SELECT all the rows where id is children of other node.

Hello,

I have a huge table (100 million rows) of relations between nodes by id in
a Postgresql 11 server. Like this:

CREATE TABLE relations (
pid INTEGER NOT NULL,
cid INTEGER NOT NULL,
)

This table has parent-child relations references between nodes by id. Like:

*pid -> cid*
n1 -> n2
n1 -> n3
n1 -> n4
n2 -> n21
n2 -> n22
n2 -> n23
n22 -> n221
n22 -> n222

I would like to get a list of all the nodes being children (direct or
indirect) of any other node.

Example. The children of:

1) n3: [] (n3 has not children)
2) n22: [n221, n222] (n22 has 2 children: n221 and n222)
3) n1: [n2, n21, n22, n23, n221, n222] (n1 has 6 children including
indirect children).

this pseudo SQL:

SELECT *
FROM relations
WHERE has_parent(myId)

It can be solved with a recursive function or stored procedure. But that
requires several passes. Is it possible to solve it in one pass? Perhaps
using some low-level function or join or some index expression or auxiliary
columns?

It is OK to create an index or similar using recursive expressions.
However, the SELECT expressions should be solved in one pass because of
speed.

Pablo