The optimization for COUNT(..) on columns that are not nullable was also used…

The optimization for COUNT(..) on columns that are not nullable was also used for COUNT(DISTINCT ..), which is incorrect.

h2/src/main/org/h2/expression/Aggregate.java

@@ -548,7 +548,10 @@ public class Aggregate extends Expression {
         if (visitor.getType() == ExpressionVisitor.OPTIMIZABLE_MIN_MAX_COUNT_ALL) {
             switch (type) {
             case COUNT:
-                return on.getNullable() == Column.NOT_NULLABLE && visitor.getTable().canGetRowCount();
+                if (!distinct && on.getNullable() == Column.NOT_NULLABLE) {
+                    return visitor.getTable().canGetRowCount();
+                }
+                return false;
             case COUNT_ALL:
                 return visitor.getTable().canGetRowCount();
             case MIN:

h2/src/test/org/h2/test/db/TestCases.java

@@ -37,6 +37,7 @@ public class TestCases extends TestBase {
     }
 
     public void test() throws Exception {
+        testCountDistinctNotNull();
         testDependencies();
         testConvertType();
         testSortedSelect();
@@ -93,6 +94,17 @@ public class TestCases extends TestBase {
         deleteDb("cases");
     }
 
+    private void testCountDistinctNotNull() throws SQLException {
+        deleteDb("cases");
+        Connection conn = getConnection("cases");
+        Statement stat = conn.createStatement();
+        stat.execute("create table test(id int not null) as select 1 from system_range(1, 10)");
+        ResultSet rs = stat.executeQuery("select count(distinct id) from test");
+        rs.next();
+        assertEquals(1, rs.getInt(1));
+        conn.close();
+    }
+
     private void testDependencies() throws SQLException {
         deleteDb("cases");
         Connection conn = getConnection("cases");