Remove duplicates from array_agg, where elements are also arrays - sql

Question

In a Postgres 11 database, I've got two arrays in two views which are joined to MAIN table:
vw_ideas_role_direction_mappings.user_direction_id - array (like {101,103,} or {101,103} or {101,} or {101,,,,104})
vw_ideas_role_category_mappings.user_direction_id - array like previous.
DDL of view vw_ideas_role_category_mappings:
category_id - int8
user_direction_id - array
-- no constraints
DDL of view vw_ideas_role_direction_mappings:
direction_id - int8
user_direction_id - array
-- no constraints
DDL table idea:
id - bigserial
-- no constraints
And the following query, where I join everything:
SELECT i.id,
array_agg(dvircm.user_direction_id || dvirdm.user_direction_id) AS directions_id
FROM idea.ideas i
LEFT JOIN vw_ideas_role_direction_mappings = i.direction_id
LEFT JOIN vw_ideas_role_category_mappings dvircm ON dvircm.category_id = i.category_id
GROUP BY i.id
So there can be NULL in arrays and duplicates.
This query does not remove them and further more it throws error - cannot accumulate arrays of different dimensionality (maybe because there are commas ' , ' in arrays before or after digits? Because when I create array user_direction_id I use this query
array_agg(distinct vw_user_data_all_roles.direction_id))
How to get rid of the error and remove duplicates and null after combining 2 arrays?

Answer 1

I think this would do what you want:
SELECT i.id, sub.directions_id
FROM idea.ideas i
LEFT JOIN LATERAL (
SELECT ARRAY (
SELECT u.id
FROM vw_ideas_role_direction_mappings d, unnest(d.user_direction_id) u(id)
WHERE d.direction_id = i.direction_id
AND u.id IS NOT NULL
UNION
SELECT u.id
FROM vw_ideas_role_category_mappings c, unnest(c.user_direction_id) u(id)
WHERE c.category_id = i.category_id
AND u.id IS NOT NULL
)
) sub(directions_id) ON sub.directions_id <> '{}'; -- exclude empty array?
UNION after unnesting removes duplicate array elements.
NULL values are removed.
About the ARRAY constructor:
Why is array_agg() slower than the non-aggregate ARRAY() constructor?
Since the ARRAY constructor always returns a row (like an aggregate function), we can use CROSS JOIN. Else, we'd use LEFT JOIN .. ON true. About the LATERAL join:
What is the difference between a LATERAL JOIN and a subquery in PostgreSQL?
If you need to preserve some original order, consider WITH ORDINALITY and ORDER BY ... See:
PostgreSQL unnest() with element number