Serial Query Execution

The optimizer does not process the following types of queries or statements in parallel:

• Queries started with an isolation mode of Cursor Stability

  Subsequent changes to the isolation mode do not affect the parallelism of queries already prepared because parallel scans scan several rows simultaneously.

• An UPDATE statement that has an update trigger in which the For Each Row section performs the updates
• Execution of data definition language (DDL) statements, such as CREATE DATABASE, CREATE TRIGGER, CREATE VIEW, and some ALTER TABLE statements

  For a complete list of DDL statements and information about ALTER TABLE statements, see the IBM Informix: Guide to SQL Syntax.

In addition, you can tune the database server or specific sessions to prefer serial plans for transactions, as described in Ensure Quick Response for Transactions.