Convert right side of join of many to many into array

To aggregate most rows

While querying all or most items, it is typically substantially faster to aggregate rows from the "many"-table first and join later:

SELECT id, i.title AS item_title, t.tag_array
FROM   items      i
JOIN  (  -- or LEFT JOIN ?
   SELECT it.item_id AS id, array_agg(t.title) AS tag_array
   FROM   items_tags it
   JOIN   tags       t  ON t.id = it.tag_id
   GROUP  BY it.item_id
   ) t USING (id);

Use LEFT [OUTER] JOIN in the outer query if there can be items without tags - which would be excluded with [INNER] JOIN.

Since that does not multiply rows in the join, we need no GROUP BY in the outer SELECT.

Joining before aggregation also gets out of hands with more than one 1:n table in the FROM list (not in this simple case). See:

• Two SQL LEFT JOINS produce incorrect result

To aggregate few rows

For a small percentage of rows, use a LATERAL join with an ARRAY constructor:

SELECT id, title AS item_title, t.tag_array
FROM   items i, LATERAL (  -- this is an implicit CROSS JOIN
   SELECT ARRAY (
      SELECT t.title
      FROM   items_tags it
      JOIN   tags       t  ON t.id = it.tag_id
      WHERE  it.item_id = i.id
      ) AS tag_array
   ) t;

Since an ARRAY constructor always produces a row (with empty array if the subquery is empty - subtle difference in the result!), LEFT JOIN LATERAL (...) ON true is not needed here. See:

• What is the difference between LATERAL and a subquery in PostgreSQL?
• Why is array_agg() slower than the non-aggregate ARRAY() constructor?
• Understanding multiple table join with aggregation

Aside

You had a typo in your query. 3rd column would be t.title. I added aliases to your original (un-aggregated) query to clarify:

SELECT i.id, i.title AS item_title, t.title AS tag_title
FROM   items      i
JOIN   items_tags it ON it.item_id = i.id
JOIN   tags       t  ON t.id = it.tag_id;

"id" or "title" are typically not very distinctive and not very useful identifiers. See:

• How to implement a many-to-many relationship in PostgreSQL?

You need to add the group by clause and use array_agg.

SELECT i.id, i.title, array_agg(i.title)
FROM items i
INNER JOIN items_tags it
ON it.item_id = i.id
INNER JOIN tags t
ON t.id = it.tag_id
GROUP BY i.id, i.title,