Re: unnest in SELECT

On 05/21/2011 03:13 AM, Karl Koster wrote:
> I have a couple of questions regarding unnest.
>
> 1) If I issue a select statement "select unnest(vector1) as v from
> some_table", I cannot seem to use the column alias v in a WHERE or
> HAVING clause. I can use it in an ORDER BY or GROUP by clause. Is this
> the way it is supposed to work?

Yes, and it's what the SQL standard requires. Otherwise, how would this
query work?

SELECT a/b FROM sometable WHERE b <> 0;

?

The SELECT list has to be processed only once the database has already
decided which rows it applies to and how.

Use unnest in a FROM clause, eg:

SELECT v1.* FROM unnest(vector) ...

This may require a join and/or subquery to obtain 'vector'.

> 2) If I issue a select statement "select unnest(vector1) as v1,
> unnest(vector2) as v2 from some_table" and vector1 has a length of 3 and
> vector2 has a length of 4, the result set will have 12 rows with the
> data of vector1 repeating 4 times and vector2 repeating 3 times.
> Shouldn't the content of the shorter array(s) simply be return null in
> it's respective column and the result set be the size of the longest array?

unnest is a set-returning function, and it doesn't really make that much
sense to have them in the SELECT list anyway. Few databases support it,
and PostgreSQL's behavior is a historical quirk that I think most people
here hope will go quietly away at some point.

Use unnest in a FROM clause.

--
Craig Ringer