Re: A question on the query planner

Jared Carr <jared@89glass.com> writes:

> Greg Stark wrote:
>
> > Well it looks like you have something strange going on. What data type is
> > car_id in each table?
> >
> car_id is a varchar(10) in both tables.

Huh. The following shows something strange. It seems joining on two varchars
no longer works well. Instead the optimizer has to convert both columns to
text.

I know some inter-type comparisons were removed a while ago, but I would not
have thought that would effect varchar-varchar comparisons. I think this is
pretty bad.


test=# create table a (x varchar primary key);
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "a_pkey" for table "a"
CREATE TABLE
test=# create table b (x varchar primary key);
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "b_pkey" for table "b"
CREATE TABLE
test=# select * from a,b where a.x=b.x;
 x | x
---+---
(0 rows)

test=# explain select * from a,b where a.x=b.x;
                            QUERY PLAN
------------------------------------------------------------------
 Merge Join  (cost=139.66..159.67 rows=1001 width=64)
   Merge Cond: ("outer"."?column2?" = "inner"."?column2?")
   ->  Sort  (cost=69.83..72.33 rows=1000 width=32)
         Sort Key: (a.x)::text
         ->  Seq Scan on a  (cost=0.00..20.00 rows=1000 width=32)
   ->  Sort  (cost=69.83..72.33 rows=1000 width=32)
         Sort Key: (b.x)::text
         ->  Seq Scan on b  (cost=0.00..20.00 rows=1000 width=32)
(8 rows)

test=# create table a2 (x text primary key);
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "a2_pkey" for table "a2"
CREATE TABLE
test=# create table b2 (x text primary key);
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "b2_pkey" for table "b2"
CREATE TABLE
test=# explain select * from a2,b2 where a2.x=b2.x;
                            QUERY PLAN
-------------------------------------------------------------------
 Hash Join  (cost=22.50..57.51 rows=1001 width=64)
   Hash Cond: ("outer".x = "inner".x)
   ->  Seq Scan on a2  (cost=0.00..20.00 rows=1000 width=32)
   ->  Hash  (cost=20.00..20.00 rows=1000 width=32)
         ->  Seq Scan on b2  (cost=0.00..20.00 rows=1000 width=32)
(5 rows)


--
greg