Add a NEWS.txt entry about LIMIT behavior change and unit tests

patch by Benjamin Lerer; reviewed by Sylvain Lebresne for CASSANDRA-10380

NEWS.txt

@@ -44,6 +44,9 @@ New features
 
 Upgrading
 ---------
+   - The LIMIT clause applies now only to the number of rows returned to the user,
+     not to the number of row queried. By consequence, queries using aggregates will not
+     be impacted by the LIMIT clause anymore.
    - The native protocol versions 1 and 2 are not supported anymore.
    - Max mutation size is now configurable via max_mutation_size_in_kb setting in
      cassandra.yaml; the default is half the size commitlog_segment_size_in_mb * 1024.

test/unit/org/apache/cassandra/cql3/validation/operations/AggregationTest.java

@@ -91,6 +91,8 @@ public class AggregationTest extends CQLTester
         assertRows(execute("SELECT COUNT(*) FROM %s"), row(4L));
         assertRows(execute("SELECT COUNT(1) FROM %s"), row(4L));
         assertRows(execute("SELECT COUNT(b), count(c), count(e), count(f) FROM %s"), row(4L, 3L, 3L, 3L));
+        // Makes sure that LIMIT does not affect the result of aggregates
+        assertRows(execute("SELECT COUNT(b), count(c), count(e), count(f) FROM %s LIMIT 2"), row(4L, 3L, 3L, 3L));
     }
 
     @Test
@@ -130,6 +132,8 @@ public class AggregationTest extends CQLTester
         assertRows(execute("SELECT COUNT(1) FROM %s"), row(4L));
         assertRows(execute("SELECT max(b), b, COUNT(*) FROM %s"), row(5, 1, 4L));
         assertRows(execute("SELECT max(b), COUNT(1), b FROM %s"), row(5, 4L, 1));
+        // Makes sure that LIMIT does not affect the result of aggregates
+        assertRows(execute("SELECT max(b), COUNT(1), b FROM %s LIMIT 2"), row(5, 4L, 1));
     }
 
     @Test