PostgreSQL Programmer's Guide
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# Visibility of Data Changes

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 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.

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.

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