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CHAPTER 7 BUILDING YOUR OWN STORAGE ENGINE
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/* return number of deleted records */ int Spartan_data::del_records() { DBUG_ENTER("Spartan_data::num_records"); DBUG_RETURN(number_del_records); } /* read header from file */ int Spartan_data::read_header() { int i; int len; DBUG_ENTER("Spartan_data::read_header"); if (number_records == -1) { my_seek(data_file, 0l, MY_SEEK_SET, MYF(0)); i = my_read(data_file, (byte *)&crashed, sizeof(bool), MYF(0)); i = my_read(data_file, (byte *)&len, sizeof(int), MYF(0)); memcpy(&number_records, &len, sizeof(int)); i = my_read(data_file, (byte *)&len, sizeof(int), MYF(0)); memcpy(&number_del_records, &len, sizeof(int)); } else my_seek(data_file, header_size, MY_SEEK_SET, MYF(0)); DBUG_RETURN(0); } /* write header to file */ int Spartan_data::write_header() { int i; DBUG_ENTER("Spartan_data::write_header"); if (number_records != -1) { my_seek(data_file, 0l, MY_SEEK_SET, MYF(0)); i = my_write(data_file, (byte *)&crashed, sizeof(bool), MYF(0)); i = my_write(data_file, (byte *)&number_records, sizeof(int), MYF(0)); i = my_write(data_file, (byte *)&number_del_records, sizeof(int), MYF(0)); } DBUG_RETURN(0); }
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CHAPTER 7 BUILDING YOUR OWN STORAGE ENGINE
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/* get position of the data file */ long long Spartan_data::cur_position() { long long pos; DBUG_ENTER("Spartan_data::cur_position"); pos = my_seek(data_file, 0L, MY_SEEK_CUR, MYF(0)); if (pos == 0) DBUG_RETURN(header_size); DBUG_RETURN(pos); } /* truncate the data file */ int Spartan_data::trunc_table() { DBUG_ENTER("Spartan_data::trunc_table"); if (data_file != -1 ) { my_chsize(data_file, 0, 0, MYF(MY_WME)); write_header(); } DBUG_RETURN(0); } /* determine the row size of the data file */ int Spartan_data::row_size(int length) { DBUG_ENTER("Spartan_data::row_size"); DBUG_RETURN(length + record_header_size); } Note the format I use to store the data. The class is designed to support reading data from disk and writing the data in memory to disk. I use a byte pointer to allocate a block of memory for storing the rows. What makes this really useful is that it provides the ability to write the rows in the table to disk using the internal MySQL row format. Likewise, I can read the data from disk and write it to a memory buffer and simply point the handler class to the block of memory to be returned to the optimizer. However, I may not be able to predict the exact amount of memory needed to store a row. Some uses of the storage engine may have tables that have variable fields or even binary large objects (BLOBs). To overcome this problem, I chose to store a single integer length field at the start of each row. This allows me to scan a file and read variable-length rows by first reading the length field and then reading the number of bytes specified into the memory buffer.
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Tip Whenever coding an extension for the MySQL server, you should always use the my_xxx utility methods. The my_xxx utility methods are encapsulations of many of the base operating systems functions and provide a better level of cross-platform support.
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CHAPTER 7 BUILDING YOUR OWN STORAGE ENGINE
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The data class is rather straightforward and can be used to implement the basic read and write operations needed for a storage engine. However, I want to make the storage engine more efficient. To achieve good performance from my data file, I need to add an index mechanism. This is where things get a lot more complicated.
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The Spartan_index Class
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To solve the problem of indexing the data file, I implement a separate index class called Spartan_index. The index class is responsible for permitting the execution of point queries (query by index for a specific record), range queries (a series of keys either ascending or descending), as well as the ability to cache the index for fast searching. Listing 7-5 includes the complete header file for the Spartan_index class. Listing 7-5. Spartan_index Class Header /* spartan_index.h This header file defines a simple index class that can be used to store file pointer indexes (long long). The class keeps the entire index in memory for fast access. The internal memory structure is a linked list. While not as efficient as a B-tree, it should be usable for most testing environments. The constructor accepts the max key length. This is used for all nodes in the index. File Layout: SOF max_key_len (int) SOF + sizeof(int) crashed (bool) SOF + sizeof(int) + sizeof(bool) DATA BEGINS HERE */ #include "my_global.h" #include "my_sys.h" const long METADATA_SIZE = sizeof(int) + sizeof(bool); /* This is the node that stores the key and the file position for the data row. */ struct SDE_INDEX { byte *key; long long pos; int length; };
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