Count of distinct values in faceting.

[Modassar Ather]

Hi,

Count of distinct values can be retrieved by following ways. Please note
that the Solr version is 5.2.1.
1. Using cardinality=true.
2. Using hll() facet function.

Kindly help me understand:
 1. How accurate are them comparatively and better performance wise with
millions of documents?
 2. Per my understanding the {!cardinality=1.0} returns the most accurate
result. Is my understanding correct and if yes is it 100% accurate?
 3. How accurate result is returned by hll() function?
 4. I am getting following exception for the query :
q=field:query&stats=true&stats.field={!cardinality=1.0}field. The exception
is not seen once the cardinality is set to 0.9 or less.
    The field is docVlaues enabled and indexed=false. The same exception I
tried to reproduce on non docVlaues field but could not. Please help me
resolve the issue.
 ERROR - 2015-08-11 12:24:00.222; [core]
org.apache.solr.common.SolrException;
null:java.lang.ArrayIndexOutOfBoundsException: 3
        at
net.agkn.hll.serialization.BigEndianAscendingWordSerializer.writeWord(BigEndianAscendingWordSerializer.java:152)
        at
net.agkn.hll.util.BitVector.getRegisterContents(BitVector.java:247)
        at net.agkn.hll.HLL.toBytes(HLL.java:917)
        at net.agkn.hll.HLL.toBytes(HLL.java:869)
        at
org.apache.solr.handler.component.AbstractStatsValues.getStatsValues(StatsValuesFactory.java:348)
        at
org.apache.solr.handler.component.StatsComponent.convertToResponse(StatsComponent.java:151)
        at
org.apache.solr.handler.component.StatsComponent.process(StatsComponent.java:62)
        at
org.apache.solr.handler.component.SearchHandler.handleRequestBody(SearchHandler.java:255)
        at
org.apache.solr.handler.RequestHandlerBase.handleRequest(RequestHandlerBase.java:143)
        at org.apache.solr.core.SolrCore.execute(SolrCore.java:2064)
        at
org.apache.solr.servlet.HttpSolrCall.execute(HttpSolrCall.java:654)
        at org.apache.solr.servlet.HttpSolrCall.call(HttpSolrCall.java:450)
        at
org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:227)
        at
org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:196)
        at
org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
        at
org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:585)
        at
org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
        at
org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:577)
        at
org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:223)
        at
org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1127)
        at
org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:515)
        at
org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:185)
        at
org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061)
        at
org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
        at
org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:215)
        at
org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:110)
        at
org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)
        at org.eclipse.jetty.server.Server.handle(Server.java:497)
        at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:310)
        at
org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:257)
        at
org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:540)
        at
org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:635)
        at
org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:555)
        at java.lang.Thread.run(Thread.java:745)

Thanks,
Modassar