field_conv.cc

Go to the documentation of this file.

/* Copyright (c) 2000, 2013, 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
   the Free Software Foundation; version 2 of the License.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA */


#include "sql_priv.h"
#include "sql_class.h"                          // THD
#include "sql_time.h"
#include <m_ctype.h>

static void do_field_eq(Copy_field *copy)
{
  memcpy(copy->to_ptr,copy->from_ptr,copy->from_length);
}

static void do_field_1(Copy_field *copy)
{
  copy->to_ptr[0]=copy->from_ptr[0];
}

static void do_field_2(Copy_field *copy)
{
  copy->to_ptr[0]=copy->from_ptr[0];
  copy->to_ptr[1]=copy->from_ptr[1];
}

static void do_field_3(Copy_field *copy)
{
  copy->to_ptr[0]=copy->from_ptr[0];
  copy->to_ptr[1]=copy->from_ptr[1];
  copy->to_ptr[2]=copy->from_ptr[2];
}

static void do_field_4(Copy_field *copy)
{
  copy->to_ptr[0]=copy->from_ptr[0];
  copy->to_ptr[1]=copy->from_ptr[1];
  copy->to_ptr[2]=copy->from_ptr[2];
  copy->to_ptr[3]=copy->from_ptr[3];
}

static void do_field_6(Copy_field *copy)
{                                               // For blob field
  copy->to_ptr[0]=copy->from_ptr[0];
  copy->to_ptr[1]=copy->from_ptr[1];
  copy->to_ptr[2]=copy->from_ptr[2];
  copy->to_ptr[3]=copy->from_ptr[3];
  copy->to_ptr[4]=copy->from_ptr[4];
  copy->to_ptr[5]=copy->from_ptr[5];
}

static void do_field_8(Copy_field *copy)
{
  copy->to_ptr[0]=copy->from_ptr[0];
  copy->to_ptr[1]=copy->from_ptr[1];
  copy->to_ptr[2]=copy->from_ptr[2];
  copy->to_ptr[3]=copy->from_ptr[3];
  copy->to_ptr[4]=copy->from_ptr[4];
  copy->to_ptr[5]=copy->from_ptr[5];
  copy->to_ptr[6]=copy->from_ptr[6];
  copy->to_ptr[7]=copy->from_ptr[7];
}

static void do_field_to_null_str(Copy_field *copy)
{
  if (*copy->null_row ||
      (copy->from_null_ptr && (*copy->from_null_ptr & copy->from_bit)))
  {
    memset(copy->to_ptr, 0, copy->from_length);
    copy->to_null_ptr[0]=1;                     // Always bit 1
  }
  else
  {
    copy->to_null_ptr[0]=0;
    memcpy(copy->to_ptr,copy->from_ptr,copy->from_length);
  }
}


type_conversion_status set_field_to_null(Field *field)
{
  if (field->real_maybe_null())
  {
    field->set_null();
    field->reset();
    return TYPE_OK;
  }
  field->reset();
  switch (field->table->in_use->count_cuted_fields) {
  case CHECK_FIELD_WARN:
    field->set_warning(Sql_condition::WARN_LEVEL_WARN, WARN_DATA_TRUNCATED, 1);
    /* fall through */
  case CHECK_FIELD_IGNORE:
    return TYPE_OK;
  case CHECK_FIELD_ERROR_FOR_NULL:
    if (!field->table->in_use->no_errors)
      my_error(ER_BAD_NULL_ERROR, MYF(0), field->field_name);
    return TYPE_ERR_NULL_CONSTRAINT_VIOLATION;
  }
  DBUG_ASSERT(false); // impossible
  return TYPE_ERR_NULL_CONSTRAINT_VIOLATION;
}


type_conversion_status
set_field_to_null_with_conversions(Field *field, bool no_conversions)
{
  if (field->real_maybe_null())
  {
    field->set_null();
    field->reset();
    return TYPE_OK;
  }
  if (no_conversions)
    return TYPE_ERR_NULL_CONSTRAINT_VIOLATION;

  /*
    Check if this is a special type, which will get a special walue
    when set to NULL (TIMESTAMP fields which allow setting to NULL
    are handled by first check).

    From the manual:

    TIMESTAMP columns [...] assigning NULL assigns the current timestamp.

    But if explicit_defaults_for_timestamp, use standard-compliant behaviour:
    no special value.
  */
  if (field->type() == MYSQL_TYPE_TIMESTAMP &&
      !field->table->in_use->variables.explicit_defaults_for_timestamp)
  {
    Item_func_now_local::store_in(field);
    return TYPE_OK;                     // Ok to set time to NULL
  }
  
  // Note: we ignore any potential failure of reset() here.
  field->reset();

  if (field == field->table->next_number_field)
  {
    field->table->auto_increment_field_not_null= FALSE;
    return TYPE_OK;                     // field is set in fill_record()
  }
  switch (field->table->in_use->count_cuted_fields) {
  case CHECK_FIELD_WARN:
    field->set_warning(Sql_condition::WARN_LEVEL_WARN, ER_BAD_NULL_ERROR, 1);
    /* fall through */
  case CHECK_FIELD_IGNORE:
    return TYPE_OK;
  case CHECK_FIELD_ERROR_FOR_NULL:
    if (!field->table->in_use->no_errors)
      my_error(ER_BAD_NULL_ERROR, MYF(0), field->field_name);
    return TYPE_ERR_NULL_CONSTRAINT_VIOLATION;
  }
  DBUG_ASSERT(false); // impossible
  return TYPE_ERR_NULL_CONSTRAINT_VIOLATION;
}


static void do_skip(Copy_field *copy __attribute__((unused)))
{
}


static void do_copy_null(Copy_field *copy)
{
  if (*copy->null_row ||
      (copy->from_null_ptr && (*copy->from_null_ptr & copy->from_bit)))
  {
    *copy->to_null_ptr|=copy->to_bit;
    copy->to_field->reset();
  }
  else
  {
    *copy->to_null_ptr&= ~copy->to_bit;
    (copy->do_copy2)(copy);
  }
}


static void do_copy_not_null(Copy_field *copy)
{
  if (*copy->null_row ||
      (copy->from_null_ptr && (*copy->from_null_ptr & copy->from_bit)))
  {
    copy->to_field->set_warning(Sql_condition::WARN_LEVEL_WARN,
                                WARN_DATA_TRUNCATED, 1);
    copy->to_field->reset();
  }
  else
    (copy->do_copy2)(copy);
}


static void do_copy_maybe_null(Copy_field *copy)
{
  *copy->to_null_ptr&= ~copy->to_bit;
  (copy->do_copy2)(copy);
}

/* timestamp and next_number has special handling in case of NULL values */

static void do_copy_timestamp(Copy_field *copy)
{
  if (*copy->null_row || (*copy->from_null_ptr & copy->from_bit))
  {
    /* Same as in set_field_to_null_with_conversions() */
    Item_func_now_local::store_in(copy->to_field);
  }
  else
    (copy->do_copy2)(copy);
}


static void do_copy_next_number(Copy_field *copy)
{
  if (*copy->null_row || (*copy->from_null_ptr & copy->from_bit))
  {
    /* Same as in set_field_to_null_with_conversions() */
    copy->to_field->table->auto_increment_field_not_null= FALSE;
    copy->to_field->reset();
  }
  else
    (copy->do_copy2)(copy);
}


static void do_copy_blob(Copy_field *copy)
{
  ulong length=((Field_blob*) copy->from_field)->get_length();
  ((Field_blob*) copy->to_field)->store_length(length);
  memcpy(copy->to_ptr, copy->from_ptr, sizeof(char*));
}

static void do_conv_blob(Copy_field *copy)
{
  copy->from_field->val_str(&copy->tmp);
  ((Field_blob *) copy->to_field)->store(copy->tmp.ptr(),
                                         copy->tmp.length(),
                                         copy->tmp.charset());
}

static void do_save_blob(Copy_field *copy)
{
  char buff[MAX_FIELD_WIDTH];
  String res(buff,sizeof(buff),copy->tmp.charset());
  copy->from_field->val_str(&res);
  copy->tmp.copy(res);
  ((Field_blob *) copy->to_field)->store(copy->tmp.ptr(),
                                         copy->tmp.length(),
                                         copy->tmp.charset());
}


static void do_field_string(Copy_field *copy)
{
  char buff[MAX_FIELD_WIDTH];
  String res(buff, sizeof(buff), copy->from_field->charset());
  res.length(0U);

  copy->from_field->val_str(&res);
  copy->to_field->store(res.c_ptr_quick(), res.length(), res.charset());
}


static void do_field_enum(Copy_field *copy)
{
  if (copy->from_field->val_int() == 0)
    ((Field_enum *) copy->to_field)->store_type((ulonglong) 0);
  else
    do_field_string(copy);
}


static void do_field_varbinary_pre50(Copy_field *copy)
{
  char buff[MAX_FIELD_WIDTH];
  copy->tmp.set_quick(buff,sizeof(buff),copy->tmp.charset());
  copy->from_field->val_str(&copy->tmp);

  /* Use the same function as in 4.1 to trim trailing spaces */
  uint length= my_lengthsp_8bit(&my_charset_bin, copy->tmp.c_ptr_quick(),
                                copy->from_field->field_length);

  copy->to_field->store(copy->tmp.c_ptr_quick(), length,
                        copy->tmp.charset());
}


static void do_field_int(Copy_field *copy)
{
  longlong value= copy->from_field->val_int();
  copy->to_field->store(value,
                        test(copy->from_field->flags & UNSIGNED_FLAG));
}

static void do_field_real(Copy_field *copy)
{
  double value=copy->from_field->val_real();
  copy->to_field->store(value);
}


static void do_field_decimal(Copy_field *copy)
{
  my_decimal value;
  copy->to_field->store_decimal(copy->from_field->val_decimal(&value));
}


inline type_conversion_status copy_time_to_time(Field *from, Field *to)
{
  MYSQL_TIME ltime;
  from->get_time(&ltime);
  return to->store_time(&ltime);
}

static void do_field_time(Copy_field *copy)
{
  (void) copy_time_to_time(copy->from_field, copy->to_field);
}


static void do_cut_string(Copy_field *copy)
{
  const CHARSET_INFO *cs= copy->from_field->charset();
  memcpy(copy->to_ptr,copy->from_ptr,copy->to_length);

  /* Check if we loosed any important characters */
  if (cs->cset->scan(cs,
                     (char*) copy->from_ptr + copy->to_length,
                     (char*) copy->from_ptr + copy->from_length,
                     MY_SEQ_SPACES) < copy->from_length - copy->to_length)
  {
    copy->to_field->set_warning(Sql_condition::WARN_LEVEL_WARN,
                                WARN_DATA_TRUNCATED, 1);
  }
}


static void do_cut_string_complex(Copy_field *copy)
{                                               // Shorter string field
  int well_formed_error;
  const CHARSET_INFO *cs= copy->from_field->charset();
  const uchar *from_end= copy->from_ptr + copy->from_length;
  uint copy_length= cs->cset->well_formed_len(cs,
                                              (char*) copy->from_ptr,
                                              (char*) from_end, 
                                              copy->to_length / cs->mbmaxlen,
                                              &well_formed_error);
  if (copy->to_length < copy_length)
    copy_length= copy->to_length;
  memcpy(copy->to_ptr, copy->from_ptr, copy_length);

  /* Check if we lost any important characters */
  if (well_formed_error ||
      cs->cset->scan(cs, (char*) copy->from_ptr + copy_length,
                     (char*) from_end,
                     MY_SEQ_SPACES) < (copy->from_length - copy_length))
  {
    copy->to_field->set_warning(Sql_condition::WARN_LEVEL_WARN,
                                WARN_DATA_TRUNCATED, 1);
  }

  if (copy_length < copy->to_length)
    cs->cset->fill(cs, (char*) copy->to_ptr + copy_length,
                   copy->to_length - copy_length, ' ');
}




static void do_expand_binary(Copy_field *copy)
{
  const CHARSET_INFO *cs= copy->from_field->charset();
  memcpy(copy->to_ptr,copy->from_ptr,copy->from_length);
  cs->cset->fill(cs, (char*) copy->to_ptr+copy->from_length,
                     copy->to_length-copy->from_length, '\0');
}



static void do_expand_string(Copy_field *copy)
{
  const CHARSET_INFO *cs= copy->from_field->charset();
  memcpy(copy->to_ptr,copy->from_ptr,copy->from_length);
  cs->cset->fill(cs, (char*) copy->to_ptr+copy->from_length,
                     copy->to_length-copy->from_length, ' ');
}

static uint get_varstring_copy_length(Field_varstring *to,
                                      const Field_varstring *from)
{
  const CHARSET_INFO * const cs= from->charset();
  const bool is_multibyte_charset= (cs->mbmaxlen != 1);
  const uint to_byte_length= to->row_pack_length();

  uint bytes_to_copy;
  if (from->length_bytes == 1)
    bytes_to_copy= *from->ptr;
  else
    bytes_to_copy= uint2korr(from->ptr);

  if (is_multibyte_charset)
  {
    int well_formed_error;
    const char *from_beg= reinterpret_cast<char*>(from->ptr + from->length_bytes);
    const uint to_char_length= (to_byte_length) / cs->mbmaxlen;
    const uint from_byte_length= bytes_to_copy;
    bytes_to_copy=
      cs->cset->well_formed_len(cs, from_beg,
                                from_beg + from_byte_length,
                                to_char_length,
                                &well_formed_error);
    if (bytes_to_copy < from_byte_length)
    {
      if (from->table->in_use->count_cuted_fields)
        to->set_warning(Sql_condition::WARN_LEVEL_WARN,
                        WARN_DATA_TRUNCATED, 1);
    }
  }
  else
  {
    if (bytes_to_copy > (to_byte_length))
    {
      bytes_to_copy= to_byte_length;
      if (from->table->in_use->count_cuted_fields)
        to->set_warning(Sql_condition::WARN_LEVEL_WARN,
                        WARN_DATA_TRUNCATED, 1);
    }
  }
  return bytes_to_copy;
}

static void copy_field_varstring(Field_varstring * const to,
                                 const Field_varstring * const from)
{
  const uint length_bytes= from->length_bytes;
  DBUG_ASSERT(length_bytes == to->length_bytes);
  DBUG_ASSERT(length_bytes == 1 || length_bytes == 2);

  const uint bytes_to_copy= get_varstring_copy_length(to, from);
  if (length_bytes == 1)
    *to->ptr= static_cast<uchar>(bytes_to_copy);
  else
    int2store(to->ptr, bytes_to_copy);

  // memcpy should not be used for overlaping memory blocks
  DBUG_ASSERT(to->ptr != from->ptr);
  memcpy(to->ptr + length_bytes, from->ptr + length_bytes, bytes_to_copy);
}

static void do_varstring(Copy_field *copy)
{
  copy_field_varstring(static_cast<Field_varstring*>(copy->to_field),
                       static_cast<Field_varstring*>(copy->from_field));
}


/***************************************************************************
** The different functions that fills in a Copy_field class
***************************************************************************/

void Copy_field::set(uchar *to,Field *from)
{
  from_ptr=from->ptr;
  to_ptr=to;
  from_length=from->pack_length();
  null_row= &from->table->null_row;
  if (from->maybe_null())
  {
    from_null_ptr=from->null_ptr;
    from_bit=     from->null_bit;
    to_ptr[0]=    1;                            // Null as default value
    to_null_ptr=  (uchar*) to_ptr++;
    to_bit=       1;
    do_copy=      do_field_to_null_str;
  }
  else
  {
    to_null_ptr=  0;                            // For easy debugging
    do_copy=      do_field_eq;
  }
}


/*
  To do: 

  If 'save' is set to true and the 'from' is a blob field, do_copy is set to
  do_save_blob rather than do_conv_blob.  The only differences between them
  appears to be:

  - do_save_blob allocates and uses an intermediate buffer before calling 
    Field_blob::store. Is this in order to trigger the call to 
    well_formed_copy_nchars, by changing the pointer copy->tmp.ptr()?
    That call will take place anyway in all known cases.
 */
void Copy_field::set(Field *to,Field *from,bool save)
{
  if (to->type() == MYSQL_TYPE_NULL)
  {
    to_null_ptr=0;                              // For easy debugging
    to_ptr=0;
    do_copy=do_skip;
    return;
  }
  from_field=from;
  to_field=to;
  from_ptr=from->ptr;
  from_length=from->pack_length();
  to_ptr=  to->ptr;
  to_length=to_field->pack_length();

  // set up null handling
  from_null_ptr=to_null_ptr=0;
  null_row= &from->table->null_row;
  if (from->maybe_null())
  {
    from_null_ptr=      from->null_ptr;
    from_bit=           from->null_bit;
    if (to_field->real_maybe_null())
    {
      to_null_ptr=      to->null_ptr;
      to_bit=           to->null_bit;
      do_copy=  do_copy_null;
    }
    else
    {
      if (to_field->type() == MYSQL_TYPE_TIMESTAMP)
        do_copy= do_copy_timestamp;               // Automatic timestamp
      else if (to_field == to_field->table->next_number_field)
        do_copy= do_copy_next_number;
      else
        do_copy= do_copy_not_null;
    }
  }
  else if (to_field->real_maybe_null())
  {
    to_null_ptr=        to->null_ptr;
    to_bit=             to->null_bit;
    do_copy= do_copy_maybe_null;
  }
  else
   do_copy=0;

  if ((to->flags & BLOB_FLAG) && save)
    do_copy2= do_save_blob;
  else
    do_copy2= get_copy_func(to,from);
  if (!do_copy)                                 // Not null
    do_copy=do_copy2;
}


Copy_field::Copy_func *
Copy_field::get_copy_func(Field *to,Field *from)
{
  bool compatible_db_low_byte_first= (to->table->s->db_low_byte_first ==
                                     from->table->s->db_low_byte_first);
  if (to->flags & BLOB_FLAG)
  {
    if (!(from->flags & BLOB_FLAG) || from->charset() != to->charset())
      return do_conv_blob;
    if (from_length != to_length || !compatible_db_low_byte_first)
    {
      // Correct pointer to point at char pointer
      to_ptr+=   to_length - portable_sizeof_char_ptr;
      from_ptr+= from_length - portable_sizeof_char_ptr;
      return do_copy_blob;
    }
  }
  else
  {
    if (to->real_type() == MYSQL_TYPE_BIT ||
        from->real_type() == MYSQL_TYPE_BIT)
      return do_field_int;
    if (to->result_type() == DECIMAL_RESULT)
      return do_field_decimal;
    // Check if identical fields
    if (from->result_type() == STRING_RESULT)
    {
      if (from->is_temporal())
      {
        if (to->is_temporal())
        {
          return do_field_time;
        }
        else
        {
          if (to->result_type() == INT_RESULT)
            return do_field_int;
          if (to->result_type() == REAL_RESULT)
            return do_field_real;
          /* Note: conversion from any to DECIMAL_RESULT is handled earlier */
        }
      }
      /*
        Detect copy from pre 5.0 varbinary to varbinary as of 5.0 and
        use special copy function that removes trailing spaces and thus
        repairs data.
      */
      if (from->type() == MYSQL_TYPE_VAR_STRING && !from->has_charset() &&
          to->type() == MYSQL_TYPE_VARCHAR && !to->has_charset())
        return do_field_varbinary_pre50;

      /*
        If we are copying date or datetime's we have to check the dates
        if we don't allow 'all' dates.
      */
      if (to->real_type() != from->real_type() ||
          to->decimals() != from->decimals() /* e.g. TIME vs TIME(6) */ ||
          !compatible_db_low_byte_first ||
          (((to->table->in_use->variables.sql_mode &
            (MODE_NO_ZERO_IN_DATE | MODE_NO_ZERO_DATE | MODE_INVALID_DATES)) &&
           to->type() == MYSQL_TYPE_DATE) ||
           to->type() == MYSQL_TYPE_DATETIME))
      {
        if (from->real_type() == MYSQL_TYPE_ENUM ||
            from->real_type() == MYSQL_TYPE_SET)
          if (to->result_type() != STRING_RESULT)
            return do_field_int;                // Convert SET to number
        return do_field_string;
      }
      if (to->real_type() == MYSQL_TYPE_ENUM ||
          to->real_type() == MYSQL_TYPE_SET)
      {
        if (!to->eq_def(from))
        {
          if (from->real_type() == MYSQL_TYPE_ENUM &&
              to->real_type() == MYSQL_TYPE_ENUM)
            return do_field_enum;
          else
            return do_field_string;
        }
      }
      else if (to->charset() != from->charset())
        return do_field_string;
      else if (to->real_type() == MYSQL_TYPE_VARCHAR)
      {
        if (((Field_varstring*) to)->length_bytes !=
            ((Field_varstring*) from)->length_bytes)
          return do_field_string;
        else
          return do_varstring;
      }
      else if (to_length < from_length)
        return (from->charset()->mbmaxlen == 1 ?
                do_cut_string : do_cut_string_complex);
      else if (to_length > from_length)
      {
        if (to->charset() == &my_charset_bin)
          return do_expand_binary;
        else
          return do_expand_string;
      }

    }
    else if (to->real_type() != from->real_type() ||
             to_length != from_length ||
             !compatible_db_low_byte_first)
    {
      if (to->real_type() == MYSQL_TYPE_DECIMAL ||
          to->result_type() == STRING_RESULT)
        return do_field_string;
      if (to->result_type() == INT_RESULT)
        return do_field_int;
      return do_field_real;
    }
    else
    {
      if (!to->eq_def(from) || !compatible_db_low_byte_first)
      {
        if (to->real_type() == MYSQL_TYPE_DECIMAL)
          return do_field_string;
        if (to->result_type() == INT_RESULT)
          return do_field_int;
        else
          return do_field_real;
      }
    }
  }
    /* Eq fields */
  switch (to_length) {
  case 1: return do_field_1;
  case 2: return do_field_2;
  case 3: return do_field_3;
  case 4: return do_field_4;
  case 6: return do_field_6;
  case 8: return do_field_8;
  }
  return do_field_eq;
}


type_conversion_status field_conv(Field *to,Field *from)
{
  if (to->real_type() == from->real_type() &&
      !(to->type() == MYSQL_TYPE_BLOB && to->table->copy_blobs) &&
      to->charset() == from->charset())
  {
    if (to->real_type() == MYSQL_TYPE_VARCHAR &&
        from->real_type() == MYSQL_TYPE_VARCHAR)
    {
      Field_varstring *to_vc= static_cast<Field_varstring*>(to);
      const Field_varstring *from_vc= static_cast<Field_varstring*>(from);
      if (to_vc->length_bytes == from_vc->length_bytes)
      {
        copy_field_varstring(to_vc, from_vc);
        return TYPE_OK;
      }
    }
    if (to->pack_length() == from->pack_length() &&
        !(to->flags & UNSIGNED_FLAG && !(from->flags & UNSIGNED_FLAG)) &&
        to->real_type() != MYSQL_TYPE_ENUM &&
        to->real_type() != MYSQL_TYPE_SET &&
        to->real_type() != MYSQL_TYPE_BIT &&
        (!to->is_temporal_with_time() ||
         to->decimals() == from->decimals()) &&
        (to->real_type() != MYSQL_TYPE_NEWDECIMAL ||
         (to->field_length == from->field_length &&
          (((Field_num*)to)->dec == ((Field_num*)from)->dec))) &&
        to->table->s->db_low_byte_first == from->table->s->db_low_byte_first &&
        (!(to->table->in_use->variables.sql_mode &
           (MODE_NO_ZERO_IN_DATE | MODE_NO_ZERO_DATE | MODE_INVALID_DATES)) ||
         (to->type() != MYSQL_TYPE_DATE &&
          to->type() != MYSQL_TYPE_DATETIME &&
          (!to->table->in_use->variables.explicit_defaults_for_timestamp ||
           to->type() != MYSQL_TYPE_TIMESTAMP))) &&
         (from->real_type() != MYSQL_TYPE_VARCHAR))
    {                                           // Identical fields
      // to->ptr==from->ptr may happen if one does 'UPDATE ... SET x=x'
      memmove(to->ptr, from->ptr, to->pack_length());
      return TYPE_OK;
    }
  }
  if (to->type() == MYSQL_TYPE_BLOB)
  {                                             // Be sure the value is stored
    Field_blob *blob=(Field_blob*) to;
    from->val_str(&blob->value);
    /*
      Copy value if copy_blobs is set, or source is not a string and
      we have a pointer to its internal string conversion buffer.
    */
    if (to->table->copy_blobs ||
        (!blob->value.is_alloced() &&
         from->real_type() != MYSQL_TYPE_STRING &&
         from->real_type() != MYSQL_TYPE_VARCHAR))
      blob->value.copy();
    return blob->store(blob->value.ptr(),blob->value.length(),from->charset());
  }
  if (from->real_type() == MYSQL_TYPE_ENUM &&
      to->real_type() == MYSQL_TYPE_ENUM &&
      from->val_int() == 0)
  {
    ((Field_enum *)(to))->store_type(0);
    return TYPE_OK;
  }
  else if (from->is_temporal() && to->result_type() == INT_RESULT)
  {
    MYSQL_TIME ltime;
    longlong nr;
    if (from->type() == MYSQL_TYPE_TIME)
    {
      from->get_time(&ltime);
      nr= TIME_to_ulonglong_time_round(&ltime);
    }
    else
    {
      from->get_date(&ltime, TIME_FUZZY_DATE);
      nr= TIME_to_ulonglong_datetime_round(&ltime);
    }
    return to->store(ltime.neg ? -nr : nr, 0);
  }
  else if (from->is_temporal() &&
           (to->result_type() == REAL_RESULT ||
            to->result_type() == DECIMAL_RESULT ||
            to->result_type() == INT_RESULT))
  {
    my_decimal tmp;
    /*
      We prefer DECIMAL as the safest precise type:
      double supports only 15 digits, which is not enough for DATETIME(6).
    */
    return to->store_decimal(from->val_decimal(&tmp));
  }
  else if (from->is_temporal() && to->is_temporal())
  {
    return copy_time_to_time(from, to);
  }
  else if ((from->result_type() == STRING_RESULT &&
            (to->result_type() == STRING_RESULT ||
             (from->real_type() != MYSQL_TYPE_ENUM &&
              from->real_type() != MYSQL_TYPE_SET))) ||
           to->type() == MYSQL_TYPE_DECIMAL)
  {
    char buff[MAX_FIELD_WIDTH];
    String result(buff,sizeof(buff),from->charset());
    from->val_str(&result);
    /*
      We use c_ptr_quick() here to make it easier if to is a float/double
      as the conversion routines will do a copy of the result doesn't
      end with \0. Can be replaced with .ptr() when we have our own
      string->double conversion.
    */
    return to->store(result.c_ptr_quick(),result.length(),from->charset());
  }
  else if (from->result_type() == REAL_RESULT)
    return to->store(from->val_real());
  else if (from->result_type() == DECIMAL_RESULT)
  {
    my_decimal buff;
    return to->store_decimal(from->val_decimal(&buff));
  }
  else
    return to->store(from->val_int(), test(from->flags & UNSIGNED_FLAG));
}