SOLR-9350: JSON Facet and method=stream: cacheDf threshold now gates use of the filter cache

(cherry picked from commit b63bb51)

solr/CHANGES.txt

@@ -171,6 +171,10 @@ Optimizations
 * SOLR-9335: Solr cache/search/update stats counters now use LongAdder which are supposed to have higher throughput
   under high contention. (Varun Thacker)
 
+* SOLR-9350: JSON Facets: method="stream" will no longer always uses & populates the filter cache, likely
+  flushing it.  'cacheDf' can be configured to set a doc frequency threshold, now defaulting to 1/16th doc count.
+  Using -1 Disables use of the cache. (David Smiley, yonik)
+
 Other Changes
 ----------------------
 

solr/core/src/java/org/apache/solr/search/facet/FacetField.java

@@ -839,9 +839,13 @@ class FacetFieldProcessorStream extends FacetFieldProcessor implements Closeable
     createAccs(-1, 1);
 
     // Minimum term docFreq in order to use the filterCache for that term.
-    int defaultMinDf = Math.max(fcontext.searcher.maxDoc() >> 4, 3);  // (minimum of 3 is for test coverage purposes)
-    int minDfFilterCache = freq.cacheDf == 0 ? defaultMinDf : freq.cacheDf;
-    if (minDfFilterCache == -1) minDfFilterCache = Integer.MAX_VALUE;  // -1 means never cache
+    if (freq.cacheDf == -1) { // -1 means never cache
+      minDfFilterCache = Integer.MAX_VALUE;
+    } else if (freq.cacheDf == 0) { // default; compute as fraction of maxDoc
+      minDfFilterCache = Math.max(fcontext.searcher.maxDoc() >> 4, 3);  // (minimum of 3 is for test coverage purposes)
+    } else {
+      minDfFilterCache = freq.cacheDf;
+    }
 
     docs = fcontext.base;
     fastForRandomSet = null;