IBM® Informix® 12.10
SQL operations that do not use PDQ
The database server does not process some types of queries in parallel.
For example, the server does not process the following types of
queries in parallel:
- Queries started with an isolation level of Cursor Stability
Subsequent changes to the isolation level do not affect the parallelism of queries already prepared. This situation results from the inherent nature of parallel scans, which scan several rows simultaneously.
- Queries that use a cursor declared as FOR UPDATE
- Queries in the FOR EACH ROW section of the Action clause of a Select trigger
- A DELETE or MERGE statement in the FOR EACH ROW section of the Action clause of a Delete trigger
- An INSERT or MERGE statement in the FOR EACH ROW section of the Action clause of an Insert trigger
- An UPDATE or MERGE statement in the FOR EACH ROW section of the Action clause of an Update trigger
- Data definition language (DDL) statements.
For a complete list of the DDL statements of SQL that Informix® supports, see the IBM® Informix Guide to SQL: Syntax.
In addition, the database server does not process sequence objects in PDQ operations. If your SQL statement includes sequencing operations, such as expressions that include the NEXTVAL or CURRVAL operators, PDQ processing is unavailable for that statement.