20 липня 2009 р. 11:02 Chris <dmag...@gmail.com> написав:
> Віталій Тимчишин wrote: > >> >> >> 2009/7/20 Robert James <srobertja...@gmail.com <mailto: >> srobertja...@gmail.com>> >> >> >> Hi. I notice that when I do a WHERE x, Postgres uses an index, and >> when I do WHERE y, it does so as well, but when I do WHERE x OR y, >> it doesn't. Why is this so? >> >> It's not clever enough. >> > > Of course it is. For simple cases > > I'm running 8.3.7. > > create table t1(id int primary key); > insert into t1(id) select a from generate_series(1, 500000) as s(a); > analyze t1; > explain analyze select * from t1 where id < 10000 "Index Scan using t1_pkey on t1 (cost=0.00..322.51 rows=9612 width=4) (actual time=0.030..3.700 rows=9999 loops=1)" " Index Cond: (id < 10000)" "Total runtime: 4.835 ms" explain analyze select * from t1 where id in (select (random() * 500000)::int4 from generate_series(0,10)) "Nested Loop (cost=32.50..1341.49 rows=200 width=4) (actual time=15.353..67.014 rows=11 loops=1)" " -> HashAggregate (cost=32.50..34.50 rows=200 width=4) (actual time=0.028..0.043 rows=11 loops=1)" " -> Function Scan on generate_series (cost=0.00..20.00 rows=1000 width=0) (actual time=0.014..0.020 rows=11 loops=1)" " -> Index Scan using t1_pkey on t1 (cost=0.00..6.52 rows=1 width=4) (actual time=6.083..6.084 rows=1 loops=11)" " Index Cond: (t1.id = (((random() * 500000::double precision))::integer))" "Total runtime: 67.070 ms" explain analyze select * from t1 where id in (select (random() * 500000)::int4 from generate_series(0,10)) or id < 10000 "Seq Scan on t1 (cost=22.50..9735.50 rows=254806 width=4) (actual time=0.049..148.947 rows=10010 loops=1)" " Filter: ((hashed subplan) OR (id < 10000))" " SubPlan" " -> Function Scan on generate_series (cost=0.00..20.00 rows=1000 width=0) (actual time=0.014..0.019 rows=11 loops=1)" "Total runtime: 150.123 ms" explain analyze select * from t1 where id in (select (random() * 500000)::int4 from generate_series(0,10)) union select * from t1 where id < 10000 "Unique (cost=2412.68..2461.74 rows=9812 width=4) (actual time=89.190..95.014 rows=10010 loops=1)" " -> Sort (cost=2412.68..2437.21 rows=9812 width=4) (actual time=89.189..91.167 rows=10010 loops=1)" " Sort Key: public.t1.id" " Sort Method: quicksort Memory: 854kB" " -> Append (cost=32.50..1762.13 rows=9812 width=4) (actual time=16.641..76.338 rows=10010 loops=1)" " -> Nested Loop (cost=32.50..1341.49 rows=200 width=4) (actual time=16.641..70.051 rows=11 loops=1)" " -> HashAggregate (cost=32.50..34.50 rows=200 width=4) (actual time=0.033..0.049 rows=11 loops=1)" " -> Function Scan on generate_series (cost=0.00..20.00 rows=1000 width=0) (actual time=0.020..0.026 rows=11 loops=1)" " -> Index Scan using t1_pkey on t1 (cost=0.00..6.52 rows=1 width=4) (actual time=6.359..6.361 rows=1 loops=11)" " Index Cond: (public.t1.id = (((random() * 500000::double precision))::integer))" " -> Index Scan using t1_pkey on t1 (cost=0.00..322.51 rows=9612 width=4) (actual time=0.023..4.075 rows=9999 loops=1)" " Index Cond: (id < 10000)" "Total runtime: 112.694 ms" So, if it founds out anything complex, it sadly falls back to Sequence scan.
