Calculate table column using over table on server side - postgresql

Question

Suppose, there are two tables in db:
Table registries:
Column | Type |
--------------------+-----------------------------+---------
registry_id | integer | not null
name | character varying | not null
...
uploaded_at | timestamp without time zone | not null
Table rows:
Column | Type | Modifiers
---------------+-----------------------------+-----------
row_id | character varying | not null
registry_id | integer | not null
row | character varying | not null
In real world registries is just a csv-file and rows is lines of the files. In my scala-slick application, I want to know how many lines in each file.
registries:
1,foo,...
2,bar,...
3,baz,...
rows:
aaa,1,...
bbb,1,...
ccc,2,...
desired result:
1,foo,... - 2
2,bar,... - 1
3,baz,... - 0
My code now is (slick-3.0):
def getRegistryWithLength(rId: Int) = {
val q1 = registries.filter(_.registryId===rId).take(1).result.headOption
val q2 = rows.filter(_.registryId===rId).length.result
val registry = Await.result(db.run(q1), 5.seconds)
val length = Await.result(db.run(q2), 5.seconds)
(registry, length)
}
(Await is bad idea, I know it)
How can I do getRegistryWithLength using single sql query?
I could add column row_n into table registries, but then I'll be forced to do updating column row_n after delete/insert query of rows table.
How can I do automatic calculation column row_n in table registries on db server side?

Answer 1

The basic query could be:
SELECT r.*, COALESCE(n.ct, 0) AS ct
FROM registry r
LEFT JOIN (
SELECT registry_id, count(*) AS ct
FROM rows
GROUP BY registry_id
) n USING (registry_id);
The LEFT [OUTER] JOIN is essential so you do not filter rows from registry without related rows in rows.
COALESCE to return 0 instead of NULL where no related rows are found.
There are many related answers on SO. One here:
SQL: How to save order in sql query?
You could wrap this in a VIEW for convenience:
CREATE VIEW reg_rn AS
SELECT ...
... which you query like a table.
Aside: It's unwise to use reserved SQL key words as identifiers. row is a no-go for a column name (even if allowed in Postgres).

Answer 2

Thanks Erwin Brandstetter for awesome answer, using it, I wrote code for my scala-slick application.
Scala code looks much more complicated than plain sql:
val registryQuery = registries.filter(_.userId === userId)
val rowQuery = rows groupBy(_.registryId) map { case (regId, rowItems) => (regId, rowItems.length)}
val q = registryQuery joinLeft rowQuery on (_.registryId === _._1) map {
case (registry, rowsCnt) => (registry, rowsCnt.map(_._2))
}
but it works!