Friday, January 27, 2006

MySQL 5 general purpose routine library - VI : Globals

(Sixth part of the MySQL 5 general purpose routine library presentation.)
Among the General Puropose Routines there are some that are more general than others. I refer to those small utility routines that don't belong to any specific module, or that are useful and required by any module. I call this group globals because it applies to all the modules, and also because one of its objectives is to create global variables that are available across sessions.

Finding existing objects

This category has some functions to report on the existence of a specific database object, mainly tables, views and routines.
Although this is a task mainly related to testing, I didn't put it among the test utilities, because they are really general, and I believe that they should be useful to many developers in several different tasks.
The list starts with three simple functions table_exists, table_or_view_exists, view_exists, which can tell you in one call if a given table or view exists.
Similar ones are routine_exists, function_exists and procedure_exists. The first one tells you if a given routine exists, providing the database name, the routine name and the routine type (procedure or function). The other two functions are just shortcuts toward calling routine_exists.

Using global variables

The main course in globals, and the reason why this module holds this name is due to the implementation of global variables, i.e. variables that keep their value across sessions. Unlike user variables, that lose their value after a disconnection, global variables can stay stored for as long as you need them.
To set a global variable, you can use either a procedure global_var_set or the homonymous function.
  set @myvar = global_var_set('varname','an important value');
call global_var_set('varname', 'an important value');
Both these calls will create a global variable named 'varname'. The first usage has the side effect of actually returning the variable's value, which is immediately assigned to a user variable. The second form will just create a global variable, assigning its value to a @global_var_set user variable.
To retrieve the value, you use global_var_get.
  set @othervar = global_var_get('varname');
Global variables are created on a user basis. With the default implementation, several users can define variables with the same name, and they will be kept separated, each one being available to their respective creator. It is possible to set the global variables to be truly global, each one available to all users, by changing the return value of library_user, which is used, incidentally, by the arrays module as well.
You can drop global variables with global_var_drop, and inquire about the existence of a global variables using global_var_exists.

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