Avoid allocation by getFunctions in SelectStatement.authorize

Memorize the result of getFunctions in SelectStatement constructor

patch by Dmitry Konstantinov; reviewed by Francisco Guerrero for CASSANDRA-21347

Author: netudima

CHANGES.txt

@@ -1,4 +1,5 @@
 6.0-alpha2
+ * Avoid allocation by getFunctions in SelectStatement.authorize (CASSANDRA-21347)
  * Avoid unit conversion in DatabaseDescriptor.getMaxValueSize() for every deserializing Cell (CASSANDRA-21295)
  * Fix single token batch atomicity with Accord/non-Accord batches by using the batch log (CASSANDRA-20588)
  * Avoid CompactionOptions parsing for every read by WithoutPurgeableTombstones (CASSANDRA-21294)

src/java/org/apache/cassandra/cql3/statements/SelectStatement.java

@@ -194,6 +194,8 @@ public class SelectStatement implements CQLStatement.SingleKeyspaceCqlStatement,
      */
     private final ColumnComparator<List<ByteBuffer>> orderingComparator;
 
+    private final List<Function> functions;
+
     public final StatementSource source;
 
     // Used by forSelection below
@@ -228,6 +230,7 @@ public SelectStatement(TableMetadata table,
         this.perPartitionLimit = perPartitionLimit;
         this.source = source;
         this.selectOptions = selectOptions;
+        this.functions = findAllFunctions();
     }
 
     @Override
@@ -244,9 +247,19 @@ public short[] getPartitionKeyBindVariableIndexes()
 
     @Override
     public Iterable<Function> getFunctions()
+    {
+        return functions;
+    }
+
+    private List<Function> findAllFunctions()
     {
         List<Function> functions = new ArrayList<>();
         addFunctionsTo(functions);
+        if (functions.isEmpty())
+        {
+            functions = Collections.emptyList(); // to avoid a new Iterator object creation during each authorization
+        }
+
         return functions;
     }