Minor Description
This ticket provides a pluggable aggregation framework through the introduction of a new Aggregator interface and a new search component called the AggregatorComponent.
The Aggregator interface extends the PostFilter interface providing methods that allow DelegatingCollectors to perform aggregation at collect time. Aggregators were designed to play nicely with the CollapsingQParserPlugin introduced in SOLR-5027.
The AggregatorComponent manages the output and distributed merging of aggregate results.
This ticket is an alternate design to SOLR-4465 which had the same basic idea but a very different implementation. This implementation resolves the caching issues in SOLR-4465 and combined with SOLR-5027 plays nicely with field collapsing. It is also much less intrusive on the core code as it's entirely implemented with plugins.
Initial Syntax for the sample SumQParserPlugin Aggregator:
../select?q=*:*&wt=xml&indent=true&fq={!sum field=popularity id=mysum}&aggregate=true
fq={!sum field=popularity id=mysum} - Calls the SumQParserPlugin telling it to sum the field popularity.
aggregate=true - turns on the AggregatorComponent
The output contains a block that looks like this:
<lst name="aggregates"> <lst name="mysum"> <long name="sum">85</long> </lst> </lst>
