Friday, 1 June 2012

Walk through of a simple SELECT (MySQL way)

Journey of a SQL SELECT statement in MySQL RDBMS engine :

Understanding the basics of a query execution is key to success in understanding core concepts of any DBMS engine. Let's examine how a simple SELECT finds its way through most of the component (at a Higher level) to a print its output to the screen (stdout). The MySQL architecture consists of the following major components :
1) MySQL Client
2) Query Cache
3) Parser
4) Preprocessor
5) Query optimizer
6) Query execution engine
7) Storage engine APIs
8) Data

a) Suppose we want to find all details of an employee having ID 999. A simple SQL query might look something like this "SELECT * FROM employee.employees WHERE id = 999;". Now to execute the query we would want to start a MySQL client session and run the query (MySQL Client is a utility to execute all MySQL commands and return an output).

b) MySQL Client once started will show a prompt something like "mysql >". When a client session is established a "MySQL THREAD" is created in MySQL to handle all the command executions and sessions.

c) Upon entering the above SELECT query, the query is submitted to the MySQL server which is a "mysqld" process running on the remote or local server (mysql process is a client and mysqld process is a server in this case and they need not be on the same host).

d) Once entering these server process the query is checked against a list of previously executed SELECTs maintained by MySQL DBMS engine in an area of memory known as "QUERY CACHE". Query cache holds the output of SELECT statements (exactly the same statements) executed earlier. So if another user had executed "SELECT * FROM employee.employees WHERE id = 999;" previously, the result set will be stored in the Query cache. And if we again issue the same query to MySQL again it will directly output the result set stored in Query cache. This saves MySQL from doing any work of parsing, preprocessing and retrieving data from disk or memory resulting in lightning fast output. However there are some limitations which can be studied in depth later.

e) If the same exact query is not present in QUERY CACHE, MySQL will move to the next component i.e "PARSER" to parse the Query Syntax. Syntax is very important part of query execution. What if the query we executed was something like this "SELECT * employee.employees WHERE id = 999;", MySQL parser will parse the query thoroughly to find out that the "FROM" is missing in the statement and immediately return us an error. Parsing at such an early stage in executions avoids MySQL from consuming resources to scan the tables and object on disk as well as memory.

f) Once the syntax checking is done (and hopefully the syntax is correct), MySQL will move onto next component i.e. "QUERY PREPROCESSOR". As the name suggest, the preprocessor will check the GRANTS of user on the objects he/she wants to access through the query (We want to retrieve data from `employee` database's `employees` table). If the user doesn't have the appropriate GRANTS, MySQL will report an error. Also the preprocessor is responsible for object checking (Checking to see if `employee` database exist and `employees` table exist), in case object specified doesnt exist MySQL will report an error.

g) After preprocessing if we have the required GRANTS and the also object exists, then MySQL will take the query to the most important component i.e "QUERY OPTIMIZER". This is the place where MySQL will decide on the execution plan to access and retrieve the data as fast as possible. Query Optimizer is responsible for checking which indexes exist on the specified table and if any exist whether it can use the index to retrieve data faster. An index here on `id` column of `employees` table will help the storage engine layer to locate and retrieve data faster. Once confirmed MySQL will create an "EXECUTION PLAN" and pass on the plan to "Storage engine" layer.

NOTE : All of the above steps are independent of the Storage engine used by MySQL. Hence we can refer the above components as part of "SERVER layer" and below referred components as the "STORAGE engine layer". This is one of biggest advantage of MySQL over any RDBMS product available. For more info on Storage engines refer the MySQL reference manual.

h) Moving ahead, the STORAGE engine APIs now have the query execution plan, which it can use to access and retrieve the required indexes into the memory and also retrieve the "DATA" from exact memory address specified in the indexes. This index lookup in memory is of magnitudes faster as compared to random disk access.

i) DATA can be stored differently depending upon the storage engine used, for eg : MyISAM and InnoDB storage engines will have DATA stored in files (on DISK), whereas Memory storage engine will store data in Memory (RAM).

Once the desired data block is located on the disk by the Storage engine API, MySQL or O.S. will cache the data in memory (RAM) and display the result set to client's screen (stdout).

Please refer to the diagram above.

Processing a SELECT is different from an INSERT, UPDATE and DELETE, hence do not refer this for the same. Hope you found it useful.


  1. Hi Akshay,

    Thank you for such a wonderful article.

    This article has really pinched me in believing the fact that strong basics are really a great foundation for building the castle of knowledge.

    Looking forward for more such interesting article.

    Thanks again,

  2. Hi Akshay, Thank you, it is very easy to understand for beginners like me.