Postgres data changes visibility rule: during a query execution, data changes made by the query itself (via SQL-function, SPI-function, triggers) are invisible to the query scan. For example, in query
INSERT INTO a SELECT * FROM atuples inserted are invisible for SELECT' scan. In effect, this duplicates the database table within itself (subject to unique index rules, of course) without recursing.
But keep in mind this notice about visibility in the SPI documentation:
Changes made by query Q are visible by queries which are started after query Q, no matter whether they are started inside Q (during the execution of Q) or after Q is done.
This is true for triggers as well so, though a tuple being inserted (tg_trigtuple) is not visible to queries in a BEFORE trigger, this tuple (just inserted) is visible to queries in an AFTER trigger, and to queries in BEFORE/AFTER triggers fired after this!