Add more tests and fixup nullable

x-pack/plugin/esql/qa/testFixtures/src/main/resources/null.csv-spec

@@ -109,3 +109,29 @@ emp_no:integer | first_name:keyword
          10010 | X
 ;
 
+coalesceEndsInNull
+# ending in null is sill because it'll noop but it shouldn't break things.
+FROM employees
+| EVAL first_name = COALESCE(first_name, last_name, null)
+| SORT first_name DESC, emp_no ASC
+| KEEP emp_no, first_name
+| limit 3;
+
+emp_no:integer | first_name:keyword
+         10047 | Zvonko
+         10081 | Zhongwei
+         10026 | Yongqiao
+;
+
+coalesceFolding
+FROM employees
+| EVAL foo=COALESCE(true, false, null)
+| SORT emp_no ASC
+| KEEP emp_no, first_name, foo
+| limit 3;
+
+emp_no:integer | first_name:keyword | foo:boolean
+         10001 | Georgi             | true
+         10002 | Bezalel            | true
+         10003 | Parto              | true
+;

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/nulls/Coalesce.java

@@ -74,7 +74,18 @@ protected TypeResolution resolveType() {
 
     @Override
     public Nullability nullable() {
-        return children().get(children().size() - 1).nullable();
+        // If any of the children aren't nullable then this isn't.
+        for (Expression c : children()) {
+            if (c.nullable() == Nullability.FALSE) {
+                return Nullability.FALSE;
+            }
+        }
+        /*
+         * Otherwise let's call this one "unknown". If we returned TRUE here
+         * an optimizer rule would replace this with null if any of our children
+         * fold to null. We don't want that at all.
+         */
+        return Nullability.UNKNOWN;
     }
 
     @Override

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/expression/function/scalar/nulls/CoalesceTests.java

@@ -18,6 +18,8 @@
 import org.elasticsearch.xpack.esql.planner.Layout;
 import org.elasticsearch.xpack.ql.expression.Expression;
 import org.elasticsearch.xpack.ql.expression.FieldAttribute;
+import org.elasticsearch.xpack.ql.expression.Literal;
+import org.elasticsearch.xpack.ql.expression.Nullability;
 import org.elasticsearch.xpack.ql.tree.Source;
 import org.elasticsearch.xpack.ql.type.DataType;
 import org.elasticsearch.xpack.ql.type.DataTypes;
@@ -151,4 +153,24 @@ public void testCoalesceIsLazy() {
             return EvalMapper.toEvaluator(child, layout);
         }).get().eval(row(testCase.getDataValues())), 0), testCase.getMatcher());
     }
+
+    public void testCoalesceNullabilityIsUnknown() {
+        assertThat(buildFieldExpression(testCase).nullable(), equalTo(Nullability.UNKNOWN));
+    }
+
+    public void testCoalesceKnownNullable() {
+        List<Expression> sub = new ArrayList<>(testCase.getDataAsFields());
+        sub.add(between(0, sub.size()), new Literal(Source.EMPTY, null, sub.get(0).dataType()));
+        Coalesce exp = build(Source.EMPTY, sub);
+        // Still UNKNOWN - if it were TRUE then an optimizer would replace it with null
+        assertThat(exp.nullable(), equalTo(Nullability.UNKNOWN));
+    }
+
+    public void testCoalesceNotNullable() {
+        List<Expression> sub = new ArrayList<>(testCase.getDataAsFields());
+        sub.add(between(0, sub.size()), randomLiteral(sub.get(0).dataType()));
+        Coalesce exp = build(Source.EMPTY, sub);
+        // Known not to be nullable because it contains a non-null literal
+        assertThat(exp.nullable(), equalTo(Nullability.FALSE));
+    }
 }