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15.15.4. Parsing Key Information

Many of the index methods pass a byte array named *key that identifies the index entry to be read in a standard format. Your storage engine will need to extract the information stored in the key and translate it into its internal index format to identify the row associated with the index.

The information in the key is obtained by iterating through the key, which is formatted the same as the definition in table->key_info[index]->key_part[part_num]. The following example from ha_berkeley.cc shows how the BerkeleyDB storage engine takes a key defined in *key and converts it to internal key format:

/*
Create a packed key from a MySQL unpacked key (like the one that is
sent from the index_read()
  
This key is to be used to read a row
*/
  
DBT *ha_berkeley::pack_key(DBT *key, uint keynr, char *buff,
                                                const byte *key_ptr, uint key_length)
{
  KEY *key_info=table->key_info+keynr;
  KEY_PART_INFO *key_part=key_info->key_part;
  KEY_PART_INFO *end=key_part+key_info->key_parts;
  DBUG_ENTER("bdb:pack_key");
  
  bzero((char*) key,sizeof(*key));
  key->data=buff;
  key->app_private= (void*) key_info;
  
  for (; key_part != end && (int) key_length > 0 ; key_part++)
  {
      uint offset=0;
      if (key_part->null_bit)
      {
          if (!(*buff++ = (*key_ptr == 0)))         // Store 0 if NULL
          {
              key_length-= key_part->store_length;
              key_ptr+=   key_part->store_length;
              key->flags|=DB_DBT_DUPOK;
              continue;
          }
          offset=1;                                 // Data is at key_ptr+1
      }
      buff=key_part->field->pack_key_from_key_image(buff,(char*) key_ptr+offset,
      key_part->length);
      key_ptr+=key_part->store_length;
      key_length-=key_part->store_length;
  }
  key->size= (buff  - (char*) key->data);
  DBUG_DUMP("key",(char*) key->data, key->size);
  DBUG_RETURN(key);
}

 
 
  Published under the terms of the GNU General Public License Design by Interspire