Query Dispatcher

 

The function of the Dispatcher is to route the query request to either CQE or SQE, depending on the attributes of the query. All queries are processed by the Dispatcher and you cannot bypass it.

Currently, the Dispatcher will route an SQL statement to CQE if it find that the statement references or contains any of the following:

• INSERT WITH VALUES statement or the target of an INSERT with subselect statement

• NLSS or CCSID translation between columns

• Lateral correlation

• Logical files

• Datalink columns

• Tables with Read Triggers

• User-defined table functions

• Read-only queries with more than 1000 dataspaces or updateable queries with more than 256 dataspaces.

• DB2^® Multisystem tables

• non-SQL queries, for example the QQQQry API, Query/400, or OPNQRYF

The Dispatcher also has the built-in capability to re-route an SQL query to CQE that was initially routed to SQE. Unless the IGNORE_DERIVED_INDEX option with a parameter value of *YES is specified, a query will typically be reverted back to CQE from SQE whenever the Optimizer processes table objects that have any of the following logical files or indexes defined:

• Logical files with the SELECT/OMIT DDS keyword specified

• Non-standard indexes or derived keys, for example logical files specifying the DDS keywords RENAME or Alternate Collating Sequence (ACS) on any field referenced in the key

• Sort Sequence NLSS specified for the index or logical file

As new functionality is added in the future, the Dispatcher will route more queries to SQE and increasingly fewer to CQE.

 

Parent topic:

Query Engine Overview

 

Related reference

MQT supported function