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's scan. In effect, this
duplicates the database table within itself (subject to unique index
rules, of course) without recursing.
Changes made by query Q are visible to 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.