Question

I have this query for an application that I am designing. There is a table of references, an authors table and a reference_authors table. There is a sub query to return all authors for a given reference which I then display formatted in php. The subquery and query run individually are both nice and speedy. However as soon as the subquery is put into the main query the whole thing takes over 120s to run. I would apprecaite some fresh eyes on this one.
Thanks.

SELECT
rf.reference_id,
rf.reference_type_id,
rf.article_title,
rf.publication,
rf.annotation,
rf.publication_year,
(SELECT GROUP_CONCAT(a.author_name)
FROM authors_final AS a
INNER JOIN reference_authors AS ra2 ON ra2.author_id = a.author_id
WHERE ra2.reference_id = rf.reference_id
GROUP BY ra2.reference_id) AS authors
FROM
references_final AS rf
INNER JOIN reference_authors AS ra ON rf.reference_id = ra.reference_id
LEFT JOIN reference_institutes AS ri ON rf.reference_id = ri.reference_id;

Here is the fixed query. Thanks guys for the recommendations.

SELECT
rf.reference_id,
rf.reference_type_id,
rf.article_title,
rf.publication,
rf.annotation,
rf.publication_year,
GROUP_CONCAT(a.author_name) AS authors
FROM
references_final as rf
INNER JOIN (reference_authors AS ra INNER JOIN authors_final AS a ON ra.author_id = a.author_id)
ON rf.reference_id = ra.reference_id
LEFT JOIN reference_institutes AS ri ON rf.reference_id = ri.reference_id
GROUP BY rf.reference_id

Answer 1

Although not every subquery can be rewritten as an inner join, I think yours can.

From 120 seconds to 78 milliseconds is not a bad improvement–about three orders of magnitude. Take the rest of the day off.

When you come back tomorrow, start looking for other subqueries in your source code.