SQL: Fix issue with common type resolution (#46565)

Many scalar functions try to find out the common type between their
arguments in order to set it as their return time, e.g.:
for `float + double` the common type which is set as the return type
of the + operation is `double`.

Previously, for data types TEXT and KEYWORD (string data types) there
was no common data type found and null was returned causing NPEs when
the function was trying to resolve the return data type.

Fixes: #46551
(cherry picked from commit 291017d)

x-pack/plugin/sql/src/main/java/org/elasticsearch/xpack/sql/type/DataTypeConversion.java

@@ -23,6 +23,7 @@
 import static org.elasticsearch.xpack.sql.type.DataType.DATETIME;
 import static org.elasticsearch.xpack.sql.type.DataType.LONG;
 import static org.elasticsearch.xpack.sql.type.DataType.NULL;
+import static org.elasticsearch.xpack.sql.type.DataType.TEXT;
 import static org.elasticsearch.xpack.sql.type.DataType.TIME;
 
 /**
@@ -50,6 +51,12 @@ public static DataType commonType(DataType left, DataType right) {
         if (DataTypes.isNull(right)) {
             return left;
         }
+        if (left.isString() && right.isString()) {
+            if (left == TEXT) {
+                return TEXT;
+            }
+            return right;
+        }
         if (left.isNumeric() && right.isNumeric()) {
             // if one is int
             if (left.isInteger()) {

x-pack/plugin/sql/src/test/java/org/elasticsearch/xpack/sql/type/DataTypeConversionTests.java

@@ -621,12 +621,15 @@ public void testCommonType() {
         assertEquals(NULL, commonType(NULL, NULL));
         assertEquals(INTEGER, commonType(INTEGER, KEYWORD));
         assertEquals(LONG, commonType(TEXT, LONG));
-        assertNull(commonType(TEXT, KEYWORD));
         assertEquals(SHORT, commonType(SHORT, BYTE));
         assertEquals(FLOAT, commonType(BYTE, FLOAT));
         assertEquals(FLOAT, commonType(FLOAT, INTEGER));
         assertEquals(DOUBLE, commonType(DOUBLE, FLOAT));
 
+        // strings
+        assertEquals(TEXT, commonType(TEXT, KEYWORD));
+        assertEquals(TEXT, commonType(KEYWORD, TEXT));
+
         // numeric and intervals
         assertEquals(INTERVAL_YEAR_TO_MONTH, commonType(INTERVAL_YEAR_TO_MONTH, LONG));
         assertEquals(INTERVAL_HOUR_TO_MINUTE, commonType(INTEGER, INTERVAL_HOUR_TO_MINUTE));