Re: Incomprehensible dogged sort in Merge Join

Aleksandr Vinokurov <aleksandr(dot)vin(at)gmail(dot)com> writes:
> Trying to get an extra time savings in my query, I stopped at an unusual
> doggedness of the planner.

> Merge Left Join
> Merge Cond: ("outer".name = "inner".name)
> -> Sort
> Sort Key: log.name
> -> Seq Scan on log_example_3 log
> -> Sort
> Sort Key: uh.name
> -> Subquery Scan uh
> -> Sort
> Sort Key: name
> -> Seq Scan on user_history

> The strange thing is that planner can combine two sorts by uh.name key
> in one, but it seems it can't see this.

> May be this can be recorded as a needed feature for future releases?

When criticizing planner deficiencies, it's considered polite to use
something that's less than two major releases back ;-)

CVS HEAD gets this right, although I need to go look at why it's
sticking a Materialize in there:

regression=# explain select * from (select * from tenk1 order by twothousand) uh right join tenk1 log on log.thousand = uh.twothousand;
QUERY PLAN
-----------------------------------------------------------------------------------
Merge Right Join (cost=4575.77..6225.77 rows=100000 width=488)
Merge Cond: (tenk1.twothousand = log.thousand)
-> Sort (cost=2287.89..2312.89 rows=10000 width=244)
Sort Key: tenk1.twothousand
-> Seq Scan on tenk1 (cost=0.00..458.00 rows=10000 width=244)
-> Materialize (cost=2287.89..2412.89 rows=10000 width=244)
-> Sort (cost=2287.89..2312.89 rows=10000 width=244)
Sort Key: log.thousand
-> Seq Scan on tenk1 log (cost=0.00..458.00 rows=10000 width=244)
(9 rows)

[ pokes at it a bit more... ] 8.1 and 8.2 get it right for a plain
join, but not for an outer join. Strange, I need to check that too.

regards, tom lane