Locks and types of application processing

For the purpose of determining lock attributes, application processing can be classified as one of the following types: read-only, intent to change, change, and cursor controlled.

• Read-only

  This processing type includes all SELECT statements that are intrinsically read-only, have an explicit FOR READ ONLY clause, or are ambiguous, but the query compiler assumes that they are read-only because of the BLOCKING option value that the PREP or BIND command specifies. This type requires only share locks (IS, NS, or S).

• Intent to change

  This processing type includes all SELECT statements that have a FOR UPDATE clause, a USE AND KEEP UPDATE LOCKS clause, a USE AND KEEP EXCLUSIVE LOCKS clause, or are ambiguous, but the query compiler assumes that change is intended. This type uses share and update locks (S, U, or X for rows; IX, S, U, or X for blocks; and IX, U, or X for tables).

• Change

  This processing type includes UPDATE, INSERT, and DELETE statements, but not UPDATE WHERE CURRENT OF or DELETE WHERE CURRENT OF. This type requires exclusive locks (IX or X).

• Cursor controlled

  This processing type includes UPDATE WHERE CURRENT OF and DELETE WHERE CURRENT OF. This type requires exclusive locks (IX or X).

A statement that inserts, updates, or deletes data in a target table, based on the result from a subselect statement, does two types of processing. The rules for read-only processing determine the locks for the tables that return data in the subselect statement. The rules for change processing determine the locks for the target table.