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CHAPTER 3 A TOUR OF THE MYSQL SOURCE CODE
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The first thing to notice is the creation of a packet buffer and a NET structure. This packet buffer is a character array and stores the raw query string as it is read from the network and stored in the NET structure. The next item that is created is a command structure, which will be used to route control to the appropriate parser functions. The my_net_read() function reads the packets from the network and stores them in the NET structure. The length of the packet is also stored in the packet_length variable of the NET structure. The last thing you see occurring in this function is a call to dispatch_command(), the point at which you can begin to see how commands are routed through the server code. OK, so now you re starting to get somewhere. The job of the dispatch_command() function is to route control to a portion of the server that can best process the incoming command. Since you have a normal SELECT query on the way, the system has identified it as a query by setting the command variable to COM_QUERY. Other command types are used to identify statements, change user, generate statistics, and many other server functions. For this chapter, I will only look at query commands (COM_QUERY). Listing 3-6 shows a condensed view of the function. I have omitted the code for all of the other commands in the switch for the sake of brevity (I m omitting the comment break too) but I m leaving in the case statements for most of the commands. Take a moment and scan through the list. Most of the names are self-explanatory. If you were to conduct this exploration for another type of query, you could find your way by looking in this function for the type identified and following the code along in that case statement. I have also included the large function comment block that appears before the function code. Take a moment to look at that. I ll be getting more into that later in this chapter. Listing 3-6. The dispatch_command() Function /* Perform one connection-level (COM_XXXX) command. SYNOPSIS dispatch_command() thd connection handle command type of command to perform packet data for the command, packet is always null-terminated packet_length length of packet + 1 (to show that data is null-terminated) except for COM_SLEEP, where it can be zero. RETURN VALUE 0 ok 1 request of thread shutdown, i. e. if command is COM_QUIT/COM_SHUTDOWN */ bool dispatch_command(enum enum_server_command command, THD *thd, char* packet, uint packet_length) { ...
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CHAPTER 3 A TOUR OF THE MYSQL SOURCE CODE
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switch (command) { case COM_INIT_DB: ... case COM_REGISTER_SLAVE: ... case COM_TABLE_DUMP: ... case COM_CHANGE_USER: ... case COM_STMT_EXECUTE: ... case COM_STMT_FETCH: ... case COM_STMT_SEND_LONG_DATA: ... case COM_STMT_PREPARE: ... case COM_STMT_CLOSE: ... case COM_STMT_RESET: ... case COM_QUERY: { if (alloc_query(thd, packet, packet_length)) break; // fatal error is set ... general_log_print(thd, command, "%s", thd->query); ... mysql_parse(thd,thd->query, thd->query_length); ... } case ... case ... case ... case ...
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COM_FIELD_LIST: COM_QUIT: COM_BINLOG_DUMP: COM_REFRESH:
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CHAPTER 3 A TOUR OF THE MYSQL SOURCE CODE
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case COM_STATISTICS: ... case COM_PING: ... case COM_PROCESS_INFO: ... case COM_PROCESS_KILL: ... case COM_SET_OPTION: ... case COM_DEBUG: ... case COM_SLEEP: ... case COM_DELAYED_INSERT: ... case COM_END: ... default: ... } The first thing that happens when control passes to the COM_QUERY handler is the query is copied from the packet array to the thd->query member variable via the alloc_query() function. In this way, the thread now has a copy of the query, which will stay with it all through its execution. Notice also that the code writes the command to the general log. This will help with debugging system problems and query issues later on. The last function call of interest in Listing 3-6 is the mysql_parse() function call. It is at this point that the code can officially transfer from the SQL Interface subsystem to the Query Parser subsystem. As you can see, this distinction is one of semantics rather than syntax.
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