PostgreSQL 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 a;
tuples 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 that 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!