MDEV-7260: Crash in get_best_combination when executing multi-table UPDATE with nested views

Do not use merge_for_insert for commands which use SELECT because optimizer can't work with such tables.

Fixes which makes multi-delete working with normally merged views.

mysql-test/r/view.result

@@ -5398,6 +5398,19 @@ DROP VIEW v1;
 DROP TABLE t1, t2;
 create view v1 as select 1;
 drop view v1;
+#
+# MDEV-7260: Crash in get_best_combination when executing multi-table
+# UPDATE with nested views
+#
+CREATE TABLE `t1` (`id` bigint(20));
+INSERT INTO `t1` VALUES (1),(2);
+CREATE TABLE `t2` (`id` bigint(20));
+CREATE TABLE `t3` (`id` bigint(20), `flag` tinyint(4));
+create view v1 as select id from t1;
+create view v2 as select t2.* from (t2 left join v1 using (id));
+update t3 left join v2 using (id) set flag=flag+1;
+drop view v2, v1;
+drop table t1, t2, t3;
 # -----------------------------------------------------------------
 # -- End of 5.5 tests.
 # -----------------------------------------------------------------

mysql-test/t/view.test

@@ -5345,6 +5345,28 @@ create view v1 as select 1;
 
 drop view v1;
 
+--echo #
+--echo # MDEV-7260: Crash in get_best_combination when executing multi-table
+--echo # UPDATE with nested views
+--echo #
+
+CREATE TABLE `t1` (`id` bigint(20));
+
+INSERT INTO `t1` VALUES (1),(2);
+
+CREATE TABLE `t2` (`id` bigint(20));
+
+CREATE TABLE `t3` (`id` bigint(20), `flag` tinyint(4));
+
+create view v1 as select id from t1;
+
+create view v2 as select t2.* from (t2 left join v1 using (id));
+
+update t3 left join v2 using (id) set flag=flag+1;
+
+drop view v2, v1;
+drop table t1, t2, t3;
+
 --echo # -----------------------------------------------------------------
 --echo # -- End of 5.5 tests.
 --echo # -----------------------------------------------------------------

sql/sql_base.cc

@@ -2028,6 +2028,9 @@ unique_table(THD *thd, TABLE_LIST *table, TABLE_LIST *table_list,
              bool check_alias)
 {
   TABLE_LIST *dup;
+
+  table= table->find_table_for_update();
+
   if (table->table && table->table->file->ht->db_type == DB_TYPE_MRG_MYISAM)
   {
     TABLE_LIST *child;

sql/sql_delete.cc

@@ -657,9 +657,10 @@ multi_delete::initialize_tables(JOIN *join)
   delete_while_scanning= 1;
   for (walk= delete_tables; walk; walk= walk->next_local)
   {
-    tables_to_delete_from|= walk->table->map;
+    TABLE_LIST *tbl= walk->correspondent_table->find_table_for_update();
+    tables_to_delete_from|= tbl->table->map;
     if (delete_while_scanning &&
-        unique_table(thd, walk, join->tables_list, false))
+        unique_table(thd, tbl, join->tables_list, false))
     {
       /*
         If the table we are going to delete from appears

sql/sql_derived.cc

@@ -507,6 +507,9 @@ bool mysql_derived_merge_for_insert(THD *thd, LEX *lex, TABLE_LIST *derived)
     DBUG_RETURN(FALSE);
   if (derived->is_materialized_derived())
     DBUG_RETURN(mysql_derived_prepare(thd, lex, derived));
+  if ((thd->lex->sql_command == SQLCOM_UPDATE_MULTI ||
+       thd->lex->sql_command == SQLCOM_DELETE_MULTI))
+    DBUG_RETURN(FALSE);
   if (!derived->is_multitable())
   {
     if (!derived->single_table_updatable())

sql/table.h

@@ -1984,6 +1984,24 @@ struct TABLE_LIST
   TABLE_LIST *find_underlying_table(TABLE *table);
   TABLE_LIST *first_leaf_for_name_resolution();
   TABLE_LIST *last_leaf_for_name_resolution();
+  /**
+     @brief
+       Find the bottom in the chain of embedded table VIEWs.
+
+     @detail
+       This is used for single-table UPDATE/DELETE when they are modifying a
+       single-table VIEW.
+  */
+  TABLE_LIST *find_table_for_update()
+  {
+    TABLE_LIST *tbl= this;
+    while(!tbl->is_multitable() && tbl->single_table_updatable() &&
+        tbl->merge_underlying_list)
+    {
+      tbl= tbl->merge_underlying_list;
+    }
+    return tbl;
+  }
   TABLE *get_real_join_table();
   bool is_leaf_for_name_resolution();
   inline TABLE_LIST *top_table()