Bug #21143080: UPDATE ON VARCHAR AND TEXT COLUMNS PRODUCE

               INCORRECT RESULTS

Issue:
-----
Updating varchar and text fields in the same update
statement can produce incorrect results. When a varchar
field is assigned to the text field and the varchar field
is then set to a different value, the text field's result
contains the varchar field's new value.

SOLUTION:
---------
Currently the blob type does not allocate space for the
string to be stored. Instead it contains a pointer to the
varchar string. So when the varchar field is changed as
part of the update statement, the value contained in the
blob also changes.

The fix would be to actually store the value by allocating
space for the blob's string. We can avoid allocating this
space when the varchar field is not being written into.

1 files changed, 17 insertions, 1 deletions

diff --git a/sql/field.h b/sql/field.h
index 6a181b7ae91..73922460037 100644
--- a/sql/field.h
+++ b/sql/field.h
@@ -1,7 +1,7 @@
 #ifndef FIELD_INCLUDED
 #define FIELD_INCLUDED
 
-/* Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
+/* Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -511,6 +511,17 @@ public:
   }
   /* Hash value */
   virtual void hash(ulong *nr, ulong *nr2);
+
+/**
+  Checks whether a string field is part of write_set.
+
+  @return
+    FALSE  - If field is not char/varchar/....
+           - If field is char/varchar/.. and is not part of write set.
+    TRUE   - If field is char/varchar/.. and is part of write set.
+*/
+  virtual bool is_updatable() const { return FALSE; }
+
   friend int cre_myisam(char * name, register TABLE *form, uint options,
 			ulonglong auto_increment_value);
   friend class Copy_field;
@@ -798,6 +809,11 @@ public:
 
   int store_decimal(const my_decimal *d);
   uint32 max_data_length() const;
+  bool is_updatable() const
+  {
+    DBUG_ASSERT(table && table->write_set);
+    return bitmap_is_set(table->write_set, field_index);
+  }
 };
 
 /* base class for float and double and decimal (old one) */