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Let s assume you want to add a command to the server to show the current disk usage of all of the databases in the server. Although there are external tools that can retrieve this information, you desire a SQL equivalent function that you can easily use in your own databasedriven applications. Let s also assume you want to add this as a SHOW command. Specifically, you want to be able to execute the command SHOW DISK_USAGE and retrieve a result set that has each database listed as a row along with the total size of all of the files (tables) listed in kilobytes. Adding a new SQL command involves adding symbols to the lexical analyzer and adding the SHOW DISK_USAGE command syntax to the YACC parser (sql_yacc.yy). The new parser must be compiled into a C program by Bison and then a new lexical hash created using the gen_lex_hash utility described earlier. The code for the parser to direct control to the new command is placed in the large case statement in sql_parse.cc with a case for the new command symbol. Let s begin with adding the symbols to the lexical analyzer. Open the lex.h file and locate the static SYMBOL symbols[] array. You can make the symbol anything you want, but it should be something meaningful (like all good variable names). Be sure to choose a symbol that isn t already in use. In this case, use the symbol DISK_USAGE. This acts like a label to the parser identifying it as a token. Place a statement in the array to direct the lexical analyzer to generate the symbol and call it DISK_USAGE_SYM. The list is in (roughly) alphabetic order, so place it in the proper location. Listing 8-17 shows an excerpt of the array with the symbols added. Listing 8-17. Updates to the lex.h File for the SHOW DISK_USAGE Command static SYMBOL symbols[] = { { "&&", SYM(AND_AND_SYM)}, ... { "DISK", SYM(DISK_SYM)}, /* BEGIN CAB MODIFICATION */ /* Reason for Modification: */ /* This section identifies the tokens for the SHOW DISK_USAGE command*/ { "DISK_USAGE", SYM(DISK_USAGE_SYM)}, /* END CAB MODIFICATION */ { "DISTINCT", SYM(DISTINCT)}, ... The next thing you need to do is add a mnemonic to identify the command. This mnemonic will be used in the parser to assign to the internal query structure and to control the flow of execution via a case in the large switch statement in the sql_parse.cc file. Open the sql_lex.h file and add the new command to the enum_sql_command enumeration. Listing 8-18 shows the modifications with the new command mnemonic. Listing 8-18. Changes to the sql_lex.h File for the SHOW DISK_USAGE Command enum enum_sql_command { SQLCOM_SELECT, SQLCOM_CREATE_TABLE, SQLCOM_CREATE_INDEX, SQLCOM_ALTER_TABLE, SQLCOM_UPDATE, SQLCOM_INSERT, SQLCOM_INSERT_SELECT, SQLCOM_DELETE, SQLCOM_TRUNCATE, SQLCOM_DROP_TABLE, SQLCOM_DROP_INDEX, ...

SQLCOM_SHOW_COLUMN_TYPES, SQLCOM_SHOW_STORAGE_ENGINES, SQLCOM_SHOW_PRIVILEGES, /* BEGIN CAB MODIFICATION */ /* Reason for Modification: */ /* This section captures the enumerations for the SHOW DISK_USAGE command tokens */ SQLCOM_SHOW_DISK_USAGE, /* END CAB MODIFICATION */ SQLCOM_HELP, SQLCOM_CREATE_USER, SQLCOM_DROP_USER, SQLCOM_RENAME_USER, ... Now that you have the new symbol and the command mnemonic, you now need to add code to the sql_yacc.yy file to define the new token that you used in the lex.h file and add the source code for the new SHOW DISK_USAGE SQL command. Open the sql_yacc.yy file and add the new token to the list of tokens (near the top). These are defined (roughly) in alphabetical order, so place the new token in the proper order. Listing 8-19 shows the modifications to the sql_yacc.yy file. Listing 8-19. Adding the Token to the sql_yacc.yy File ... %token DISK_SYM /* BEGIN CAB MODIFICATION */ /* Reason for Modification: */ /* This section defines the tokens for the SHOW DISK_USAGE command */ %token DISK_USAGE_SYM /* END CAB MODIFICATION */ %token DISTINCT ...

Note If you use Windows, the sql_yacc.yy file is not included in the source code distribution. You must

download one of the Linux source code distributions and retrieve the file from there. Make sure you use the same version of the Linux source code that matches your Windows source code.

You also need to add the command syntax to the parser YACC code (also in sql_yacc.yy). Locate the show: label and add the command as shown in Listing 8-20. Listing 8-20. Parser Syntax Source Code for the SHOW DISK_USAGE Command show: /* BEGIN CAB MODIFICATION */ /* Reason for Modification: */ /* This section captures (parses) the SHOW ALV statement */ SHOW DISK_USAGE_SYM { LEX *lex=Lex; lex->sql_command= SQLCOM_SHOW_DISK_USAGE;