MDEV-31224 Crash with EXPLAIN EXTENDED for multi-table update of system table

EXPLAIN EXTENDED should always print the field item used in the left part
of an equality expression from the SET clause of an update statement as a
reference to table column.

Approved by Oleksandr Byelkin <sanja@mariadb.com>

mysql-test/main/explain_non_select.result

@@ -277,3 +277,22 @@ EXECUTE stmt;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	2	
 drop table t1,t2;
+#
+# MDEV-31224: EXPLAIN EXTENDED for multi-table update of system table
+#
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t2 (b INT) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (3);
+EXPLAIN EXTENDED UPDATE t1, t2 SET b = 4 WHERE a IN (6,2);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t2	system	NULL	NULL	NULL	NULL	1	100.00	
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
+Warnings:
+Note	1003	update `test`.`t1` set `test`.`t2`.`b` = 4 where `test`.`t1`.`a` in (6,2)
+UPDATE t1, t2 SET b = 4 WHERE a IN (6,2);
+SELECT * from t2;
+b
+4
+DROP TABLE t1, t2;
+# End of 10.4 tests

mysql-test/main/explain_non_select.test

@@ -250,3 +250,23 @@ PREPARE stmt FROM 'EXPLAIN INSERT INTO t1 SELECT * FROM t2';
 EXECUTE stmt;
 drop table t1,t2;
 
+--echo #
+--echo # MDEV-31224: EXPLAIN EXTENDED for multi-table update of system table
+--echo #
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(2);
+
+CREATE TABLE t2 (b INT) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (3);
+
+let $q=
+UPDATE t1, t2 SET b = 4 WHERE a IN (6,2);
+
+eval EXPLAIN EXTENDED $q;
+eval $q;
+SELECT * from t2;
+
+DROP TABLE t1, t2;
+
+--echo # End of 10.4 tests

mysql-test/main/myisam_explain_non_select_all.result

@@ -2689,7 +2689,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t2	system	NULL	NULL	NULL	NULL	0	0.00	Const row not found
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	100.00	
 Warnings:
-Note	1003	update `test`.`t1` set NULL = 10
+Note	1003	update `test`.`t1` set `test`.`t2`.`c2` = 10
 # Status of EXPLAIN EXTENDED query
 Variable_name	Value
 Handler_read_key	7
@@ -2734,7 +2734,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t2	system	NULL	NULL	NULL	NULL	0	0.00	Const row not found
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
 Warnings:
-Note	1003	update `test`.`t1` set NULL = 10 where `test`.`t1`.`c3` = 10
+Note	1003	update `test`.`t1` set `test`.`t2`.`c2` = 10 where `test`.`t1`.`c3` = 10
 # Status of EXPLAIN EXTENDED query
 Variable_name	Value
 Handler_read_key	7

sql/sql_select.cc

@@ -28138,7 +28138,7 @@ void st_select_lex::print_set_clause(THD *thd, String *str,
     else
       str->append(',');
 
-    item->print(str, query_type);
+    item->print(str, (enum_query_type) (query_type | QT_NO_DATA_EXPANSION));
     str->append(STRING_WITH_LEN(" = "));
     val->print(str, query_type);
   }