MySQL’s Logical Architecture

A good mental picture of how MySQL’s components work together will help you
understand the server. The topmost layer contains the services that aren’t unique to MySQL. They’re services most network-based client/server tools or servers need: connection handling, authentication, security, and so forth.

The second layer is where things get interesting. Much of MySQL’s brains are here, including the code for query parsing, analysis, optimization, caching, and all the built-in functions (e.g., dates, times, math, and encryption). Any functionality provided across storage engines lives at this level: stored procedures, triggers, and views, for example.

The third layer contains the storage engines. They are responsible for storing and retrieving all data stored “in” MySQL. Like the various filesystems available for GNU/Linux, each storage engine has its own benefits and drawbacks. The server communicates with them through the storage engine API. This interface hides differences between storage engines and makes them largely transparent at the query layer. The API contains a couple of dozen low-level functions that perform operations such as “begin a transaction” or “fetch the row that has this primary key.” The storage engines don’t parse SQL* or communicate with each other; they simply respond to requests from the server.

Connection Management and Security
Each client connection gets its own thread within the server process. The connection’s queries execute within that single thread, which in turn resides on one core or CPU. The server caches threads, so they don’t need to be created and destroyed for each new connection.

When clients (applications) connect to the MySQL server, the server needs to authenticate them. Authentication is based on username, originating host, and password. X.509 certificates can also be used across an Secure Sockets Layer (SSL) connection. Once a client has connected, the server verifies whether the client has privileges for each query it issues (e.g., whether the client is allowed to issue a SELECT statement that accesses the Country table in the world database).

Optimization and Execution
MySQL parses queries to create an internal structure (the parse tree), and then applies a variety of optimizations. These may include rewriting the query, determining the order in which it will read tables, choosing which indexes to use, and so on. You can pass hints to the optimizer through special keywords in the query, affecting its decision-making process. You can also ask the server to explain various aspects of optimization. This lets you know what decisions the server is making and gives you a reference point for reworking queries, schemas, and settings to make everything run as efficiently as possible. The optimizer does not really care what storage engine a particular table uses, but
the storage engine does affect how the server optimizes query. The optimizer asks the
storage engine about some of its capabilities and the cost of certain operations, and
for statistics on the table data. For instance, some storage engines support index
types that can be helpful to certain queries.

Before even parsing the query, though, the server consults the query cache, which can store only SELECT statements, along with their result sets. If anyone issues a query that’s identical to one already in the cache, the server doesn’t need to parse, optimize, or execute the query at all—it can simply pass back the stored result set!

Source of Information : MySQL (4th Edition)


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