Re: Plan for relatively simple query seems to be very inefficient

On Wed, Apr 06, 2005 at 06:52:35PM +0200, Arjen van der Meijden wrote:
> Hi list,
>
> I noticed on a forum a query taking a surprisingly large amount of time
> in MySQL. Of course I wanted to prove PostgreSQL 8.0.1 could do it much
> better. To my surprise PostgreSQL was ten times worse on the same
> machine! And I don't understand why.
>
> I don't really need this query to be fast since I don't use it, but the
> range-thing is not really an uncommon query I suppose. So I'm wondering
> why it is so slow and this may point to a wrong plan being chosen or
> generated.

That's the wrong index type for fast range queries. You really need
something like GiST or rtree for that. I do something similar in
production and queries are down at the millisecond level with the
right index.


Cheers,
  Steve

> Here are table definitions:
>
>         Table "public.postcodes"
>    Column    |     Type      | Modifiers
> -------------+---------------+-----------
>  postcode_id | smallint      | not null
>  range_from  | smallint      |
>  range_till  | smallint      |
> Indexes:
>     "postcodes_pkey" PRIMARY KEY, btree (postcode_id)
>     "range" UNIQUE, btree (range_from, range_till)
>
>    Table "public.data_main"
>  Column |   Type   | Modifiers
> --------+----------+-----------
>  userid | integer  | not null
>  range  | smallint |
> Indexes:
>     "data_main_pkey" PRIMARY KEY, btree (userid)
>
> And here's the query I ran:
>
> SELECT COUNT(*) FROM
> data_main AS dm,
> postcodes AS p
> WHERE dm.range BETWEEN p.range_from AND p.range_till