select for update

select for update exclusively locks rows in datarows-locked tables for subsequent updates within the same transaction, and for updatable cursors.

functionality is automatically available to clients when the for update clause is added to a select statement and to any updatable cursors opened within the clients. select for update is supported at isolation levels 1, 2, and 3.

select for update can be issued as a language statement outside of a cursor context. With both language statements and cursors, you must execute select for update within a begin transaction command or in chained mode.

If you run select for update within a cursor context, the cursor open and fetch statements must be within the context of a transaction.

The synax is:

select <col-list> from … where …
	[for update[ of col-list ]

• Use select for update in Cursors and DML
  select for update behavior is based on the value of the configuration parameter select for update.
• Concurrency Issues
  If a session has an open transaction that is executing select for update at isolation levels 1, 2, or 3, a second concurrent session issuing data manipulation language (DML) statements on the same table may be blocked, depending on the type of transaction issued by the second session and its isolation level.