how to print barcode in c# net BUILDING YOUR OWN STORAGE ENGINE in Font

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CHAPTER 7 BUILDING YOUR OWN STORAGE ENGINE
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Caution This is the point at which we start adding functionality beyond that of the example engine. Be
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sure to correctly specify your return codes. The example engine tells the optimizer a function is not supported by issuing the return statement DBUG_RETURN(HA_ERR_WRONG_COMMAND);. Be sure to change these to something other than the wrong command return code (e.g., 0).
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The next method you need to change is rnd_next(), which is responsible for getting the next record from the file and detecting the end of the file. In this method, you can call the data class read_row() method, passing in the record buffer, the length of the buffer, and the current position in the file. Notice the return for the end of the file and the setting of more statistics. The method also records the current position so the next call to the method will advance the file to the next record. Listing 7-22 shows the updated method with the changes. Listing 7-22. Changes to the rnd_next() Method in ha_spartan.cc int ha_spartan::rnd_next(byte *buf) { int rc; DBUG_ENTER("ha_spartan::rnd_next"); ha_statistic_increment(&SSV::ha_read_rnd_next_count); /* Read the row from the data file. */ rc = share->data_class->read_row(buf, table->s->rec_buff_length, current_position); if (rc != -1) current_position = (off_t)share->data_class->cur_position(); else DBUG_RETURN(HA_ERR_END_OF_FILE); records++; DBUG_RETURN(0); } The Spartan_data class is nice because it stores the records in the same format as the MySQL internal buffer. In fact, it just writes a few bytes of a header for each record storing a deleted flag and the record length (for use in scanning and repairing). If you were working on a storage engine that stored the data in a different format, you would need to perform the translation at this point. A sample of how that translation could be accomplished is found in the ha_tina.cc file. The process looks something like this: for (Field **field=table->field ; *field ; field++) { /* copy field data to your own storage type */ my_value = (*field)->val_str(); my_store_field(my_value); }
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CHAPTER 7 BUILDING YOUR OWN STORAGE ENGINE
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In this example, you are iterating through the field array, writing out the data in your own format. Look for the ha_tina::find_current_row() method for an example. The next method you need to change is position(), which records the current position of the file in the MySQL pointer storage mechanism. It is called after each call to rnd_next(). The methods for storing and retrieving these pointers are my_store_ptr() and my_get_ptr(). The store pointer method takes a reference variable (the place you want to store something), the length of what you want to store, and the thing you want to store as parameters. The get pointer method takes a reference variable and the length of what you are retrieving and returns the item stored. These methods are used in the case of an order by rows where the data will need to be sorted. Take a look at the changes for the position() method shown in Listing 7-23 to see how you can call the store pointer method. Listing 7-23. Changes to the position() Method in ha_spartan.cc void ha_spartan::position(const byte *record) { DBUG_ENTER("ha_spartan::position"); my_store_ptr(ref, ref_length, current_position); DBUG_VOID_RETURN; } The next method you need to change is rnd_pos(), which is where you ll retrieve the current position stored and then read in the row from that position. Notice in this method we also increment the read statistic ha_read_rnd_next_count. This provides the optimizer information about how many rows there are in the table and can be helpful in optimizing later queries. Listing 7-24 shows the updated method with the changes. Listing 7-24. Changes to the rnd_pos() Method in ha_spartan.cc int ha_spartan::rnd_pos(byte * buf, byte *pos) { DBUG_ENTER("ha_spartan::rnd_pos"); ha_statistic_increment(&SSV::ha_read_rnd_next_count); current_position = (off_t)my_get_ptr(pos,ref_length); share->data_class->read_row(buf, current_position, -1); DBUG_RETURN(0); } The next method you need to change is info(), which returns information to the optimizer to help choose an optimal execution path. This is an interesting method to implement, and when you read the comments in the source code it ll seem humorous. What you need to do in this method is to return the number of records. MySQL AB states that you should always return a value of 2 or more. This disengages portions of the optimizer that are wasteful for a record set of one row. Listing 7-25 shows the updated info() method.
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