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

On Sat, May 18, 2013 at 1:47 AM, Jeff Amiel <becauseimjeff(at)yahoo(dot)com> wrote:
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> ----- 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!
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>> 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)
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>> 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.
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>> Am I missing something obvious here? I an see the nested loop row estimate being LESS but certainly not more.
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>> Can you also post the output of explain analyze <your-query>?
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> 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:
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> 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
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> 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

Have you tried analyze (it's probably a case of insufficient/outdated
statistics to planner's disposal) or probably consider changing
default_statistics_target?

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Amit Langote