I have 2 tables – course that contains id and name of the courses and tagCourse that contains tags for each course.
course tagcourse
------------ ----------------
PK id_course PK tag
name PK, FK id_course
I’d like to write a function that searches courses by given array of tags and returns them ordered by quantity of matching tags. However I don’t know how to write it correctly and in an efficient way. Please help me.
ie.
CREATE OR REPLACE FUNCTION searchByTags(tags varchar[])
RETURNS SETOF ..
RETURN QUERY SELECT * FROM course c INNER JOIN tagcourse tc ON c.id_course = tc.id_course
WHERE ??? ORDER BY ???
END .
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Answer
CREATE OR REPLACE FUNCTION search_by_tags(tags varchar[])
RETURNS TABLE (id_course integer, name text, tag_ct integer)
LANGUAGE sql AS
$func$
SELECT id_course, c.name, ct.tag_ct
FROM (
SELECT tc.id_course, count(*)::int AS tag_ct
FROM unnest($1) x(tag)
JOIN tagcourse tc USING (tag)
GROUP BY 1 -- first aggregate ..
) AS ct
JOIN course c USING (id_course) -- .. then join
ORDER BY ct.tag_ct DESC -- more columns to break ties?
$func$;
Use unnest()
to produce a table from your input array, like already demonstrated by @Clodoaldo.
You don’t need plpgsql for this. Simpler with a plain SQL function.
I use unnest($1)
(with positional parameter) instead of unnest(tags)
, since the later is only valid for PostgreSQL 9.2+ in SQL functions (unlike plpgsql). The manual:
In the older numeric approach, arguments are referenced using the syntax
$n
:$1
refers to the first input argument,$2
to the second, and so on. This will work whether or not the particular argument was declared with a name.
count()
returns bigint
. You need to cast it to int
to match the declared return type or declare the the returned column as bigint
to begin with.
Perfect occasion to simplify the syntax a bit with USING
(equi-joins): USING (tag)
instead of ON tc.tag = c.tag
.
It’s regularly faster to first aggregate, then join to another table. Reduces the needed join operations.
To address @Clodoaldo’s comments, here is a fiddle demonstrating the difference:
OTOH, if you aggregate after the join, you don’t need a subquery. Shorter, but probably slower:
SELECT c.id_course, c.name, count(*)::int AS tag_ct
FROM unnest($1) x(tag)
JOIN tagcourse tc USING (tag)
JOIN course c USING (id_course)
GROUP BY 1
ORDER BY 3 DESC; -- more columns to break ties?