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MySQL Query Parsing

One can identify the following modules in the server:
• Server Initialization Module
• Connection Manager
• Thread Manager
• Connection Thread
• User Authentication Module
• Access Control Module
• Parser
• Command Dispatcher
• Query Cache Module
• Optimizer
• Table Manager
  Table Modification Modules
• Table Maintenance Module
• Status Reporting Module
• Abstracted Storage Engine Interface (Table Handler)
• Storage Engine Implementations (MyISAM, InnoDB, MEMORY, Berkeley DB)
• Logging Module
• Replication Master Module
• Replication Slave Module
• Client/Server Protocol API
• Low-Level Network I/O API
• Core API


When the server is started on the command line, the Initialization Module takes control.
It parses the configuration file and the command-line arguments, allocates global memory buffers, initializes global variables and structures, loads the access control tables, and performs a number of other initialization tasks. Once the initialization job is complete, the Initialization Module passes control to the Connection Manager, which starts listening for connections from clients in a loop.
When a client connects to the database server, the Connection Manager performs a number of low-level network protocol tasks and then passes control to the Thread Manager, which in turn supplies a thread to handle the connection (which from now on will be referred to as the Connection Thread). The Connection Thread might be created a new, or retrieved from the thread cache and called to active duty. Once the Connection Thread receives control, it first invokes the User Authentication Module.The credentials of the connecting user are verified, and the client may now issue requests.
The Connection Thread passes the request data to the Command Dispatcher. Some requests, known in the MySQL code terminology as commands, can be accommodated by the Command Dispatcher directly, while more complex ones need to be redirected to another module. A typical command may request the server to run a query, change the active database, report the status, send a continuous dump of the replication updates, close the connection, or perform some other operation.
In MySQL server terminology, there are two types of client requests: a query and a command. A query is anything that has to go through the parser. A command is a request that can be executed without the need to invoke the parser. We will use the term query in the context of MySQL internals. Thus, not only a SELECT but also a DELETE or INSERT in our terminology would be called a query. What we would call a query is sometimes called an SQL statement.

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