Hazelcast distributed queries will run on each member in parallel and will return only the results to the caller. Then, on the caller side, the results will be merged.
When a query runs on a
member, Hazelcast will iterate through all the owned entries and find the matching ones. This can be made faster by indexing
the mostly queried fields, just like you would do for your database. Indexing will add overhead for each
operation but queries will be a lot faster. If you query your map a lot, make sure to add indexes for the most frequently
queried fields. For example, if you do an
active and age < 30 query, make sure you add an index for the
age fields. The following example code does that by:
- getting the map from the Hazelcast instance, and
- adding indexes to the map with the IMap
IMap map = hazelcastInstance.getMap( "employees" ); // ordered, since we have ranged queries for this field map.addIndex( "age", true ); // not ordered, because boolean field cannot have range map.addIndex( "active", false );
Indexing Ranged Queries
IMap.addIndex(fieldName, ordered) is used for adding index. For each indexed field, if you have ranged queries such as
age BETWEEN 40 AND 60, then you should set the
ordered parameter to
true. Otherwise, set it to
Configuring IMap Indexes
Also, you can define
IMap indexes in configuration. An example is shown below.
<map name="default"> ... <indexes> <index ordered="false">name</index> <index ordered="true">age</index> </indexes> </map>
You can also define
IMap indexes using programmatic configuration, as in the example below.
mapConfig.addMapIndexConfig( new MapIndexConfig( "name", false ) ); mapConfig.addMapIndexConfig( new MapIndexConfig( "age", true ) );
The following is the Spring declarative configuration for the same sample.
<hz:map name="default"> <hz:indexes> <hz:index attribute="name"/> <hz:index attribute="age" ordered="true"/> </hz:indexes> </hz:map>
NOTE: Non-primitive types to be indexed should implement
Indexing Attributes with ValueExtractor
You can also define custom attributes that may be referenced in predicates, queries and indexes. Custom attributes can be defined by implementing a
ValueExtractor. Please see the Indexing Custom Attributes section for details.