Re: different unnest function

2011/9/16 David Johnston <polobo(at)yahoo(dot)com>:
> -----Original Message-----
> From: pgsql-general-owner(at)postgresql(dot)org [mailto:pgsql-general-owner(at)postgresql(dot)org] On Behalf Of Ondrej Ivanic
> Sent: Friday, September 16, 2011 12:54 AM
> To: pgsql-general(at)postgresql(dot)org general
> Subject: [GENERAL] different unnest function
>
> Hi,
>
> I need function which unnest array in a different way. Input table has ineger[][] column:
> col1
> ----------
> {{1,2,3,4}, {5,6,7,8}, {9, 10, 11, 12}}
> {{11,12,13,14}, {15,16,17,18}, {19, 110, 111, 112}} ...
>
> and output should be:
>
> select unnest2(col1) from T
> unnest2
> -----------------
> {1,2,3,4}
> {5,6,7,8}
> {9, 10, 11, 12}
> {11,12,13,14}
> {15,16,17,18}
> {19, 110, 111, 112}
>
> My function is:
> create or replace function unnest2(anyarray) returns setof anyarray AS $BODY$ select $1[i:i] from generate_series(array_lower($1,1), array_upper($1,1)) i; $BODY$ language 'sql';
>
> and the result is:
> {{1,2,3,4}}
> {{5,6,7,8}}
> {{9, 10, 11, 12}}
> {{11,12,13,14}}
> {{15,16,17,18}}
> {{19, 110, 111, 112}}
>
> which is almost what I need...(or I'm at the beginning :))  Any ideas?
>
> ----------------------------------------------------------------------------
>
> Untested by try something like:
>
> SELECT ($1[i:i])[1] FROM generate_series(...) i;
>
> Basically you want to take the first (and only) array element from your slice.  You might need to use sub-queries to get the syntax to work but the idea should be possible.

nope -- that doesn't work. there is no way to manipulate array
dimensionality with the [] operator except in the special case from
D1->D0 (datum). this is a fundamental awkwardness that can be weird
to most people.

to get the output OP wants, you need to expand and rewrap:
create or replace function unnest2(anyarray) returns setof anyarray AS
$BODY$
select array(select unnest($1[i:i])) from
generate_series(array_lower($1,1), array_upper($1,1)) i;
$BODY$
language 'sql';

-- ugh.

9.1 has a vastly improved (plpgsql only) way to do this -- FOREACH SLICE...

http://www.postgresql.org/docs/9.1/interactive/plpgsql-control-structures.html#PLPGSQL-FOREACH-ARRAY

merlin