2. Why do both HashAggregate and GroupAggregate say the cost estimate
is 40000 rows?
I've reproduced this :
CREATE TABLE popo AS SELECT (x%1000) AS a,(x%1001) AS b FROM
generate_series( 1,1000000 ) AS x;
VACUUM ANALYZE popo;
EXPLAIN ANALYZE SELECT a,b,count(*) FROM (SELECT * FROM popo UNION ALL
SELECT * FROM popo) AS foo GROUP BY a,b;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=43850.00..44350.00 rows=40000 width=8) (actual
time=1893.441..2341.780 rows=1000000 loops=1)
-> Append (cost=0.00..28850.00 rows=2000000 width=8) (actual
time=0.025..520.581 rows=2000000 loops=1)
-> Seq Scan on popo (cost=0.00..14425.00 rows=1000000 width=8)
(actual time=0.025..142.639 rows=1000000 loops=1)
-> Seq Scan on popo (cost=0.00..14425.00 rows=1000000 width=8)
(actual time=0.003..114.257 rows=1000000 loops=1)
Total runtime: 2438.741 ms
(5 lignes)
Temps : 2439,247 ms
I guess the row count depends on the correlation of a and b, which pg has
no idea about. In the first example, there is no correlation, now with
full correlation :
UPDATE popo SET a=b;
VACUUM FULL popo;
VACUUM FULL popo;
ANALYZE popo;
EXPLAIN ANALYZE SELECT a,b,count(*) FROM (SELECT * FROM popo UNION ALL
SELECT * FROM popo) AS foo GROUP BY a,b;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=43850.00..44350.00 rows=40000 width=8) (actual
time=1226.201..1226.535 rows=1001 loops=1)
-> Append (cost=0.00..28850.00 rows=2000000 width=8) (actual
time=0.008..518.068 rows=2000000 loops=1)
-> Seq Scan on popo (cost=0.00..14425.00 rows=1000000 width=8)
(actual time=0.007..128.609 rows=1000000 loops=1)
-> Seq Scan on popo (cost=0.00..14425.00 rows=1000000 width=8)
(actual time=0.005..128.502 rows=1000000 loops=1)
Total runtime: 1226.797 ms
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