MySQL stored programs can often add to application functionality and developer efficiency, and there are certainly many cases where the use of a procedural language such as the MySQL stored program language can do things that a nonprocedural language like SQL cannot. There are also a number of reasons why a MySQL stored program approach may offer performance improvements over a traditional SQL approach:
It provides a procedural approach
SQL is a declarative, nonprocedural language: this means that in SQL you don't specify how to retrieve datayou only specify the data that you want to retrieve (or change). It's up to MySQL itselfspecifically, the MySQL query optimizerto determine how to go about identifying the result set.
From time to time, we might have a very good idea about the most efficient way to retrieve the data, but find that the MySQL optimizer chooses anotherless efficientpath.
When we think we know how the data should be retrieved but can't get the optimizer to play ball, we can sometimes use MySQL stored programs to force the desired approach.
It reduces client-server traffic
In a traditional SQL-based application, SQL statements and data flow back and forth between the client and the server. This traffic can cause delays even when both the client and the server programs are on the same machine. If the client and server are on different machines, then the overhead is even higher.
We can use MySQL stored programs to eliminate much of this overhead, particularly when we need to execute a series of related SQL statements. A succinct message is sent from the client to the server (the stored program execution request) and a minimal response is sent from the server to the client (perhaps only a return code). Furthermore, we can take advantage of database triggers to automatically execute statements in the database without any network interaction at all.
The resulting reduction in network traffic can significantly enhance performance.
It allows us to divide and conquer complex statements
As SQL statements become more complex, they also get harder and harder to fully optimize, both for the MySQL optimizer and for the programmer. We have all seen (and some of us have written) massive SQL statements with multiple subqueries, UNION operations, and complex joins. Tuning these "monster" SQL statements can be next to impossible for both humans and software optimizers.
It's often a winning strategy to break these massive SQL statements into smaller individual statements and optimize each individually. For instance, subqueries could be run outside of the SQL statement and the results forwarded to subsequent steps as query parameters or through temporary tables.
Having said that, we don't want to give you the impression that we think you should rewrite all of your non-trivial SQL statements in MySQL stored programs. In fact, it is usually the case that if you can express your needs in "straight" SQL, that will be the most efficient approach. And do remember that complex arithmetic computations will usually be slower in a stored program than in an equivalent SQL statement.