Re: [SQL] To use a VIEW or not to use a View.....

[Tomasz Myrta]

Stephan Szabo wrote:

That's not the same join for optimization purposes
since postgresql treats explicit join syntax as a
constraint on the ordering of joins.

The same join would be something like:

przystanki p1 join miasta m1 using (id_miasta)
join (przystanki p2 join miasta m2 using (id_miasta))
using (id_przystanku)

minus the fact I think you'd need some explicit naming in
there.

You are right.

The result of your query is:
explain select * from
przystanki p1 join miasta m1 using (id_miasta)
join (przystanki p2 join miasta m2 using (id_miasta))
using (id_przystanku)
where id_przystanku=1230

Hash Join (cost=13.00..30.10 rows=1 width=128)
-> Hash Join (cost=6.74..21.02 rows=374 width=64)
-> Seq Scan on przystanki p2 (cost=0.00..7.74 rows=374 width=41)
-> Hash (cost=5.99..5.99 rows=299 width=23)
-> Seq Scan on miasta m2 (cost=0.00..5.99 rows=299 width=23)
-> Hash (cost=6.26..6.26 rows=1 width=64)
-> Nested Loop (cost=0.00..6.26 rows=1 width=64)
-> Index Scan using przystanki_pkey on przystanki p1 (cost=0.00..3.14 rows=1 width=41)
-> Index Scan using miasta_pkey on miasta m1 (cost=0.00..3.10 rows=1 width=23)

Anyway - is it possible to expose table "przystanki alias p2" to get valid result?

The problem is similiar to my problem "sub-select with aggregate" dated on 2002-10-23 and the answer (which doesn't satisfy me) is the same:
if we pass static values to "przystanki p2 join miasta m2", the query will work ok:
explain select * from
przystanki p1 join miasta m1 using (id_miasta)
cross join (przystanki p2 join miasta m2 using (id_miasta)) X
where p1.id_przystanku=1230 and X.id_przystanku=1230

Nested Loop (cost=0.00..12.52 rows=1 width=128)
-> Nested Loop (cost=0.00..6.26 rows=1 width=64)
-> Index Scan using przystanki_pkey on przystanki p1 (cost=0.00..3.14 rows=1 width=41)
-> Index Scan using miasta_pkey on miasta m1 (cost=0.00..3.10 rows=1 width=23)
-> Materialize (cost=6.26..6.26 rows=1 width=64)
-> Nested Loop (cost=0.00..6.26 rows=1 width=64)
-> Index Scan using przystanki_pkey on przystanki p2 (cost=0.00..3.14 rows=1 width=41)
-> Index Scan using miasta_pkey on miasta m2 (cost=0.00..3.10 rows=1 width=23)

Stephan - I have some problems with mail relay to you. Does my mail server have any open-relay problem, or something like this (213.25.37.66) ?

Regards,
Tomasz Myrta





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