Which is better, correlated subqueries or joins?

От: Jeffrey Tenny
Тема: Which is better, correlated subqueries or joins?
Дата: ,
Msg-id: 428D0A16.2010709@comcast.net
(см: обсуждение, исходный текст)
Ответы: Re: Which is better, correlated subqueries or joins?  ("Joshua D. Drake")
Список: pgsql-performance

Looking for some general advice on correlated subqueries vs. joins.

Which of these plans is likely to perform better.  One table is a master
record table for entities and their IDs (nv_products), the other
represents a transitive closure of parent/child relationships (for a
tree) of ID's in the master record table (and so is larger)
(ssv_product_children).

The query is, in english: for direct children of an ID, return the ones
for which isrel is true.

I have only a tiny demo table set for which there is only one record
matched by the queries below, it's hard to guess at how deep or branchy
a production table might be, so I'm trying to develop a general query
strategy and learn a thing or two about pgsql.


Here's the join:

# explain select child_pid from ssv_product_children, nv_products where
nv_products.id = ssv_product_children.child_pid and
ssv_product_children.pid = 1 and nv_products.isrel = 't';
                                 QUERY PLAN
--------------------------------------------------------------------------
  Hash Join  (cost=1.22..2.47 rows=2 width=8)
    Hash Cond: ("outer".child_pid = "inner".id)
    ->  Seq Scan on ssv_product_children  (cost=0.00..1.18 rows=9 width=4)
          Filter: (pid = 1)
    ->  Hash  (cost=1.21..1.21 rows=4 width=4)
          ->  Seq Scan on nv_products  (cost=0.00..1.21 rows=4 width=4)
                Filter: (isrel = true)
(7 rows)


Here's the correlated subquery:


# explain select child_pid from ssv_product_children where pid = 1 and
child_pid = (select nv_products.id from nv_products where nv_products.id
= child_pid and isrel = 't');
                              QUERY PLAN
---------------------------------------------------------------------
  Seq Scan on ssv_product_children  (cost=0.00..18.78 rows=1 width=4)
    Filter: ((pid = 1) AND (child_pid = (subplan)))
    SubPlan
      ->  Seq Scan on nv_products  (cost=0.00..1.26 rows=1 width=4)
            Filter: ((id = $0) AND (isrel = true))
(5 rows)


Thanks for any advice.


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