> On 11 Oct 2017, at 21.46, David Rowley <david.rowley@xxxxxxxxxxxxxxx> wrote:
>
>> On 12 October 2017 at 08:37, Kim Rose Carlsen <krc@xxxxxxxx> wrote:
>>
>>> Yeah. The ORDER BY creates a partial optimization fence, preventing
>>> any such plan from being considered.
>>>>
>>
>> I can see in the general case it semanticly means different things If you allow the WHERE to pass through ORDER BY.
>>
>> A special case can be allowed for WHERE to pass the ORDER BY if the column is part of DISTINCT ON.
>
> Yeah, we do allow predicates to be pushed down in that case.
>
Let's ignore that it's not a very useful query I have written.
Why don't I see that predicate (customer_id) pushed into the outer nested loop so we don't have to sort the whole table on each loop.
(See original post and follow up for definitions)
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------
Nested Loop Left Join (cost=139.00..10392.96 rows=668 width=16) (actual time=0.528..35.120 rows=200 loops=1)
Join Filter: (c.customer_id = product.customer_id)
Rows Removed by Join Filter: 199900
-> Nested Loop (cost=0.28..199.21 rows=334 width=12) (actual time=0.075..1.146 rows=100 loops=1)
-> Seq Scan on customer (cost=0.00..21.51 rows=334 width=8) (actual time=0.067..0.282 rows=100 loops=1)
Filter: (age < 20)
Rows Removed by Filter: 901
-> Index Only Scan using customer_pkey on customer c (cost=0.28..0.53 rows=1 width=4) (actual time=0.006..0.006 rows=1 loops=100)
Index Cond: (customer_id = customer.customer_id)
Heap Fetches: 100
-> Materialize (cost=138.73..173.75 rows=2001 width=8) (actual time=0.005..0.130 rows=2001 loops=100)
-> Sort (cost=138.73..143.73 rows=2001 width=8) (actual time=0.448..0.588 rows=2001 loops=1)
Sort Key: product.customer_id, product.product_id
Sort Method: quicksort Memory: 142kB
-> Seq Scan on product (cost=0.00..29.01 rows=2001 width=8) (actual time=0.006..0.215 rows=2001 loops=1)
Planning time: 0.214 ms
Execution time: 35.284 ms
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