SQL: Fix bug with optimization of null related conditionals (#41355)

The SimplifyConditional rule is removing NULL literals from those
functions to simplify their evaluation. This happens in the Optimizer
and a new instance of the conditional function is generated. Previously,
the dataType was not set properly (defaulted to DataType.NULL) for
those new instances and since the resolveType() wasn't called again
it resulted in returning always null.

E.g.:

SELECT COALESCE(null, 'foo', null, 'bar')

COALESCE(null, 'foo', null, 'bar')
-----------------
null

This issue was not visible before because the tests always used an alias
for the conditional function which caused the resolveType() to be
called which sets the dataType properly.

E.g.:

SELECT COALESCE(null, 'foo', null, 'bar') as c

c
-----------------
foo

(cherry picked from commit c39980a65dd593363f1d8d1b038b26cb0ce02aaf)

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

@@ -61,6 +61,13 @@ c:i
 ;
 
 coalesceMixed
+SELECT COALESCE(null, 123, null, 321);
+
+COALESCE(null, 123, null, 321):i
+123
+;
+
+coalesceMixedWithAlias
 SELECT COALESCE(null, 123, null, 321) AS c;
 
 c:i

x-pack/plugin/sql/src/main/java/org/elasticsearch/xpack/sql/expression/predicate/conditional/ConditionalFunction.java

@@ -25,7 +25,7 @@ import static org.elasticsearch.xpack.sql.util.StringUtils.ordinal;
  */
 public abstract class ConditionalFunction extends ScalarFunction {
 
-    protected DataType dataType = DataType.NULL;
+    protected DataType dataType = null;
 
     ConditionalFunction(Source source, List<Expression> fields) {
         super(source, fields);
@@ -33,6 +33,12 @@ public abstract class ConditionalFunction extends ScalarFunction {
 
     @Override
     public DataType dataType() {
+        if (dataType == null) {
+            dataType = DataType.NULL;
+            for (Expression exp : children()) {
+                dataType = DataTypeConversion.commonType(dataType, exp.dataType());
+            }
+        }
         return dataType;
     }
 
@@ -61,7 +67,6 @@ public abstract class ConditionalFunction extends ScalarFunction {
                         child.dataType().typeName));
                 }
             }
-            dataType = DataTypeConversion.commonType(dataType, child.dataType());
         }
         return TypeResolution.TYPE_RESOLVED;
     }

x-pack/plugin/sql/src/test/java/org/elasticsearch/xpack/sql/optimizer/OptimizerTests.java

@@ -501,6 +501,7 @@ public class OptimizerTests extends ESTestCase {
                         randomListOfNulls())));
         assertEquals(1, e.children().size());
         assertEquals(TRUE, e.children().get(0));
+        assertEquals(DataType.BOOLEAN, e.dataType());
     }
 
     private List<Expression> randomListOfNulls() {
@@ -514,6 +515,7 @@ public class OptimizerTests extends ESTestCase {
         assertEquals(Coalesce.class, e.getClass());
         assertEquals(1, e.children().size());
         assertEquals(TRUE, e.children().get(0));
+        assertEquals(DataType.BOOLEAN, e.dataType());
     }
 
     public void testSimplifyIfNullNulls() {
@@ -527,11 +529,13 @@ public class OptimizerTests extends ESTestCase {
         assertEquals(IfNull.class, e.getClass());
         assertEquals(1, e.children().size());
         assertEquals(ONE, e.children().get(0));
+        assertEquals(DataType.INTEGER, e.dataType());
 
         e = new SimplifyConditional().rule(new IfNull(EMPTY, ONE, NULL));
         assertEquals(IfNull.class, e.getClass());
         assertEquals(1, e.children().size());
         assertEquals(ONE, e.children().get(0));
+        assertEquals(DataType.INTEGER, e.dataType());
     }
 
     public void testFoldNullNotAppliedOnNullIf() {
@@ -559,6 +563,7 @@ public class OptimizerTests extends ESTestCase {
         assertEquals(2, e.children().size());
         assertEquals(ONE, e.children().get(0));
         assertEquals(TWO, e.children().get(1));
+        assertEquals(DataType.INTEGER, e.dataType());
     }
 
     public void testSimplifyLeastNulls() {
@@ -580,6 +585,7 @@ public class OptimizerTests extends ESTestCase {
         assertEquals(2, e.children().size());
         assertEquals(ONE, e.children().get(0));
         assertEquals(TWO, e.children().get(1));
+        assertEquals(DataType.INTEGER, e.dataType());
     }
     
     public void testConcatFoldingIsNotNull() {