Merge moonbone.local:/work/15351-bug-4.1-mysql

into moonbone.local:/home/evgen/bk-trees/mysql-4.1-opt


sql/item_strfunc.cc:
  Auto merged

13 files changed, 291 insertions, 52 deletions

diff --git a/sql/field.cc b/sql/field.cc
index a64eaad7308..ec4d4b4e4f5 100644
--- a/sql/field.cc
+++ b/sql/field.cc
@@ -6841,7 +6841,11 @@ create_field::create_field(Field *old_field,Field *orig_field)
 bool 
 Field::set_warning(const uint level, const uint code, int cuted_increment)
 {
-  THD *thd= table->in_use;
+  /*
+    If this field was created only for type conversion purposes it
+    will have table == NULL.
+  */
+  THD *thd= table ? table->in_use : current_thd;
   if (thd->count_cuted_fields)
   {
     thd->cuted_fields+= cuted_increment;
@@ -6876,7 +6880,8 @@ Field::set_datetime_warning(const uint level, const uint code,
                             timestamp_type ts_type, int cuted_increment)
 {
   if (set_warning(level, code, cuted_increment))
-    make_truncated_value_warning(table->in_use, str, str_length, ts_type);
+    make_truncated_value_warning(table ? table->in_use : current_thd,
+                                 str, str_length, ts_type);
 }
 
 
@@ -6905,8 +6910,8 @@ Field::set_datetime_warning(const uint level, const uint code,
   {
     char str_nr[22];
     char *str_end= longlong10_to_str(nr, str_nr, -10);
-    make_truncated_value_warning(table->in_use, str_nr, str_end - str_nr, 
-                                 ts_type);
+    make_truncated_value_warning(table ? table->in_use : current_thd,
+                                 str_nr, str_end - str_nr, ts_type);
   }
 }
 
@@ -6935,7 +6940,8 @@ Field::set_datetime_warning(const uint level, const uint code,
     /* DBL_DIG is enough to print '-[digits].E+###' */
     char str_nr[DBL_DIG + 8];
     uint str_len= my_sprintf(str_nr, (str_nr, "%g", nr));
-    make_truncated_value_warning(table->in_use, str_nr, str_len, ts_type);
+    make_truncated_value_warning(table ? table->in_use : current_thd,
+                                 str_nr, str_len, ts_type);
   }
 }
 
diff --git a/sql/item.h b/sql/item.h
index 66da7ad4e74..eca8ee184a1 100644
--- a/sql/item.h
+++ b/sql/item.h
@@ -327,6 +327,14 @@ public:
     cleanup();
     delete this;
   }
+  /*
+    result_as_longlong() must return TRUE for Items representing DATE/TIME
+    functions and DATE/TIME table fields.
+    Those Items have result_type()==STRING_RESULT (and not INT_RESULT), but
+    their values should be compared as integers (because the integer
+    representation is more precise than the string one).
+  */
+  virtual bool result_as_longlong() { return FALSE; }
 };
 
 
@@ -450,6 +458,10 @@ public:
   Item *get_tmp_table_item(THD *thd);
   void cleanup();
   inline uint32 max_disp_length() { return field->max_length(); }
+  bool result_as_longlong()
+  {
+    return field->can_be_compared_as_longlong();
+  }
   friend class Item_default_value;
   friend class Item_insert_value;
   friend class st_select_lex_unit;
@@ -973,6 +985,10 @@ public:
   }
   Item *real_item() { return *ref; }
   void print(String *str);
+  bool result_as_longlong()
+  {
+    return (*ref)->result_as_longlong();
+  }
 };
 
 
diff --git a/sql/item_cmpfunc.cc b/sql/item_cmpfunc.cc
index 3c41fb56d89..e92e1d30ca4 100644
--- a/sql/item_cmpfunc.cc
+++ b/sql/item_cmpfunc.cc
@@ -25,6 +25,8 @@
 #include <m_ctype.h>
 #include "sql_select.h"
 
+static bool convert_constant_item(THD *thd, Field *field, Item **item);
+
 static Item_result item_store_type(Item_result a,Item_result b)
 {
   if (a == STRING_RESULT || b == STRING_RESULT)
@@ -58,12 +60,155 @@ static void agg_result_type(Item_result *type, Item **items, uint nitems)
   }
 }
 
-static void agg_cmp_type(Item_result *type, Item **items, uint nitems)
+
+/*
+  Aggregates result types from the array of items.
+
+  SYNOPSIS:
+    agg_cmp_type()
+    thd          thread handle
+    type   [out] the aggregated type
+    items        array of items to aggregate the type from
+    nitems       number of items in the array
+
+  DESCRIPTION
+    This function aggregates result types from the array of items. Found type
+    supposed to be used later for comparison of values of these items.
+    Aggregation itself is performed by the item_cmp_type() function.
+
+  NOTES
+    Aggregation rules:
+    If all items are constants the type will be aggregated from all items.
+    If there are some non-constant items then only types of non-constant
+    items will be used for aggregation.
+    If there are DATE/TIME fields/functions in the list and no string
+    fields/functions in the list then:
+      The INT_RESULT type will be used for aggregation instead of orginal
+      result type of any DATE/TIME field/function in the list
+      All constant items in the list will be converted to a DATE/TIME using
+      found field or result field of found function.
+
+    Implementation notes:
+      The code is equvalent to:
+      1. Check the list for presense of a STRING field/function.
+         Collect the is_const flag.
+      2. Get a Field* object to use for type coercion
+      3. Perform type conversion.
+      1 and 2 are implemented in 2 loops. The first searches for a DATE/TIME
+      field/function and checks presense of a STRING field/function.
+      The second loop works only if a DATE/TIME field/function is found.
+      It checks presense of a STRING field/function in the rest of the list.
+
+  TODO
+    1) The current implementation can produce false comparison results for
+    expressions like:
+      date_time_field BETWEEN string_field_with_dates AND string_constant
+    if the string_constant will omit some of leading zeroes.
+    In order to fully implement correct comparison of DATE/TIME the new
+    DATETIME_RESULT result type should be introduced and agg_cmp_type()
+    should return the DATE/TIME field used for the conversion. Later
+    this field can be used by comparison functions like Item_func_between to
+    convert string values to ints on the fly and thus return correct results.
+    This modification will affect functions BETWEEN, IN and CASE.
+
+    2) If in the list a DATE field/function and a DATETIME field/function
+    are present in the list then the first found field/function will be
+    used for conversion. This may lead to wrong results and probably should
+    be fixed.
+*/
+
+static void agg_cmp_type(THD *thd, Item_result *type, Item **items, uint nitems)
 {
   uint i;
-  type[0]= items[0]->result_type();
-  for (i=1 ; i < nitems ; i++)
-    type[0]= item_cmp_type(type[0], items[i]->result_type());
+  Item::Type res;
+  char *buff= NULL;
+  uchar null_byte;
+  Field *field= NULL;
+
+  /* Search for date/time fields/functions */
+  for (i= 0; i < nitems; i++)
+  {
+    if (!items[i]->result_as_longlong())
+    {
+      /* Do not convert anything if a string field/function is present */
+      if (!items[i]->const_item() && items[i]->result_type() == STRING_RESULT)
+      {
+        i= nitems;
+        break;
+      }
+      continue;
+    }
+    if ((res= items[i]->real_item()->type()) == Item::FIELD_ITEM)
+    {
+      field= ((Item_field *)items[i]->real_item())->field;
+      break;
+    }
+    else if (res == Item::FUNC_ITEM)
+    {
+      field= items[i]->tmp_table_field_from_field_type(0);
+      if (field)
+        buff= alloc_root(thd->mem_root, field->max_length());
+      if (!buff || !field)
+      {
+        if (field)
+          delete field;
+        if (buff)
+          my_free(buff, MYF(MY_WME));
+        field= 0;
+      }
+      else
+        field->move_field(buff, &null_byte, 0);
+      break;
+    }
+  }
+  if (field)
+  {
+    /* Check the rest of the list for presense of a string field/function. */
+    for (i++ ; i < nitems; i++)
+    {
+      if (!items[i]->const_item() && items[i]->result_type() == STRING_RESULT &&
+          !items[i]->result_as_longlong())
+      {
+        field= 0;
+        break;
+      }
+    }
+  }
+  /* Reset to 0 on first occurence of non-const item. 1 otherwise */
+  bool is_const= items[0]->const_item();
+  /*
+    If the first item is a date/time function then its result should be
+    compared as int
+  */
+  if (field)
+  {
+    /* Suppose we are comparing dates and some non-constant items are present. */
+    type[0]= INT_RESULT;
+    is_const= 0;
+  }
+  else
+    type[0]= items[0]->result_type();
+
+  for (i= 0; i < nitems ; i++)
+  {
+    if (!items[i]->const_item())
+    {
+      Item_result result= field && items[i]->result_as_longlong() ?
+                            INT_RESULT : items[i]->result_type();
+      type[0]= is_const ? result : item_cmp_type(type[0], result);
+      is_const= 0;
+    }
+    else if (is_const)
+      type[0]= item_cmp_type(type[0], items[i]->result_type());
+    else if (field)
+      convert_constant_item(thd, field, &items[i]);
+  }
+
+  if (res == Item::FUNC_ITEM && field)
+  {
+    delete field;
+    my_free(buff, MYF(MY_WME));
+  }
 }
 
 static void my_coll_agg_error(DTCollation &c1, DTCollation &c2,
@@ -896,31 +1041,9 @@ void Item_func_between::fix_length_and_dec()
   */
   if (!args[0] || !args[1] || !args[2])
     return;
-  agg_cmp_type(&cmp_type, args, 3);
-  if (cmp_type == STRING_RESULT &&
-      agg_arg_charsets(cmp_collation, args, 3, MY_COLL_CMP_CONV))
-    return;
-
-  /*
-    Make a special case of compare with date/time and longlong fields.
-    They are compared as integers, so for const item this time-consuming
-    conversion can be done only once, not for every single comparison
-  */
-  if (args[0]->type() == FIELD_ITEM)
-  {
-    Field *field=((Item_field*) args[0])->field;
-    if (field->can_be_compared_as_longlong())
-    {
-      /*
-        The following can't be recoded with || as convert_constant_item
-        changes the argument
-      */
-      if (convert_constant_item(thd, field,&args[1]))
-	cmp_type=INT_RESULT;			// Works for all types.
-      if (convert_constant_item(thd, field,&args[2]))
-	cmp_type=INT_RESULT;			// Works for all types.
-    }
-  }
+  agg_cmp_type(thd, &cmp_type, args, 3);
+  if (cmp_type == STRING_RESULT)
+      agg_arg_charsets(cmp_collation, args, 3, MY_COLL_CMP_CONV);
 }
 
 
@@ -1444,7 +1567,7 @@ void Item_func_case::fix_length_and_dec()
     for (nagg= 0; nagg < ncases/2 ; nagg++)
       agg[nagg+1]= args[nagg*2];
     nagg++;
-    agg_cmp_type(&cmp_type, agg, nagg);
+    agg_cmp_type(current_thd, &cmp_type, agg, nagg);
     if ((cmp_type == STRING_RESULT) &&
         agg_arg_charsets(cmp_collation, agg, nagg, MY_COLL_CMP_CONV))
     return;
@@ -1925,7 +2048,7 @@ void Item_func_in::fix_length_and_dec()
   uint const_itm= 1;
   THD *thd= current_thd;
   
-  agg_cmp_type(&cmp_type, args, arg_count);
+  agg_cmp_type(thd, &cmp_type, args, arg_count);
 
   if (cmp_type == STRING_RESULT &&
       agg_arg_charsets(cmp_collation, args, arg_count, MY_COLL_CMP_CONV))
diff --git a/sql/item_cmpfunc.h b/sql/item_cmpfunc.h
index ade09113c63..08d9de6ffd6 100644
--- a/sql/item_cmpfunc.h
+++ b/sql/item_cmpfunc.h
@@ -43,8 +43,11 @@ public:
   int set_compare_func(Item_bool_func2 *owner, Item_result type);
   inline int set_compare_func(Item_bool_func2 *owner_arg)
   {
-    return set_compare_func(owner_arg, item_cmp_type((*a)->result_type(),
-						     (*b)->result_type()));
+    Item_result ar= (*a)->result_as_longlong() && (*b)->const_item() ?
+                      INT_RESULT : (*a)->result_type();
+    Item_result br= (*b)->result_as_longlong() && (*a)->const_item() ?
+                      INT_RESULT : (*b)->result_type();
+    return set_compare_func(owner_arg, item_cmp_type(ar, br));
   }
   inline int set_cmp_func(Item_bool_func2 *owner_arg,
 			  Item **a1, Item **a2,
@@ -57,8 +60,11 @@ public:
   inline int set_cmp_func(Item_bool_func2 *owner_arg,
 			  Item **a1, Item **a2)
   {
-    return set_cmp_func(owner_arg, a1, a2, item_cmp_type((*a1)->result_type(),
-							 (*a2)->result_type()));
+    Item_result ar= (*a1)->result_as_longlong() && (*a2)->const_item() ?
+                      INT_RESULT : (*a1)->result_type();
+    Item_result br= (*a2)->result_as_longlong() && (*a1)->const_item() ?
+                      INT_RESULT : (*a2)->result_type();
+    return set_cmp_func(owner_arg, a1, a2, item_cmp_type(ar, br));
   }
   inline int compare() { return (this->*func)(); }
 
diff --git a/sql/item_strfunc.cc b/sql/item_strfunc.cc
index e817edac6c0..deb3542f4a5 100644
--- a/sql/item_strfunc.cc
+++ b/sql/item_strfunc.cc
@@ -268,11 +268,13 @@ String *Item_func_concat::val_str(String *str)
   DBUG_ASSERT(fixed == 1);
   String *res,*res2,*use_as_buff;
   uint i;
+  bool is_const= 0;
 
   null_value=0;
   if (!(res=args[0]->val_str(str)))
     goto null;
   use_as_buff= &tmp_value;
+  is_const= args[0]->const_item();
   for (i=1 ; i < arg_count ; i++)
   {
     if (res->length() == 0)
@@ -295,7 +297,7 @@ String *Item_func_concat::val_str(String *str)
 			    current_thd->variables.max_allowed_packet);
 	goto null;
       }
-      if (res->alloced_length() >= res->length()+res2->length())
+      if (!is_const && res->alloced_length() >= res->length()+res2->length())
       {						// Use old buffer
 	res->append(*res2);
       }
@@ -350,6 +352,7 @@ String *Item_func_concat::val_str(String *str)
 	res= &tmp_value;
 	use_as_buff=str;
       }
+      is_const= 0;
     }
   }
   res->set_charset(collation.collation);
diff --git a/sql/item_timefunc.cc b/sql/item_timefunc.cc
index 5fdbd968df1..f7b4c9dd630 100644
--- a/sql/item_timefunc.cc
+++ b/sql/item_timefunc.cc
@@ -2306,6 +2306,20 @@ String *Item_datetime_typecast::val_str(String *str)
 }
 
 
+longlong Item_datetime_typecast::val_int()
+{
+  DBUG_ASSERT(fixed == 1);
+  TIME ltime;
+  if (get_arg0_date(&ltime,1))
+  {
+    null_value= 1;
+    return 0;
+  }
+
+  return TIME_to_ulonglong_datetime(&ltime);
+}
+
+
 bool Item_time_typecast::get_time(TIME *ltime)
 {
   bool res= get_arg0_time(ltime);
@@ -2320,6 +2334,17 @@ bool Item_time_typecast::get_time(TIME *ltime)
 }
 
 
+longlong Item_time_typecast::val_int()
+{
+  TIME ltime;
+  if (get_time(&ltime))
+  {
+    null_value= 1;
+    return 0;
+  }
+  return ltime.hour * 10000L + ltime.minute * 100 + ltime.second;
+}
+
 String *Item_time_typecast::val_str(String *str)
 {
   DBUG_ASSERT(fixed == 1);
@@ -2359,6 +2384,14 @@ String *Item_date_typecast::val_str(String *str)
   return 0;
 }
 
+longlong Item_date_typecast::val_int()
+{
+  DBUG_ASSERT(fixed == 1);
+  TIME ltime;
+  if (args[0]->get_date(&ltime, TIME_FUZZY_DATE))
+    return 0;
+  return (longlong) (ltime.year * 10000L + ltime.month * 100 + ltime.day);
+}
 
 /*
   MAKEDATE(a,b) is a date function that creates a date value 
@@ -2395,6 +2428,33 @@ err:
 }
 
 
+longlong Item_func_makedate::val_int()
+{
+  DBUG_ASSERT(fixed == 1);
+  TIME l_time;
+  long daynr=  (long) args[1]->val_int();
+  long yearnr= (long) args[0]->val_int();
+  long days;
+
+  if (args[0]->null_value || args[1]->null_value ||
+      yearnr < 0 || daynr <= 0)
+    goto err;
+
+  days= calc_daynr(yearnr,1,1) + daynr - 1;
+  /* Day number from year 0 to 9999-12-31 */
+  if (days >= 0 && days < MAX_DAY_NUMBER)
+  {
+    null_value=0;
+    get_date_from_daynr(days,&l_time.year,&l_time.month,&l_time.day);
+    return (longlong) (l_time.year * 10000L + l_time.month * 100 + l_time.day);
+  }
+
+err:
+  null_value= 1;
+  return 0;
+}
+
+
 void Item_func_add_time::fix_length_and_dec()
 {
   enum_field_types arg0_field_type;
diff --git a/sql/item_timefunc.h b/sql/item_timefunc.h
index 163b1591e52..95f1dfa75d1 100644
--- a/sql/item_timefunc.h
+++ b/sql/item_timefunc.h
@@ -339,6 +339,7 @@ public:
   {
     return (new Field_date(maybe_null, name, t_arg, &my_charset_bin));
   }  
+  bool result_as_longlong() { return TRUE; }
 };
 
 
@@ -354,6 +355,7 @@ public:
   {
     return (new Field_datetime(maybe_null, name, t_arg, &my_charset_bin));
   }
+  bool result_as_longlong() { return TRUE; }
 };
 
 
@@ -383,6 +385,7 @@ public:
     TIME representation using UTC-SYSTEM or per-thread time zone.
   */
   virtual void store_now_in_TIME(TIME *now_time)=0;
+  bool result_as_longlong() { return TRUE; }
 };
 
 
@@ -588,6 +591,7 @@ public:
   {
     return (new Field_time(maybe_null, name, t_arg, &my_charset_bin));
   }
+  bool result_as_longlong() { return TRUE; }
 };
 
 /*
@@ -715,6 +719,8 @@ public:
     max_length= 10;
     maybe_null= 1;
   }
+  bool result_as_longlong() { return TRUE; }
+  longlong val_int();
 };
 
 
@@ -731,6 +737,8 @@ public:
   {
     return (new Field_time(maybe_null, name, t_arg, &my_charset_bin));
   }
+  bool result_as_longlong() { return TRUE; }
+  longlong val_int();
 };
 
 
@@ -746,6 +754,8 @@ public:
   {
     return (new Field_datetime(maybe_null, name, t_arg, &my_charset_bin));
   }
+  bool result_as_longlong() { return TRUE; }
+  longlong val_int();
 };
 
 class Item_func_makedate :public Item_str_func
@@ -764,6 +774,8 @@ public:
   {
     return (new Field_date(maybe_null, name, t_arg, &my_charset_bin));
   }
+  bool result_as_longlong() { return TRUE; }
+  longlong val_int();
 };
 
 
diff --git a/sql/opt_range.cc b/sql/opt_range.cc
index 71f937f90c6..67141aab6ce 100644
--- a/sql/opt_range.cc
+++ b/sql/opt_range.cc
@@ -64,7 +64,7 @@ public:
 	  uint8 min_flag, uint8 max_flag, uint8 maybe_flag);
   SEL_ARG(enum Type type_arg)
     :elements(1),use_count(1),left(0),next_key_part(0),color(BLACK),
-     type(type_arg)
+     type(type_arg),min_flag(0)
   {}
   inline bool is_same(SEL_ARG *arg)
   {
diff --git a/sql/opt_sum.cc b/sql/opt_sum.cc
index cfb5b3695a3..97e271121d3 100644
--- a/sql/opt_sum.cc
+++ b/sql/opt_sum.cc
@@ -582,7 +582,8 @@ static bool matching_cond(bool max_fl, TABLE_REF *ref, KEY *keyinfo,
     }
     else
     {
-      store_val_in_field(part->field, args[between && max_fl ? 2 : 1]);
+      store_val_in_field(part->field, args[between && max_fl ? 2 : 1],
+                         CHECK_FIELD_IGNORE);
       if (part->null_bit) 
         *key_ptr++= (byte) test(part->field->is_null());
       part->field->get_key_image((char*) key_ptr, part->length,
@@ -638,6 +639,8 @@ static bool matching_cond(bool max_fl, TABLE_REF *ref, KEY *keyinfo,
         field BETWEEN const1 AND const2
      3. all references to the columns from the same table as column field
         occur only in conjucts mentioned above.
+     4. each of k first components the index is not partial, i.e. is not
+        defined on a fixed length proper prefix of the field.
 
      If such an index exists the function through the ref parameter
      returns the key value to find max/min for the field using the index,
@@ -647,8 +650,8 @@ static bool matching_cond(bool max_fl, TABLE_REF *ref, KEY *keyinfo,
       of the whole search key)
 
   NOTE
-   This function may set table->key_read to 1, which must be reset after
-   index is used! (This can only happen when function returns 1)
+    This function may set table->key_read to 1, which must be reset after
+    index is used! (This can only happen when function returns 1)
 
   RETURN
     0   Index can not be used to optimize MIN(field)/MAX(field)
@@ -682,6 +685,10 @@ static bool find_key_for_maxmin(bool max_fl, TABLE_REF *ref,
       if (!(table->file->index_flags(idx, jdx, 0) & HA_READ_ORDER))
         return 0;
 
+        /* Check whether the index component is partial */
+      if (part->length < table->field[part->fieldnr-1]->pack_length())
+        break;
+
       if (field->eq(part->field))
       {
         ref->key= idx;
diff --git a/sql/sql_parse.cc b/sql/sql_parse.cc
index 51ef3f31b26..06005f31198 100644
--- a/sql/sql_parse.cc
+++ b/sql/sql_parse.cc
@@ -3047,8 +3047,8 @@ unsent_create_error:
       }
     }
 
-    if (!thd->is_fatal_error && (result= new multi_delete(thd,aux_tables,
-							  table_count)))
+    if (!res && !thd->is_fatal_error &&
+        (result= new multi_delete(thd,aux_tables, table_count)))
     {
       res= mysql_select(thd, &select_lex->ref_pointer_array,
 			select_lex->get_table_list(),
diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index 57fb9738612..5a7e9e52aed 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -3434,7 +3434,7 @@ get_store_key(THD *thd, KEYUSE *keyuse, table_map used_tables,
 */
 
 bool
-store_val_in_field(Field *field,Item *item)
+store_val_in_field(Field *field, Item *item, enum_check_fields check_flag)
 {
   bool error;
   THD *thd=current_thd;
@@ -3445,7 +3445,7 @@ store_val_in_field(Field *field,Item *item)
     with select_insert, which make count_cuted_fields= 1
    */
   enum_check_fields old_count_cuted_fields= thd->count_cuted_fields;
-  thd->count_cuted_fields= CHECK_FIELD_WARN;
+  thd->count_cuted_fields= check_flag;
   error= item->save_in_field(field, 1);
   thd->count_cuted_fields= old_count_cuted_fields;
   return error || cuted_fields != thd->cuted_fields;
@@ -7097,7 +7097,7 @@ static bool test_if_ref(Item_field *left_item,Item *right_item)
 	    field->real_type() != FIELD_TYPE_VAR_STRING &&
 	    (field->type() != FIELD_TYPE_FLOAT || field->decimals() == 0))
 	{
-	  return !store_val_in_field(field,right_item);
+	  return !store_val_in_field(field, right_item, CHECK_FIELD_WARN);
 	}
       }
     }
diff --git a/sql/sql_select.h b/sql/sql_select.h
index 94cc8839466..75cd0b4d797 100644
--- a/sql/sql_select.h
+++ b/sql/sql_select.h
@@ -335,7 +335,7 @@ extern const char *join_type_str[];
 void TEST_join(JOIN *join);
 
 /* Extern functions in sql_select.cc */
-bool store_val_in_field(Field *field,Item *val);
+bool store_val_in_field(Field *field, Item *val, enum_check_fields check_flag);
 TABLE *create_tmp_table(THD *thd,TMP_TABLE_PARAM *param,List<Item> &fields,
 			ORDER *group, bool distinct, bool save_sum_fields,
 			ulong select_options, ha_rows rows_limit,
diff --git a/sql/structs.h b/sql/structs.h
index 081ada88bf7..2037496635a 100644
--- a/sql/structs.h
+++ b/sql/structs.h
@@ -70,7 +70,13 @@ typedef struct st_key_part_info {	/* Info about a key part */
   Field *field;
   uint	offset;				/* offset in record (from 0) */
   uint	null_offset;			/* Offset to null_bit in record */
-  uint16 length;			/* Length of key_part */
+  uint16 length;                        /* Length of keypart value in bytes */
+  /* 
+    Number of bytes required to store the keypart value. This may be
+    different from the "length" field as it also counts
+     - possible NULL-flag byte (see HA_KEY_NULL_LENGTH)
+     - possible HA_KEY_BLOB_LENGTH bytes needed to store actual value length.
+  */
   uint16 store_length;
   uint16 key_type;
   uint16 fieldnr;			/* Fieldnum in UNIREG */