Re: How to alias attributes in an ARRAY_AGG expression

On Wed, Nov 13, 2013 at 2:50 AM, Claudio Poli <masterkain(at)gmail(dot)com> wrote:
> Hello,
> I'm playing with this query, my goal is return an entire json response with
> some column names changed in the resultset of ARRAY_AGG::
>
> SELECT ARRAY_TO_JSON(ARRAY_AGG(ALBUM_ROW))
> FROM (
> SELECT
> album,
> MAX(release_year) AS release_year,
> MAX(artwork_path) AS artwork_path,
> MAX(MD5(CONCAT(album, release_year, artist))) AS token,
> ARRAY_AGG((media_files.position, media_files.token) ORDER BY
> media_files.position) as media_files
> FROM media_files
> INNER JOIN playlist_media_files ON playlist_media_files.media_file_id =
> media_files.id
> WHERE playlist_media_files.playlist_id = 1
> GROUP BY album, release_year, artist
> ORDER BY artist, release_year
> ) as ALBUM_ROW
>
> This works fairly well, however I need to alias the attribute names in the
> ARRAY_AGG:
> ARRAY_AGG((media_files.position, media_files.token) ...
>
> ARRAY_AGG apparently does not support AS, so I have to resort to a subquery,
> which works but is totally inefficient:

Well, for starters, define "totally inefficient".

> SELECT ARRAY_TO_JSON(ARRAY_AGG(ALBUM_ROW))
> FROM (
> SELECT
> album,
> MAX(release_year) AS release_year,
> MAX(artwork_path) AS artwork_path,
> MAX(MD5(CONCAT(album, release_year, artist))) AS token,
> (
> SELECT ARRAY_AGG(d)
> FROM (
> SELECT mf.position AS myalias
> FROM media_files AS mf
> INNER JOIN playlist_media_files ON
> playlist_media_files.media_file_id = mf.id
> WHERE playlist_media_files.playlist_id = #{playlist_id}
> AND mf.album = media_files.album
> ORDER BY mf."position" ASC
> ) d
> ) as media_files
> FROM media_files
> INNER JOIN playlist_media_files ON
> playlist_media_files.media_file_id = media_files.id
> WHERE playlist_media_files.playlist_id = #{playlist_id}
> GROUP BY album, release_year, artist
> ORDER BY artist, release_year
> ) as ALBUM_ROW
>
> Does anyone have better ideas?

Try creating a composite type and caseting:

> ARRAY_AGG((media_files.position, media_files.token) ORDER BY
> media_files.position) as media_files

could become

ARRAY_AGG((media_files.position, media_files.token) ORDER BY
media_files.position)::foo[] as media_files

where foo is the type with the names as you want them. Also, when not
grouping, don't be afraid to try the array() constructor syntax:

select
a,
array(
select b from b where b.id = a.id order by ...
)
...

merlin