MDEV-14551 Can't find record in table on multi-table update with ORDER BY

preserve positions if the multi-update join is using tmp table:

* store positions in the tmp table if needed
  JOIN::add_fields_for_current_rowid()

* take positions from the tmp table, not from file->position():
  multi_update::prepare2()

1 files changed, 56 insertions, 0 deletions

diff --git a/sql/sql_update.cc b/sql/sql_update.cc
index 829bfa546ce..a8b86448d5e 100644
--- a/sql/sql_update.cc
+++ b/sql/sql_update.cc
@@ -2198,10 +2198,66 @@ loop_end:
       DBUG_RETURN(1);
     tmp_tables[cnt]->file->extra(HA_EXTRA_WRITE_CACHE);
   }
+  join->tmp_table_keep_current_rowid= TRUE;
   DBUG_RETURN(0);
 }
 
 
+static TABLE *item_rowid_table(Item *item)
+{
+  if (item->type() != Item::FUNC_ITEM)
+    return NULL;
+  Item_func *func= (Item_func *)item;
+  if (func->functype() != Item_func::TEMPTABLE_ROWID)
+    return NULL;
+  Item_temptable_rowid *itr= (Item_temptable_rowid *)func;
+  return itr->table;
+}
+
+
+/*
+  multi_update stores a rowid and new field values for every updated row in a
+  temporary table (one temporary table per updated table).  These rowids are
+  obtained via Item_temptable_rowid's by calling handler::position().  But if
+  the join is resolved via a temp table, rowids cannot be obtained from
+  handler::position() in the multi_update::send_data().  So, they're stored in
+  the join's temp table (JOIN::add_fields_for_current_rowid()) and here we
+  replace Item_temptable_rowid's (that would've done handler::position()) with
+  Item_field's (that will simply take the corresponding field value from the
+  temp table).
+*/
+int multi_update::prepare2(JOIN *join)
+{
+  if (!join->need_tmp || !join->tmp_table_keep_current_rowid)
+    return 0;
+
+  // there cannot be many tmp tables in multi-update
+  JOIN_TAB *tmptab= join->join_tab + join->exec_join_tab_cnt();
+
+  for (Item **it= tmptab->tmp_table_param->items_to_copy; *it ; it++)
+  {
+    TABLE *tbl= item_rowid_table(*it);
+    if (!tbl)
+      continue;
+    for (uint i= 0; i < table_count; i++)
+    {
+      for (Item **it2= tmp_table_param[i].items_to_copy; *it2; it2++)
+      {
+        if (item_rowid_table(*it2) != tbl)
+          continue;
+        Item *fld= new (thd->mem_root)
+                        Item_field(thd, (*it)->get_tmp_table_field());
+        if (!fld)
+          return 1;
+        fld->set_result_field((*it2)->get_tmp_table_field());
+        *it2= fld;
+      }
+    }
+  }
+  return 0;
+}
+
+
 multi_update::~multi_update()
 {
   TABLE_LIST *table;