Hello

I try to look on one slow query with correlated subquery:

create table xx(a int primary key);
create table yy(a int);

insert into xx select generate_series(1,1000000);
insert into yy select (random()*1000000)::int from generate_series(1,100000);

create index on yy(a);

Query A
select a, (select true from yy where xx.a = yy.a limit 1) from xx
limit 10 offset 0;

postgres=> explain select a, (select true from yy where xx.a = yy.a
limit 1) from xx;
                                      QUERY PLAN
--------------------------------------------------------------------------------------
 Seq Scan on xx  (cost=0.00..4392325.00 rows=1000000 width=4)
   SubPlan 1
     ->  Limit  (cost=0.00..4.38 rows=1 width=0)
           ->  Index Only Scan using yy_a_idx on yy  (cost=0.00..4.38
rows=1 width=0)
                 Index Cond: (a = xx.a)
(5 rows)

plan for this query is expected

But when I rewrote query I got strange plan (query B):

postgres=> explain select a, exists(select 1 from yy where xx.a =
yy.a)  from xx limit 10 offset 0;
QUERY PLAN
--------------------------------------------------------------------------------------
 Limit  (cost=0.00..43.92 rows=10 width=4)
   ->  Seq Scan on xx  (cost=0.00..4392325.00 rows=1000000 width=4)
         SubPlan 1
           ->  Index Only Scan using yy_a_idx on yy  (cost=0.00..4.38
rows=1 width=0)
                 Index Cond: (a = xx.a)
         SubPlan 2
           ->  Seq Scan on yy  (cost=0.00..1443.00 rows=100000 width=4)
(7 rows)

Why there are a SubPlan 2?

But query B is two time faster than query A

 public | xx                            | table | pavel    | 35 MB      |
 public | yy                            | table | pavel    | 3576 kB    |

regards

Pavel

Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
> Why there are a SubPlan 2?

http://git.postgresql.org/gitweb/?p=postgresql.git&a=commitdiff&h=bd3daddaf232d95b0c9ba6f99b0170a0147dd8af

			regards, tom lane

2012/2/28 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
> Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
>> Why there are a SubPlan 2?
>
> http://git.postgresql.org/gitweb/?p=postgresql.git&a=commitdiff&h=bd3daddaf232d95b0c9ba6f99b0170a0147dd8af
>
>                        regards, tom lane

Thank you - I can verify so it works well, but a EXPLAIN result is
really strange

Pavel