Hello,
if i have the following (simple) table layout:
create table a (
id serial primary key
);
create table b (
id integer references a,
test text
);
create view c as
select a.id,b.test from a
left join b
on a.id = b.id;
So if i do a select * from c i get the following:
test=# EXPLAIN SELECT * from g;
QUERY PLAN
----------------------------------------------------------------
Hash Left Join (cost=2.45..8.91 rows=8 width=36)
Hash Cond: ("outer".id = "inner".id)
-> Seq Scan on a (cost=0.00..1.08 rows=8 width=4)
-> Hash (cost=2.16..2.16 rows=116 width=36)
-> Seq Scan on b (cost=0.00..2.16 rows=116 width=36)
and a select id from c executes as
test=# EXPLAIN SELECT id from g;
QUERY PLAN
---------------------------------------------------------------
Hash Left Join (cost=2.45..7.02 rows=8 width=4)
Hash Cond: ("outer".id = "inner".id)
-> Seq Scan on a (cost=0.00..1.08 rows=8 width=4)
-> Hash (cost=2.16..2.16 rows=116 width=4)
-> Seq Scan on b (cost=0.00..2.16 rows=116 width=4)
so the only difference is the width estimation.
But why is the scan on table b performed?
If i understand it correctly this is unnecessary because the
result contains only rows from table a.
Is there a way to tell postgres not to do the extra work.
My aim is to speed up lookup to complex joins.
Thanks
Sebastian
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