SNAPSHOT TABLE STABILITY

The SNAPSHOT TABLE STABILITY — or SNAPSHOT TABLE — isolation level is the most restrictive.As in SNAPSHOT, a transaction in SNAPSHOT TABLE STABILITY isolation sees only those changes that were committed before the current transaction was started.After a SNAPSHOT TABLE STABILITY is started, no other transactions can make any changes to any table in the database that has changes pending for this transaction.Other transactions can read other data, but any attempt at inserting, updating or deleting by a parallel process will cause conflict exceptions.

The RESERVING clause can be used to allow other transactions to change data in some tables.

If any other transaction has an uncommitted change pending in any (non-SHARED) table listed in the RESERVING clause, trying to start a SNAPSHOT TABLE STABILITY transaction will result in an indefinite wait (default or explicit WAIT), or an exception (NO WAIT or after expiration of the LOCK TIMEOUT).

The SNAPSHOT TABLE STABILITY isolation level is also known as “consistency”.

The READ COMMITTED isolation level allows all data changes that other transactions have committed since it started to be seen immediately by the uncommitted current transaction.Uncommitted changes are not visible to a READ COMMITTED transaction.

To retrieve the updated list of rows in the table you are interested in — “refresh” — the SELECT statement needs to be executed again, whilst still in the uncommitted READ COMMITTED transaction.