OK, check...
test=> CREATE TABLE test (id INTEGER PRIMARY KEY);
test=> INSERT INTO test SELECT random()*5 + n*10 FROM
generate_series( 1,100000 ) AS n;
test=> SELECT * FROM test LIMIT 10;
id
-----
11
23
31
41
52
63
70
85
94
103
test=> ANALYZE test;
ANALYZE
-- Self Join 1
test=> EXPLAIN ANALYZE SELECT t1.id AS current_id, t2.id AS next_id
FROM test t1, test t2
WHERE t2.id = ( SELECT min(id) FROM test AS t3 WHERE t3.id > t1.id )
ORDER BY t1.id;
QUERY
PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------
Sort (cost=26703.19..26953.19 rows=100000 width=8) (actual
time=5240.392..5271.529 rows=99999 loops=1)
Sort Key: t1.id
-> Hash Join (cost=2691.00..18398.37 rows=100000 width=8) (actual
time=106.588..5179.737 rows=99999 loops=1)
Hash Cond: ((subplan) = t2.id)
-> Seq Scan on test t1 (cost=0.00..1441.00 rows=100000 width=4)
(actual time=0.013..34.782 rows=100000 loops=1)
-> Hash (cost=1441.00..1441.00 rows=100000 width=4) (actual
time=106.420..106.420 rows=100000 loops=1)
-> Seq Scan on test t2 (cost=0.00..1441.00 rows=100000
width=4) (actual time=0.007..43.077 rows=100000 loops=1)
SubPlan
-> Result (cost=0.03..0.04 rows=1 width=0) (actual
time=0.023..0.023 rows=1 loops=199999)
InitPlan
-> Limit (cost=0.00..0.03 rows=1 width=4) (actual
time=0.021..0.022 rows=1 loops=199999)
-> Index Scan using test_pkey on test t3
(cost=0.00..1029.59 rows=33333 width=4) (actual time=0.020..0.020 rows=1
loops=199999)
Index Cond: (id > $0)
Filter: (id IS NOT NULL)
Total runtime: 5295.677 ms
-- Self Join 2
test=> set enable_hashjoin TO 0;
test=> EXPLAIN ANALYZE SELECT t1.id AS current_id, t2.id AS next_id
FROM test t1, test t2
WHERE t2.id = ( SELECT min(id) FROM test AS t3 WHERE t3.id > t1.id )
ORDER BY t1.id;
QUERY
PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
Sort (cost=30806.48..31056.48 rows=100000 width=8) (actual
time=2876.249..2903.011 rows=99999 loops=1)
Sort Key: t1.id
-> Merge Join (cost=9745.82..22501.66 rows=100000 width=8) (actual
time=2547.830..2820.347 rows=99999 loops=1)
Merge Cond: (t2.id = "inner"."?column2?")
-> Index Scan using test_pkey on test t2 (cost=0.00..2828.26
rows=100000 width=4) (actual time=0.035..67.747 rows=100000 loops=1)
-> Sort (cost=9745.82..9995.82 rows=100000 width=4) (actual
time=2547.779..2582.889 rows=100000 loops=1)
Sort Key: (subplan)
-> Seq Scan on test t1 (cost=0.00..1441.00 rows=100000
width=4) (actual time=0.060..2487.728 rows=100000 loops=1)
SubPlan
-> Result (cost=0.03..0.04 rows=1 width=0)
(actual time=0.023..0.023 rows=1 loops=100000)
InitPlan
-> Limit (cost=0.00..0.03 rows=1 width=4)
(actual time=0.021..0.022 rows=1 loops=100000)
-> Index Scan using test_pkey on
test t3 (cost=0.00..1029.59 rows=33333 width=4) (actual time=0.020..0.020
rows=1 loops=100000)
Index Cond: (id > $0)
Filter: (id IS NOT NULL)
Total runtime: 2923.804 ms
-- DISTINCT ON
test=> EXPLAIN SELECT DISTINCT ON (t1.id) t1.id AS current_id, t2.id AS
next_id
FROM test t1 JOIN test t2 ON t2.id > t1.id
ORDER BY t1.id, t2.id;
QUERY PLAN
-------------------------------------------------------------------------------------------------
Unique (cost=729806679.75..746473346.41 rows=100000 width=8)
-> Sort (cost=729806679.75..738140013.08 rows=3333333333 width=8)
Sort Key: t1.id, t2.id
-> Nested Loop (cost=0.00..100028973.00 rows=3333333333 width=8)
-> Seq Scan on test t1 (cost=0.00..1441.00 rows=100000
width=4)
-> Index Scan using test_pkey on test t2
(cost=0.00..583.61 rows=33333 width=4)
Index Cond: (t2.id > t1.id)
(7 lignes)
This one takes much longer (I interrupted it).
-- Using a function
CREATE TYPE test_type AS ( current_id INTEGER, next_id INTEGER );
CREATE OR REPLACE FUNCTION testfunc( )
RETURNS SETOF test_type
LANGUAGE plpgsql
AS
$$
DECLARE
_row test_type;
BEGIN
_row.current_id = NULL;
FOR _row.next_id IN SELECT id FROM test ORDER BY id LOOP
IF _row.current_id IS NOT NULL THEN
RETURN NEXT _row;
END IF;
_row.current_id = _row.next_id;
END LOOP;
END;
$$;
test=> EXPLAIN ANALYZE SELECT * FROM testfunc();
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------
Function Scan on testfunc (cost=0.00..12.50 rows=1000 width=8) (actual
time=211.702..238.322 rows=100000 loops=1)
Total runtime: 262.369 ms
Same results, at least 10x faster on large datasets...
