Fix vtexplain not handling UNION queries with weight_string res…

…ults correctly.

Signed-off-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 dt.c0 as id, dt.c1 as `name`, weight_string(dt.c0), weight_string(dt.c1) from (select distinct `user`.id, `user`.`name` from `user` where `user`.id = 2) as dt(c0, c1) limit 10001
+2 ks_sharded/-40: select dt.c0 as id, dt.c1 as `name`, weight_string(dt.c0), weight_string(dt.c1) from (select distinct `user`.id, `user`.`name` from `user` where `user`.id = 1) as dt(c0, c1) limit 10001
+
+----------------------------------------------------------------------

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

@@ -864,9 +864,15 @@ 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 {