IBM® Informix® 12.10
Database server operations that use PDQ
Informix® processes some types of SQL operations that the database server processes in parallel. However some situations limit the degree of parallelism that Informix can use.
In the topics on database server operations that use PDQ in this section, a query is any SELECT statement.
- Parallel update and delete operations
Informix performs some types of update and delete operations in parallel. - Parallel insert operations
Informix performs some types of insert operations in parallel. - Parallel index builds
Index builds can take advantage of PDQ and can be parallelized. The database server performs both scans and sorts in parallel for index builds. - Parallel user-defined routines
If a query contains a user-defined routine (UDR) in an expression, the database server can execute a query in parallel when you turn on PDQ. - Hold cursors that use PDQ
When hold cursors that are created by declaring the WITH HOLD qualifier have no locks, PDQ is enabled. - SQL operations that do not use PDQ
The database server does not process some types of queries in parallel. - Update statistics operations affected by PDQ
An SQL UPDATE STATISTICS statement that is not processed in parallel, is affected by PDQ because it must allocate the memory used for sorting. Thus the behavior of the UPDATE STATISTICS statement is affected by the memory management associated with PDQ. - SPL routines and triggers and PDQ
Statements that involve SPL routines are not executed in parallel. However, statements within procedures are executed in parallel. - Correlated and uncorrelated subqueries
The database server does not use PDQ to process correlated subqueries. Only one thread at a time can execute a correlated subquery. While one thread executes a correlated subquery, other threads that request to execute the subquery are blocked until the first one completes. - OUTER index joins and PDQ
The database server reduces the PDQ priority of queries that contain OUTER index joins to LOW (if the priority is set to a higher value) for the duration of the query. If a subquery or a view contains OUTER index joins, the database server lowers the PDQ priority of only that subquery or view, not of the parent query or any other subquery. - Remote tables used with PDQ
Although the database server can process the data stored in a remote table in parallel, the data is communicated serially because the database server allocates a single thread to submit and receive the data from the remote table. The database server lowers the PDQ priority of queries that require access to a remote database to LOW.
Parent topic: Parallel database query (PDQ)
Related concepts:
To find the PDF, see Publications for the IBM Informix 12.10 family of products.
For the release notes, documentation notes, and/or machine notes, see the Release Notes page.
For the release notes, documentation notes, and/or machine notes, see the Release Notes page.