An UPDATE operation sets a WRITE lock for the table, partitions, or row being updated. For a nonconcurrent isolated update on load isolated table, the update operation sets an EXCLUSIVE lock.
The lock set for SELECT subquery operations depends on the isolation level for the session, the setting of the AccessLockForUncomRead DBS Control field, and whether the subquery is embedded within a SELECT operation or within an UPDATE request.
| Transaction Isolation Level | DBS Control AccessLockForUncomRead Field Setting | Default Locking Severity for Outer SELECT and Ordinary SELECT Subquery Operations | Default Locking Severity for SELECT Operations Embedded Within an UPDATE Request |
|---|---|---|---|
| SERIALIZABLE | FALSE | READ | READ |
| TRUE | READ | ||
| READ UNCOMMITTED | FALSE | READ | |
| TRUE | ACCESS |
For more information, see:
- “SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL” in Teradata Vantage™ SQL Data Definition Language Detailed Topics , B035-1184
- Teradata Vantage™ SQL Request and Transaction Processing, B035-1142
- Teradata Vantage™ - Database Utilities , B035-1102