title: Solr sort by the value of function query does not work
body: <p>I'm working on a feature which relies on sorting the value of function query. </p>
  
  <p>Function query is as below : </p>
  
  <pre><code>http://localhost:8983/solr/collection/select?q=*:*&amp;sort=if(exists(f1_t), f1_t, theme)+ASC&amp;wt=json&amp;indent=true
  </code></pre>
  
  <p>The sort function is "<code>if(exists(f1_t), f1_t, theme) ASC</code>"</p>
  
  <p>So basically what I'm doing is if the field "<code>f1_t</code>" (which is a dynamic field of type TextField) exists then the sorting is based on the value of the field else the sorting is based on the value of "theme" which is also a TextField (but it is not a dynamic field)</p>
  
  <p>But when I run this it fails with the below exception.
  I tried the "if" function with integer values and it works fine.
  I was just wondering if you could help me figure out why the above function query fails? How can I get this working?</p>
  
  <p>Solr version I'm using is 4.6.1</p>
  
  <pre><code>"trace": "java.lang.UnsupportedOperationException\n\tat
  org.apache.lucene.queries.function.FunctionValues.doubleVal(FunctionValues.java:47)\n\tat 
  org.apache.lucene.queries.function.valuesource.IfFunction$1.doubleVal(IfFunction.java:83)\n\tat 
  org.apache.lucene.queries.function.ValueSource$ValueSourceComparator.copy(ValueSource.java:152)\n\tat 
  org.apache.lucene.search.TopFieldCollector$OneComparatorNonScoringCollector.collect(TopFieldCollector.java:86)\n\tat 
  org.apache.lucene.search.Scorer.score(Scorer.java:65)\n\tat 
  org.apache.lucene.search.IndexSearcher.search(IndexSearcher.java:621)\n\tat 
  org.apache.lucene.search.IndexSearcher.search(IndexSearcher.java:297)\n\tat 
  org.apache.solr.search.SolrIndexSearcher.getDocListNC(SolrIndexSearcher.java:1529)\n\tat 
  org.apache.solr.search.SolrIndexSearcher.getDocListC(SolrIndexSearcher.java:1395)\n\tat 
  org.apache.solr.search.SolrIndexSearcher.search(SolrIndexSearcher.java:474)\n\tat 
  org.apache.solr.handler.component.QueryComponent.process(QueryComponent.java:438)\n\tat 
  org.apache.solr.handler.component.SearchHandler.handleRequestBody(SearchHandler.java:208)\n\tat 
  org.apache.solr.handler.RequestHandlerBase.handleRequest(RequestHandlerBase.java:135)\n\tat org.apache.solr.core.SolrCore.execute(SolrCore.java:1859)\n\tat 
  org.apache.solr.servlet.SolrDispatchFilter.execute(SolrDispatchFilter.java:723)\n\tat 
  org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:419)\n\tat 
  org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:203)\n\tat 
  org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1419)\n\tat 
  org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:455)\n\tat org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:137)\n\tat 
  org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:557)\n\tat 
  org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:231)\n\tat 
  org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1075)\n\tat 
  org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:384)\n\tat 
  org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:193)\n\tat 
  org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1009)\n\tat 
  org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:135)\n\tat 
  org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:255)\n\tat 
  org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:154)\n\tat 
  org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:116)\n\tat org.eclipse.jetty.server.Server.handle(Server.java:368)\n\tat 
  org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:489)\n\tat 
  org.eclipse.jetty.server.BlockingHttpConnection.handleRequest(BlockingHttpConnection.java:53)\n\tat 
  org.eclipse.jetty.server.AbstractHttpConnection.headerComplete(AbstractHttpConnection.java:942)\n\tat 
  org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.headerComplete(AbstractHttpConnection.java:1004)\n\tat 
  org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:640)\n\tat 
  org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:235)\n\tat 
  org.eclipse.jetty.server.BlockingHttpConnection.handle(BlockingHttpConnection.java:72)\n\tat 
  org.eclipse.jetty.server.bio.SocketConnector$ConnectorEndPoint.run(SocketConnector.java:264)\n\tat 
  org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:608)\n\tat 
  org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:543)\n\tat java.lang.Thread.run(Thread.java:662)\n",
      "code": 500 
  </code></pre>