[CALCITE-6559] Query with measure that applies AVG to SMALLINT throws…

… AssertionError "Cannot add expression of different type to set"

When a query has a measure that is AVG applied to a SMALLINT or TINYINT
column, we get the following assertion failure:

  java.lang.AssertionError: Cannot add expression of different type to set:
  set type is RecordType(VARCHAR(9) JOB, MEASURE<SMALLINT NOT NULL> NOT NULL $f1, SMALLINT $f0) NOT NULL
  expression type is RecordType(VARCHAR(9) JOB, MEASURE<SMALLINT NOT NULL> NOT NULL $f1, SMALLINT NOT NULL $f0) NOT NULL

The problem is that a cast is added internally to the result of
SUM / COUNT, but this cast prevents a cast to nullable for the
null-generating side of the outer join. (The outer join comes from
the expansion of the measure as a correlated scalar subquery.)

Add method RexBuilder.makeNullable, sharing implementation with
existing method makeNotNull.

Add tests to scalar.iq and lateral.iq for some tricky query patterns
that already work.

Close #3950

core/src/main/java/org/apache/calcite/rex/RexBuilder.java

@@ -960,17 +960,33 @@ public RexNode makeReinterpretCast(
   }
 
   /**
-   * Makes a cast of a value to NOT NULL;
-   * no-op if the type already has NOT NULL.
+   * Makes a cast of an expression to nullable;
+   * returns the expression unchanged if its type is already nullable.
+   */
+  public RexNode makeNullable(RexNode exp) {
+    return makeNullable(exp, true);
+  }
+
+  /**
+   * Makes a cast of an expression to NOT NULL;
+   * returns the expression unchanged if its type already has NOT NULL.
    */
   public RexNode makeNotNull(RexNode exp) {
+    return makeNullable(exp, false);
+  }
+
+  /**
+   * Makes a cast of an expression to the required nullability; returns
+   * the expression unchanged if its type already has the desired nullability.
+   */
+  public RexNode makeNullable(RexNode exp, boolean nullability) {
     final RelDataType type = exp.getType();
-    if (!type.isNullable()) {
+    if (type.isNullable() == nullability) {
       return exp;
     }
-    final RelDataType notNullType =
-        typeFactory.createTypeWithNullability(type, false);
-    return makeAbstractCast(notNullType, exp, false);
+    final RelDataType type2 =
+        typeFactory.createTypeWithNullability(type, nullability);
+    return makeAbstractCast(type2, exp, false);
   }
 
   /**

core/src/main/java/org/apache/calcite/sql2rel/RelDecorrelator.java

@@ -412,7 +412,13 @@ protected RexNode removeCorrelationExpr(
     RemoveCorrelationRexShuttle shuttle =
         new RemoveCorrelationRexShuttle(relBuilder.getRexBuilder(),
             projectPulledAboveLeftCorrelator, null, isCount);
-    return exp.accept(shuttle);
+    RexNode exp2 = exp.accept(shuttle);
+
+    // Fix the nullability.
+    if (projectPulledAboveLeftCorrelator) {
+      exp2 = relBuilder.getRexBuilder().makeNullable(exp2);
+    }
+    return exp2;
   }
 
   /** Fallback if none of the other {@code decorrelateRel} methods match. */
@@ -1432,7 +1438,6 @@ private RelNode projectJoinOutputWithNullability(
               pair.left,
               projectPulledAboveLeftCorrelator,
               nullIndicator);
-
       newProjExprs.add(newProjExpr, pair.right);
     }
 

core/src/test/resources/sql/lateral.iq

@@ -219,4 +219,31 @@ from t,
 
 !ok
 
+# LEFT JOIN LATERAL
+select ename, deptno, ename2
+from emp as o
+  left join lateral (select ename as ename2
+                     from emp
+                     where deptno = o.deptno + 10) on true
+where job = 'MANAGER';
++-------+--------+--------+
+| ENAME | DEPTNO | ENAME2 |
++-------+--------+--------+
+| BLAKE |     30 |        |
+| CLARK |     10 | ADAMS  |
+| CLARK |     10 | FORD   |
+| CLARK |     10 | JONES  |
+| CLARK |     10 | SCOTT  |
+| CLARK |     10 | SMITH  |
+| JONES |     20 | ALLEN  |
+| JONES |     20 | BLAKE  |
+| JONES |     20 | JAMES  |
+| JONES |     20 | MARTIN |
+| JONES |     20 | TURNER |
+| JONES |     20 | WARD   |
++-------+--------+--------+
+(12 rows)
+
+!ok
+
 # End lateral.iq

core/src/test/resources/sql/measure.iq

@@ -179,6 +179,57 @@ from empm;
 
 !ok
 
+# [CALCITE-6559] Query with measure that applies AVG to SMALLINT
+# throws AssertionError "Cannot add expression of different type to set"
+#
+# Occurs when AVG measure is applied to SMALLINT or TINYINT column.
+# A CAST is added internally to the result of SUM / COUNT, but this cast
+# prevents a cast to nullable for the null-generating side of an outer join.
+WITH empm AS (
+    SELECT *, AVG(empno) AS MEASURE avg_empno
+    FROM emp)
+SELECT job, avg_empno
+FROM empm
+GROUP BY job;
++-----------+-----------+
+| JOB       | AVG_EMPNO |
++-----------+-----------+
+| ANALYST   |      7845 |
+| CLERK     |      7769 |
+| MANAGER   |      7682 |
+| PRESIDENT |      7839 |
+| SALESMAN  |      7629 |
++-----------+-----------+
+(5 rows)
+
+!ok
+
+!if (false) {
+# [CALCITE-6564] Support queries with measures on top of a VALUES relation
+#
+# The following querey is the original test case for [CALCITE-6559]; that
+# bug is fixed, but this query now fails during planning due to VALUES.
+WITH tbl_with_null_dim AS (
+    SELECT e.deptno, e.grade, AVG(e.grade) AS MEASURE avg_grade
+    FROM (VALUES (1, 70),
+             (1, 50),
+             (NULL, 50),
+             (3, 82)) AS e (deptno, grade))
+SELECT deptno, avg_grade
+FROM tbl_with_null_dim
+GROUP BY deptno;
++--------+-----------+
+| DEPTNO | AVG_GRADE |
++--------+-----------+
+|      1 |        60 |
+|      3 |        82 |
+|        |        50 |
++--------+-----------+
+(3 rows)
+
+!ok
+!}
+
 # CTE with literal measure
 with emp2 as (
   select *, 2 as measure two

core/src/test/resources/sql/scalar.iq

@@ -239,4 +239,66 @@ select deptno, (select empno from "scott".emp order by empno limit 1) as x from
 
 !ok
 
+# Scalar subquery on VALUES; based on a query mentioned in
+# [CALCITE-3244] RelDecorrelator unable to decorrelate expression with filter and aggregate on top
+select
+    (select count(t2.id)
+        from (values (1), (2)) t2(id)
+        where t2.id = t1.id) as c
+from (values (1), (2)) t1(id);
++---+
+| C |
++---+
+| 1 |
+| 1 |
++---+
+(2 rows)
+
+!ok
+
+select
+    (select count(t2.id)
+        from (values (1), (2), (2), (4)) t2(id)
+        where t2.id = t1.id) as c
+from (values (1), (3)) t1(id);
++---+
+| C |
++---+
+| 1 |
+| 0 |
++---+
+(2 rows)
+
+!ok
+
+# Scalar subquery using EXISTS on VALUES
+select exists (select count(t2.id)
+               from (values (1), (2), (2), (4)) t2(id)
+               where t2.id = t1.id) as e
+from (values (1), (3)) t1(id);
++------+
+| E    |
++------+
+| true |
+| true |
++------+
+(2 rows)
+
+!ok
+
+# Variation on previous query
+select exists (select *
+               from (values (1), (2), (2), (4)) t2(id)
+               where t2.id = t1.id) as e
+from (values (1), (3)) t1(id);
++-------+
+| E     |
++-------+
+| false |
+| true  |
++-------+
+(2 rows)
+
+!ok
+
 # End scalar.iq