2011/3/9 Vlad Arkhipov <arhi...@dc.baikal.ru>: > Let there are two transactions that were created with read commited > isolation level. In the first one we're executing a SELECT query: > SELECT * FROM t UNION ALL SELECT * FROM t; > > In the second transaction we're modifying the same table: > INSERT INTO t DEFAULT VALUES; > COMMIT; > > Is it possible that the last UNION part in the first query will retrieve not > the same rows as the first one?
No, because statements never see changes made by other transactions while they are in flight. > Another scenario is where we're executing two SELECT queries in a stored > procedure: > BEGIN > ... > SELECT * FROM t; > SELECT * FROM t; > END; > > Is it possible to get different results in the second query? Yes, because they are separate statements, and in READ COMMITTED mode, a new snapshot is taken when a statement starts. See: <URL:http://www.postgresql.org/docs/9.0/static/transaction-iso.html#XACT-READ-COMMITTED>. > Does SQL standard define the behaviour in such cases? The first one certainly. The standard doesn't describe PL/PgSQL, so the question is moot in the second case; nonetheless, I assume that the answer would be yes in the case of SQL/PSM. Note that the standard defines things that must never happen in the case of READ COMMITTED, it does not specify that one *must* be able to see the stuff as committed by previous transactions, for example. Nicolas -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers