTER INDEX vs_idx SET (vacuum_segments = 128);
VACUUM vs;
SELECT count(*) AS unchanged FROM sift.segments('vs_idx') s JOIN before b USING (gen);
 unchanged 
-----------
         1
(1 row)

SELECT sift.verify('vs_idx', true);
 verify 
--------
 
(1 row)

-- out of range
ALTER INDEX vs_idx SET (vacuum_segments = 0);
ERROR:  value 0 out of bounds for option "vacuum_segments"
DETAIL:  Valid values are between "1" and "128".
ALTER INDEX vs_idx SET (vacuum_segments = 129);
ERROR:  value 129 out of bounds for option "vacuum_segments"
DETAIL:  Valid values are between "1" and "128".
ALTER INDEX vs_idx RESET (vacuum_segments);
SELECT sift.verify('vs_idx');
 verify 
--------
 
(1 row)

-- a segment far smaller than the rest stays above the target: merging it would rewrite the whole index at every VACUUM
DROP INDEX vs_idx;
CREATE INDEX vs_idx ON vs USING sift (body) WITH (build_segments = 1, write_buffer_limit = 64);
INSERT INTO vs (body) SELECT 'k' || (i % 7) || ' g' || (i % 1000) || ' common word' || (i % 50) FROM generate_series(1, 400) i;
SELECT sift.promote('vs_idx');
 promote 
---------
       1
(1 row)

CREATE TEMP TABLE small AS SELECT gen FROM sift.segments('vs_idx');
VACUUM vs;
SELECT count(*) AS kept, (SELECT count(*) FROM sift.segments('vs_idx')) AS segments FROM sift.segments('vs_idx') JOIN small USING (gen);
 kept | segments 
------+----------
    3 |        3
(1 row)

SELECT sift.verify('vs_idx', true);
 verify 
--------
 
(1 row)

-- a mostly dead segment is rewritten whatever the target: with 128 the tiered merges are not for the count, the rewrite is for the garbage
DROP INDEX vs_idx;
CREATE INDEX vs_idx ON vs USING sift (body) WITH (build_segments = 10, vacuum_segments = 128);
SELECT count(*) AS built FROM sift.segments('vs_idx');
 built 
-------
    10
(1 row)

SELECT sum(ndocs) AS docs FROM sift.segments('vs_idx');
 docs  
-------
 30400
(1 row)

DELETE FROM vs WHERE id <= 2000;
VACUUM vs;
SELECT count(*) AS segments, sum(ndocs) AS docs FROM sift.segments('vs_idx');
 segments | docs  
----------+-------
       10 | 28400
(1 row)

SELECT count(*) AS hits FROM vs WHERE body @@> 'common';
 hits  
-------
 28400
(1 row)

SELECT sift.verify('vs_idx', true);
 verify 
--------
 
(1 row)

DROP TABLE vs;
DROP EXTENSION pg_sift;