Re: Why does row estimation on nested loop make no sense to me

----- Original Message -----
From: Amit Langote <amitlangote09(at)gmail(dot)com>
To: Jeff Amiel <becauseimjeff(at)yahoo(dot)com>
Cc: "pgsql-general(at)postgresql(dot)org" <pgsql-general(at)postgresql(dot)org>
Sent: Friday, May 17, 2013 11:37 AM
Subject: Re: [GENERAL] Why does row estimation on nested loop make no sense to me

On Sat, May 18, 2013 at 1:25 AM, Jeff Amiel <becauseimjeff(at)yahoo(dot)com> wrote:
> On most nested loops that I do explain/explain analyze on, the row estimation for the nested-loop itself is a product of the inner nodes of the nested loop.
> However in this case, I am stumped!
>
> explain
> select  era.child_entity  from entity_rel era  join user_entity ue on ue.entity_id = era.parent_entity and ue.user_id=12345
>
> Nested Loop  (cost=0.00..2903.37 rows=29107 width=4)
>  ->  Index Only Scan using entity_pk on user_entity ue  (cost=0.00..62.68 rows=2 width=4)
>        Index Cond: (user_id = 10954)
>  ->  Index Scan using rel_parent on entity_rel era  (cost=0.00..1261.85 rows=317 width=8)
>        Index Cond: (parent_entity = ue.entity_id)
>
>
> How can the estimated number of rows for the nested loop node EXCEED the product of the 2 row estimates of the tables being joined?
> Not only does it exceed it - but it is orders of magnitude greater.
>
> Am I missing something obvious here?  I an see the nested loop row estimate being LESS but certainly not more.
>

> Can you also post the output of explain analyze <your-query>?

I'm not worried about performance (per se) but the row estimation issue which propagates up as part of a bigger query.  But here ya go:

explain analyze
select  era.child_entity  from entity_rel era  join user_entity ue on ue.entity_id = era.parent_entity and ue.user_id=12345

Nested Loop  (cost=0.00..2903.37 rows=29107 width=4) (actual time=0.028..0.274 rows=201 loops=1)
  ->  Index Only Scan using entity_pk on user_entity ue  (cost=0.00..62.68 rows=2 width=4) (actual time=0.011..0.012 rows=1 loops=1)
        Index Cond: (user_id = 12345)
        Heap Fetches: 1
  ->  Index Scan using rel_parent on entity_rel era  (cost=0.00..1261.85 rows=317 width=8) (actual time=0.013..0.164 rows=201 loops=1)
        Index Cond: (parent_entity = ue.entity_id)
Total runtime: 0.361 ms