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.
---------------------------(end of broadcast)---------------------------
TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match
