[release-17.0] Fix vtexplain not handling UNION queries with `wei…

…ght_string` results correctly. (#16129) (#16155)

Signed-off-by: Arthur Schreiber <arthurschreiber@github.com>
Co-authored-by: vitess-bot[bot] <108069721+vitess-bot[bot]@users.noreply.github.com>
Co-authored-by: Arthur Schreiber <arthurschreiber@github.com>

go/vt/vtexplain/testdata/multi-output/selectsharded-output.txt

@@ -207,3 +207,9 @@ SELECT id FROM orders WHERE id IN (1, "1", 1)
 2 ks_sharded/40-80: select id from orders where id in (1, '1', 1) limit 10001
 
 ----------------------------------------------------------------------
+(SELECT user.id, user.name FROM user WHERE user.id = 1) UNION (SELECT user.id, user.name FROM user WHERE user.id = 2)
+
+2 ks_sharded/-40: select distinct `user`.id, `user`.`name`, weight_string(`user`.id), weight_string(`user`.`name`) from `user` where `user`.id = 2 limit 10001 /* INT64 */
+2 ks_sharded/-40: select distinct `user`.id, `user`.`name`, weight_string(`user`.id), weight_string(`user`.`name`) from `user` where `user`.id = 1 limit 10001 /* INT64 */
+
+----------------------------------------------------------------------

go/vt/vtexplain/testdata/selectsharded-queries.sql

@@ -38,4 +38,6 @@ select id from user where not id in (select col from music where music.user_id =
 
 SELECT user.id, user.name, name_info.info FROM user INNER JOIN music ON (user.id = music.user_id) LEFT OUTER JOIN name_info ON (user.name = name_info.name);
 
-SELECT id FROM orders WHERE id IN (1, "1", 1)
+SELECT id FROM orders WHERE id IN (1, "1", 1);
+
+(SELECT user.id, user.name FROM user WHERE user.id = 1) UNION (SELECT user.id, user.name FROM user WHERE user.id = 2);

go/vt/vtexplain/vtexplain_vttablet.go

@@ -842,9 +842,15 @@ func inferColTypeFromExpr(node sqlparser.Expr, tableColumnMap map[sqlparser.Iden
 			colTypes = append(colTypes, colType)
 		}
 	case sqlparser.Callable:
-		// As a shortcut, functions are integral types
-		colNames = append(colNames, sqlparser.String(node))
-		colTypes = append(colTypes, querypb.Type_INT32)
+		switch node := node.(type) {
+		case *sqlparser.WeightStringFuncExpr:
+			colNames = append(colNames, sqlparser.String(node))
+			colTypes = append(colTypes, querypb.Type_BINARY)
+		default:
+			// As a shortcut, functions are integral types
+			colNames = append(colNames, sqlparser.String(node))
+			colTypes = append(colTypes, querypb.Type_INT32)
+		}
 	case *sqlparser.Literal:
 		colNames = append(colNames, sqlparser.String(node))
 		switch node.Type {