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15.9.6. Implementing the rnd_next() Function

After the table is initialized, the MySQL server will call the handler's rnd_next() function once for every row to be scanned until the server's search condition is satisfied or an end of file is reached, in which case the handler returns HA_ERR_END_OF_FILE.

The rnd_next() function takes a single byte array parameter named *buf. The *buf parameter must be populated with the contents of the table row in the internal MySQL format.

The server uses three data formats: fixed-length rows, variable-length rows, and variable-length rows with BLOB pointers. In each format, the columns appear in the order in which they were defined by the CREATE TABLE statement. (The table definition is stored in the .frm file, and the optimizer and the handler are both able to access table metadata from the same source, its TABLE structure).

Each format begins with a “NULL bitmap” of one bit per nullable column. A table with as many as eight nullable columns will have a one-byte bitmap; a table with nine to sixteen nullable columns will have a two-byte bitmap, and so forth. One exception is fixed-width tables, which have an additional starting bit so that a table with eight nullable columns would have a two-byte bitmap.

After the NULL bitmap come the columns, one by one. Each column is of the size indicated in Chapter 11, Data Types. In the server, column data types are defined in the sql/field.cc file. In the fixed length row format, the columns are simply laid out one by one. In a variable-length row, VARCHAR columns are coded as a one or two-byte length, followed by a string of characters. In a variable-length row with BLOB columns, each blob is represented by two parts: first an integer representing the actual size of the BLOB, and then a pointer to the BLOB in memory.

Examples of row conversion (or “packing”) can be found by starting at rnd_next() in any table handler. In ha_tina.cc, for example, the code in find_current_row() illustrates how the TABLE structure (pointed to by table) and a string object (named buffer) can be used to pack character data from a CSV file. Writing a row back to disk requires the opposite conversion, unpacking from the internal format.

The following example is from the CSV storage engine:

int ha_tina::rnd_next(byte *buf)
{
   DBUG_ENTER("ha_tina::rnd_next");
 
   statistic_increment(table->in_use->status_var.ha_read_rnd_next_count, &LOCK_status);
 
   current_position= next_position;
   if (!share->mapped_file)
     DBUG_RETURN(HA_ERR_END_OF_FILE);
   if (HA_ERR_END_OF_FILE == find_current_row(buf) )
     DBUG_RETURN(HA_ERR_END_OF_FILE);
 
   records++;
   DBUG_RETURN(0);
}  

The conversion from the internal row format to CSV row format is performed in the find_current_row() function:

int ha_tina::find_current_row(byte *buf)
{
   byte *mapped_ptr= (byte *)share->mapped_file + current_position;
   byte *end_ptr;
   DBUG_ENTER("ha_tina::find_current_row");
 
   /* EOF should be counted as new line */
   if ((end_ptr=  find_eoln(share->mapped_file, current_position,
                            share->file_stat.st_size)) == 0)
     DBUG_RETURN(HA_ERR_END_OF_FILE);
 
   for (Field **field=table->field ; *field ; field++)
   {
     buffer.length(0);
     mapped_ptr++; // Increment past the first quote
     for(;mapped_ptr != end_ptr; mapped_ptr++)
     {
       // Need to convert line feeds!
       if (*mapped_ptr == '"' &&
           (((mapped_ptr[1] == ',') && (mapped_ptr[2] == '"')) ||
            (mapped_ptr == end_ptr -1 )))
       {
         mapped_ptr += 2; // Move past the , and the "
         break;
       }
       if (*mapped_ptr == '\\' && mapped_ptr != (end_ptr - 1))
       {
         mapped_ptr++;
         if (*mapped_ptr == 'r')
           buffer.append('\r');
         else if (*mapped_ptr == 'n' )
           buffer.append('\n');
         else if ((*mapped_ptr == '\\') || (*mapped_ptr == '"'))
           buffer.append(*mapped_ptr);
         else  /* This could only happed with an externally created file */
         {
           buffer.append('\\');
           buffer.append(*mapped_ptr);
         }
       }
       else
         buffer.append(*mapped_ptr);
     }
     (*field)->store(buffer.ptr(), buffer.length(), system_charset_info);
   }
   next_position= (end_ptr - share->mapped_file)+1;
   /* Maybe use \N for null? */
   memset(buf, 0, table->s->null_bytes); /* We do not implement nulls! */
 
   DBUG_RETURN(0);
}  

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