Re: Hash Join not using hashed index?

On Wed, Jun 28, 2000 at 10:56:17AM -0400, Tom Lane wrote:
> Ang Chin Han <angch@pollcities.com> writes:
> If it was like that then a hash index wouldn't have been applicable
> anyway; hashes are only good for strict equality checks.  If you want
> something that can do ordering checks you need a btree index.
> 
> (There are good reasons why btree is the default index type ;-))

There _was_ a btree index, before I added the extra hash index:

pintoo=# \dcountry_pkeyIndex "country_pkey"Attribute  |   Type
------------+----------country_id | smallint
unique btree (primary key)

> > Original cost est:
> > Hash Join  (cost=8.85..16.76 rows=75 width=18)
> >   -> Seq Scan on city  (cost=0.00..1.75 rows=75 width=16) 
> >   -> Hash  (cost=5.53..5.53 rows=253 width=2)
> >        -> Seq Scan on country  (cost=0.00..5.53 rows=253 width=2) 
> 
> > I guess the problem is that country-city is a one-to-many relation,
> > BUT I've more countries than cities (note the # of rows above), thus
> > throwing the planner off...
> 
> Off what?  This looks like a pretty reasonable plan to me, given the
> fairly small table sizes.  Do you have evidence that another plan
> type would be quicker for this problem?

No evidence, but I was hoping that having a prehashed country_id
would speed things up a bit, since the seq scan on country could
be redundant, requring only a seq scan on city and a index (hash)
lookup on country.


Or maybe this is a related question (just curious):

pintoo=# explain select country_id from country order by country_id;
NOTICE:  QUERY PLAN:
Sort  (cost=15.63..15.63 rows=253 width=2) ->  Seq Scan on country  (cost=0.00..5.53 rows=253 width=2)

pintoo=# explain select name from country order by name;
NOTICE:  QUERY PLAN:
Sort  (cost=15.63..15.63 rows=253 width=12) ->  Seq Scan on country  (cost=0.00..5.53 rows=253 width=12)

If there is already in b-tree index on country_id, why bother
re-sorting it, when it could be output'd by traversing the tree?
Comparing with an unindexed column, we can see that the index
is not used at all.