Re: BUG #13528: LATERAL vs. correlated scalar subquery

On 7/30/15 1:48 PM, Maxim Boguk wrote:
> You could see that the new plan have lower total cost than the old plan
> (​cost=1867.28..1873.03 vs ​cost=0.00..125498.75).
> I think it's primary reason why it been selected (planner could produce the
> old plan but new plan wins on the cost basis).

I'll have to admit I could've put more time into the original report,
but I don't think that's accurate.  If I disable hashagg and hashjoin
and tune the query to tell the planner that only one row is to be
expected, the plan looks like this:

=#* explain analyze select u.elem, x.count from (SELECT u.elem FROM
unnest(array[1]) u(elem) LIMIT 1) u(elem), lateral (select counts.count
from counts where counts.a = u.elem) x;
                                                                   QUERY
PLAN

----------------------------------------------------------------------------------------------------------------------------------------------
  Nested Loop  (cost=0.29..4778.86 rows=1 width=12) (actual
time=0.060..52.380 rows=1 loops=1)
    Join Filter: (u.elem = data.a)
    Rows Removed by Join Filter: 99999
    ->  Limit  (cost=0.00..0.01 rows=1 width=4) (actual
time=0.010..0.011 rows=1 loops=1)
          ->  Function Scan on unnest u  (cost=0.00..1.00 rows=100
width=4) (actual time=0.009..0.009 rows=1 loops=1)
    ->  GroupAggregate  (cost=0.29..4774.33 rows=200 width=4) (actual
time=0.047..45.634 rows=100000 loops=1)
          Group Key: data.a
          ->  Index Only Scan using data_pkey on data
(cost=0.29..4298.32 rows=94802 width=4) (actual time=0.042..19.381
rows=100000 loops=1)
                Heap Fetches: 100000
  Planning time: 0.147 ms
  Execution time: 52.429 ms
(11 rows)


which to me suggests that the planner just doesn't realize that it can
push the condition on counts.a into the view.


.m