Usage Notes - Teradata Database

Locks and Concurrency

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