WebSphere eXtreme Scale Best Practices for Operation and Management

Chapter 6. Performance planning for application developers 135

򐂰 All composite indexes must have the RangeIndex property set to false.

򐂰 All association (relationship) indexes must have the RangeIndex property set to false.

An alternative to indexes with much better scalability and performance (at the price of having

custom code) is the reverse index pattern, which is described in 5.4.4, “Indexing plug-ins” on

page 107.

6.5.3 Using pagination

In client-server environments, the query engine transports the entire result map to the client.

The data that is returned must be divided into reasonable chunks. The EntityManager Query

and ObjectMap ObjectQuery interfaces both support the setFirstResult and setMaxResults

methods that allow the query to return a subset of the results.

6.5.4 Returning primitive values instead of entities

With the EntityManager Query API, entities are returned as query parameters. The query

engine currently returns the keys for these entities to the client. When the client iterates over

these entities using the Iterator from the getResultIterator method, each entity is automatically

inflated and managed as though it were created with the find method on the EntityManager

built from the entity ObjectMap on the client. The entity value attributes and any related

entities are eagerly resolved.

To avoid building the costly graph, modify the query to return the individual attributes with path

navigation, for example:

// Returns an entity

SELECT u FROM User u

// Returns attributes

SELECT u.userId, u.firstName, u.lastName, FROM User u

6.5.5 Query plan

All WebSphere eXtreme Scale queries have a query plan. The plan describes how the query

engine interacts with object maps and indexes. Display the query plan to determine if the

query string or indexes are being used appropriately. You can also use the query plan to

explore the differences that subtle changes in a query string make in the way that WebSphere

eXtreme Scale runs a query.

The query plan can be viewed in one of two ways:

򐂰 EntityManager Query or ObjectQuery getPlan API method

򐂰 ObjectGrid diagnostic trace

6.5.6 getPlan method

The getPlan method on the ObjectQuery and Query interfaces returns a string that describes

the query plan. This string can be displayed to standard output or a log to display a query

plan.

Distributed environment note: In a distributed environment, the getPlan method does not

run against the server and does not reflect any defined indexes. To view the plan, use an

agent to view the plan on the server.