Re: Understanding behavior of SELECT with multiple unnested columns - Mailing list pgsql-general
Basically you are getting Cartesian joins on the row output of
unnest()
Well that's what I expected too. Except look at this example, after you delete c:
testdb=# DELETE FROM t2 where val='c';
DELETE 1
testdb=# SELECT * from t1, t2;
val | val
-----+-----
1 | a
1 | b
2 | a
2 | b
(4 rows)
And compare to:
SELECT unnest(array[1,2]),unnest(array['a','b']);
unnest | unnest
--------+--------
1 | a
2 | b
(2 rows)
You can see they are not the same! Or this, which does not return the 12 rows we might both expect:
SELECT unnest(array[1,2]),unnest(array['a','b']),unnest(array[4,5,6]);
unnest | unnest | unnest
--------+--------+--------
1 | a | 4
2 | b | 5
1 | a | 6
2 | b | 4
1 | a | 5
2 | b | 6
(6 rows)
Add another element onto the third array, so they "match up" better, and you get only 4 rows:
SELECT unnest(array[1,2]),unnest(array['a','b']),unnest(array[4,5,6,7]);
unnest | unnest | unnest
--------+--------+--------
1 | a | 4
2 | b | 5
1 | a | 6
2 | b | 7
(4 rows)
(and presumably
unnest2() - I guess this is a function you defined yourself?)
Sorry for causing confusion--I meant to remove the unnest2. There was source code for the unnest function for earlier versions, which I defined as unnest2 to try to understand what was going on. It should yield the same behavior as unnest itself.
Cheers,
Ken
On Tue, Mar 26, 2013 at 11:55 PM, Ian Lawrence Barwick <barwick@gmail.com> wrote:
2013/3/27 Ken Tanzer <ken.tanzer@gmail.com>Basically you are getting Cartesian joins on the row output of
>
> I've been working on some queries involving multiple unnested columns. At first, I expected the number of rows returned would be the product of the array lengths, so that this query would return 4 rows:
>
> SELECT unnest2(array['a','b']),unnest2(array['1','2']);
>
> when in fact it returns 2:
>
> unnest2 | unnest2
> ---------+---------
> a | 1
> b | 2
>
> Which is all well and good. (Better, in fact, for my purposes.) But then this query returns 6 rows:
>
> SELECT unnest2(array['a','b','c']),unnest2(array['1','2']);
> unnest2 | unnest2
> ---------+---------
> a | 1
> b | 2
> c | 1
> a | 2
> b | 1
> c | 2
>
> Throw an unnested null column in and you get zero rows, which I also didn't expect:
>
> SELECT unnest2(array['a','b','c']),unnest2(array['1','2']),unnest(NULL::varchar[]);
> unnest2 | unnest2 | unnest
> ---------+---------+--------
> (0 rows)
>
>
> After some head scratching, I think I understand what to expect from these unnests, but I'm unclear of the logic behind what is going on. I'm hoping someone can explain it a bit.
unnest() (and presumably
unnest2() - I guess this is a function you defined yourself?)
Effectively you are doing this:
CREATE TABLE t1 (val INT);
INSERT INTO t1 VALUES (1),(2);
CREATE TABLE t2 (val CHAR(1));
INSERT INTO t2 VALUES ('a'),('b'),('c');
CREATE TABLE t3 (val INT);
testdb=# SELECT * from t1, t2;
val | val
-----+-----
1 | a
1 | b
1 | c
2 | a
2 | b
2 | c
(6 rows)
testdb=# DELETE FROM t2 where val='c';
DELETE 1
testdb=# SELECT * from t1, t2;
val | val
-----+-----
1 | a
1 | b
2 | a
2 | b
(4 rows)
testdb=# SELECT * from t1, t2, t3;
val | val | val
-----+-----+-----
(0 rows)
HTH
Ian Barwick
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