Setting this parameter to a number greater than 0 enables caching for result grouping.
Result Grouping executes two searches; this option caches the second search. The server default value is 0.
Testing has shown that group caching only improves search time with Boolean, wildcard, and fuzzy queries. For simple queries like term or "match all" queries, group caching degrades performance.
group.cache.percent parameter
SolrQuery::setGroupLimit() - Specifies the number of results to return for each group. The server default value is 1
SolrQuery::setGroupMain() - If true, the result of the first field grouping command is used as the main result list in the response, using group.format=simple
SolrQuery::setGroupNGroups() - If true, Solr includes the number of groups that have matched the query in the results
SolrQuery::setGroupTruncate() - If true, facet counts are based on the most relevant document of each group matching the query