MDEV-28965 Assertion failure when preparing UPDATE with derived table in WHERE

This patch fixes not only the assertion failure in the function
Field_iterator_table_ref::set_field_iterator() but also:
 - fixes the problem of forced materialization of derived tables used
   in subqueries contained in WHERE clauses of single-table and multi-table
   UPDATE and DELETE statements
 - fixes the problem of MDEV-17954 that prevented execution of multi-table
   DELETE statements if they use in their WHERE clauses references to
   the tables that are updated.

The patch must be considered a complement to the patch for MDEV-28883.

Approved by Oleksandr Byelkin <sanja@mariadb.com>

1 files changed, 12 insertions, 4 deletions

diff --git a/sql/sql_update.h b/sql/sql_update.h
index d0fc7cb01e1..bd7d58cf97d 100644
--- a/sql/sql_update.h
+++ b/sql/sql_update.h
@@ -46,12 +46,12 @@ class Sql_cmd_update final : public Sql_cmd_dml
 {
 public:
   Sql_cmd_update(bool multitable_arg)
-      : multitable(multitable_arg)
-  { }
+    : orig_multitable(multitable_arg), multitable(multitable_arg)
+  {}
 
   enum_sql_command sql_command_code() const override
   {
-    return multitable ? SQLCOM_UPDATE_MULTI : SQLCOM_UPDATE;
+    return orig_multitable ? SQLCOM_UPDATE_MULTI : SQLCOM_UPDATE;
   }
 
   DML_prelocking_strategy *get_dml_prelocking_strategy()
@@ -59,6 +59,12 @@ public:
     return &multiupdate_prelocking_strategy;
   }
 
+  bool processing_as_multitable_update_prohibited(THD *thd);
+
+  bool is_multitable() { return multitable; }
+
+  void set_as_multitable() { multitable= true; }
+
 protected:
   /**
     @brief Perform precheck of table privileges for update statements
@@ -82,6 +88,9 @@ private:
   */
   bool update_single_table(THD *thd);
 
+  /* Original value of the 'multitable' flag set by constructor */
+  const bool orig_multitable;
+
   /*
     True if the statement is a multi-table update or converted to such.
     For a single-table update this flag is set to true if the statement
@@ -95,7 +104,6 @@ private:
  public:
   /* The list of the updating expressions used in the set clause */
   List<Item> *update_value_list;
-
 };
 
 #endif /* SQL_UPDATE_INCLUDED */