-- The segments that are mostly dead are rewritten the most dead first: a pass that stops early (the byte budget of the background worker) has
-- freed the most it could for the bytes it copied. A rewrite makes a new segment with the next generation, so the order of the generations of
-- the rewritten segments is the order of the rewrites; each is told by the rows it keeps.
CREATE EXTENSION pg_sift;
SET sift.table_auto_defrag = off;
SET sift.table_dead_rewrite_pct = 30;
CREATE TABLE od (id int, grp int, pad text) USING sift_table WITH (autovacuum_enabled = false);
-- three segments of 1000 rows, the most dead one the largest in bytes (a rule of "smallest first" would rewrite it last)
INSERT INTO od SELECT g, 1, repeat('a', 200) FROM generate_series(1, 1000) g;
VACUUM od;
INSERT INTO od SELECT g, 2, repeat('b', 1000) FROM generate_series(1001, 2000) g;
VACUUM od;
INSERT INTO od SELECT g, 3, repeat('c', 200) FROM generate_series(2001, 3000) g;
VACUUM od;
SELECT count(*) AS segments, min(rows) AS min_rows, max(rows) AS max_rows FROM sift.table_segments('od') WHERE kind = 'columns';
-- 40%, 90% and 60% dead
DELETE FROM od WHERE grp = 1 AND id % 5 < 2;
DELETE FROM od WHERE grp = 2 AND id % 10 < 9;
DELETE FROM od WHERE grp = 3 AND id % 5 < 3;
SELECT busy, dead_rows, merged FROM sift.table_maintain('od');
SELECT array_agg(rows ORDER BY gen) AS rows_by_rewrite, sum(dead_rows) AS dead_left
	FROM sift.table_segments('od') WHERE kind = 'columns';
SELECT grp, count(*) FROM od GROUP BY grp ORDER BY grp;
DROP TABLE od;
DROP EXTENSION pg_sift;