Concatenate/merge array values during grouping/aggregation

Question

I have a table with the an array column type:

 title       tags
\"ridealong\";\"{comedy,other}\"
\"ridealong\";\"{comedy,tragedy}\"
\"freddyjason\";\"{horror         


        

Answer 1

Custom aggregate

Approach 1: define a custom aggregate. Here's one I wrote earlier.

CREATE TABLE my_test(title text, tags text[]);

INSERT INTO my_test(title, tags) VALUES
('ridealong', '{comedy,other}'),
('ridealong', '{comedy,tragedy}'),
('freddyjason', '{horror,silliness}');

CREATE AGGREGATE array_cat_agg(anyarray) (
  SFUNC=array_cat,
  STYPE=anyarray
);

select title, array_cat_agg(tags) from my_test group by title;

LATERAL query

... or since you don't want to preserve order and want to deduplicate, you could use a LATERAL query like:

SELECT title, array_agg(DISTINCT tag ORDER BY tag) 
FROM my_test, unnest(tags) tag 
GROUP BY title;

in which case you don't need the custom aggregate. This one is probably a fair bit slower for big data sets due to the deduplication. Removing the ORDER BY if not required may help, though.

Answer 2

The obvious solution would be the LATERAL join (which also suggested by @CraigRinger), but that is added to PostgreSQL in 9.3.

In 9.1 you cannot avoid the sub-query, but you can simplify it:

SELECT title, array_agg(DISTINCT tag)
FROM (SELECT title, unnest(tags) FROM my_test) AS t(title, tag)
GROUP BY title;

SQL Fiddle