By default, the standard program reads the entire main table when retrieving data for a particular form, even though some records and fields may not be required. By default also, the standard program reads only those reference table fields that are included on the form, even though the UI script and/or the DAL script may require other reference table fields.
Query extensions define conditions that the standard program adds to the SELECT, FROM, and WHERE clauses of a database query in order to filter out unwanted records and fields of the main table and in order to include in the query all reference fields required by the form, UI script, and DAL script.
In the UI script, you program query extensions to minimize the number of records and fields of the main table read by the query and to retrieve any reference tables field that are required by the UI script but are not on the form. In the DAL script, you program query extensions to retrieve reference table fields that are required by the DAL script.
When the standard program executes a database query, it automatically adds the query extensions defined in UI and DAL scripts. So, in the case of main table fields, the standard program reads only those fields and records that meet the conditions set in the query extension in the UI script. In the case of reference tables, the standard program reads those fields included on the main table and also those mentioned in the UI and DAL scripts.
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