The initial default transaction isolation level for any session is SERIALIZABLE. That is, READ is the default read-only locking severity for any session. This locking severity applies to all SELECT operations, regardless of context.
SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL permits you to set the default read-only locking severity for SELECT operations within DELETE, INSERT, or UPDATE requests for the current session to ACCESS or to set it back to READ after it has been set to ACCESS.
Therefore, changing the session-level isolation level to READ UNCOMMITTED affects the default read-only locking severity for any SELECT requests embedded within DELETE, INSERT, MERGE, or UPDATE requests, making the default lock severity ACCESS, but only when the system setting is set TRUE. When the system setting is set FALSE, the default read-only locking severity for those SELECT operations is READ.
You must specify READ UNCOMMITTED for Vantage to downgrade the default locking severity for embedded SELECT requests within data manipulating requests from READ to ACCESS for the session. In this context, a data manipulating request is any DELETE, INSERT, MERGE, or UPDATE request.
| Isolation Level | Result |
|---|---|
| READ UNCOMMITTED RU |
Lock severity of embedded SELECT requests is not downgraded from READ to ACCESS. |
| SERIALIZABLE SR |
The Optimizer implicitly specifies the locking level (either row hash, view, or table) for any read-only operation, whether the default session isolation level is SERIALIZABLE or READ UNCOMMITTED. Isolation levels affect locking severities, not locking levels.
You can only specify locking levels explicitly using the LOCKING request modifier.