Last week, I described how to use the MySQL plug-in API to write a minimal ‘Hello world!’ information schema plug-in. The main purpose of that plug-in is to illustrate the bare essentials of the MySQL information schema plug-in interface.

In this article, I’d like to take that to the next level and demonstrate how to write an information schema plug-in that can access some of the internals of the MySQL server. For this particular purpose, we will focus on a plug-in that reports all the SAVEPOINTs available in the current session. This MYSQL_SAVEPOINTS plug-in may be of some value when debugging scripts and stored routines that rely on complex scenarios using transactions and savepoints.

In a forthcoming article, I will describe a few information schema plug-ins that are arguably more interesting, such as a plug-in to list the currently existing TEMPORARY tables, user-defined variables, and the contents of the query cache. Although the plug-in described in this article may be of some use, its main purpose is to illustrate the minimal requirements for plug-ins that can access the server’s internals.

MySQL 5.1 offers an extremely useful feature called information_schema plugins. This feature allows dynamic runtime loading of a shared library into the MySQL server to implement a table in the information_schema database. The SQL standard (ISO/IEC 9075-11:2003) allows database implementations to extend the information_schema. MySQL 5.1 transfers the possibility to do this directly to privileged database users so they can extend the information_schema themselves, in any way they see fit.

In this article, we will demonstrate how to create a minimal “Hello, World!” MySQL information schema plugin. In a forthcoming article, we’ll demonstrate how information schema plugins may be used to report some of the server’s internals such as the contents of the query cache, session level objects such as the currently defined TEMPORARY tables, user-defined variables and SAVEPOINTs.