Hello Everyone, I'm looking at an easy real-time application using PostgreSQL. Looking at the pg docs, I see lots of discussion about cases where MVCC may reflect different versions of the data to different connections. For example: http://www.postgresql.org/docs/8.1/static/mvcc.html "while querying a database each transaction sees a snapshot of data (a database version) as it was some time ago, regardless of the current state of the underlying data" "Read Committed is the default isolation level ... a SELECT query sees only data committed before the query began; it never sees either uncommitted data or changes committed during query execution by concurrent transactions. (However, the SELECT does see the effects of previous updates executed within its own transaction, even though they are not yet committed.) In effect, a SELECT query sees a snapshot of the database as of the instant that that query begins to run." However, I don't actually see any statements giving guarantees about when the updated data _does_ become visible. The central question: So if I successfully commit an update transaction on one connection, then instantaneously issue a select on another previously-opened connection, under what circumstances am I guaranteed that the select will see the effects of the update? The db is in the default read committed mode. The select is being done on another long-running connection which has never done any updates, just selects within its implicit transaction. Maybe this is the statement I'm looking for: "in Read Committed mode each new command starts with a new snapshot that includes all transactions committed up to that instant, subsequent commands in the same transaction will see the effects of the committed concurrent transaction". But this statement is just an aside when making a different point, and I see other statements like "So the whole concept of "now" is somewhat ill-defined anyway." "This is not normally a big problem if the client applications are isolated from each other, but if the clients can communicate via channels outside the database then serious confusion may ensue." And communication via outside channels is exactly what the app is doing. Thanks, - Marsh -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
