Re: SELECT DISTINCT on boxes

Bruce Momjian wrote:
> Is there a way to do a SELECT DISTINCT on boxes:
>
> test=> create TABLE t3 (a box);
> CREATE TABLE
> test=> insert into t3 values ('(2,2),(1,1)');
> INSERT 17232 1
> test=> insert into t3 values ('(2,2),(1,1)');
> INSERT 17233 1
> test=> insert into t3 values ('(3,3),(2,2)');
> INSERT 17234 1
> test=> insert into t3 values ('(3,3),(2,2)');
> INSERT 17235 1
> test=> select distinct * from t3;
> ERROR: could not identify an ordering operator for type box
> HINT: Use an explicit ordering operator or modify the query.
>
> I tried doing subqueries and using oids but that didn't help.
>
> I don't understand why this doesn't work:
>
> SELECT a
> FROM t3 t2
> WHERE t2.oid = (SELECT MIN(t.oid) FROM t3 t WHERE t2.a = t.a);
>
> a
> -------------
> (2,2),(1,1)
> (1 row)
>
> If finds only the duplicate.

I found the cause. Equals for boxes compares only area, \do:

pg_catalog | = | box | box | boolean | equal by area

The proper fix is to use ~= which compares boxes for similarity:

pg_catalog | ~= | box | box | boolean | same as?

The correct query for DISTINCT is:

test=> SELECT oid, a FROM t3 t2 WHERE t2.oid = (SELECT MIN(t.oid) FROM
t3 t WHERE t2.a ~= t.a);
oid | a
-------+-------------
17232 | (2,2),(1,1)
17234 | (3,3),(2,2)
(2 rows)

--
Bruce Momjian | http://candle.pha.pa.us
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