Concatenate/merge array values during grouping/aggregation

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.