An UPDATE operation sets a WRITE lock for the table, partitions, or row being updated.
The lock set for SELECT subquery operations depends on the isolation level for the session and whether the subquery is embedded within a SELECT operation or within an UPDATE request.
| Transaction Isolation Level | Default Locking Severity for Outer SELECT and Ordinary SELECT Subquery Operations | Default Locking Severity for SELECT Operations Embedded Within an UPDATE Request |
|---|---|---|
| SERIALIZABLE | READ | READ |
| READ | ||
| READ UNCOMMITTED | READ | |
| READ UNCOMMITTED | ACCESS |
For more information, see SET SESSION TRANSACTION ISOLATION LEVEL.