Implicit database connection

Understanding the implicit database connection with DATABASE instruction used before MAIN.

The DATABASE statement can be used in two distinct ways, depending on the context of the statement within its source module:

To specify a default database: Typically used in a GLOBALS module, to define variables with the DEFINE ... LIKE, but it is also used for the INITIALIZE and VALIDATE statements. Using the DATABASE statement in this way results in that database being opened automatically at run time.
To specify a current database: In MAIN or in a FUNCTION, used to connect to a database. A variable can be used in this context ( DATABASE varname ).

A default database is almost always used, because many programs contain DEFINE ... LIKE statements. A problem occurs when the production database name differs from the development database name, because the default database specification will result in an automatic connection (just after MAIN):

DATABASE stock_dev  -- Default database, used at compile time
DEFINE
   p_cust RECORD LIKE customer.*
MAIN -- Connection to default database occurs at MAIN
   DEFINE dbname CHAR(30)
   LET dbname = "stock1"
   DATABASE dbname -- Real database used in production
   ...
END MAIN

In order to avoid the implicit connection, you can use the SCHEMA instruction instead of DATABASE:

SCHEMA stock_dev -- Schema specification only
DEFINE
   p_cust RECORD LIKE customer.*
MAIN -- No default connection occurs...
   DEFINE dbname CHAR(30)
   LET dbname = "stock1"
   DATABASE dbname
END MAIN

This instruction will define the database schema for compilation only, and will not make an implicit connection at runtime.