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6.2.2 Catalog Representation

Each database (in an implementation) has a system catalog which describes all the user accessible tables in that database (including itself).

The system catalog base table has the following fields. PRI indicates a primary key for that table.

     PRI table-name
         column-limit            the highest column number
         coltab-name             descriptor table name
         bastab-id               data base table identifier
         user-integrity-rule
         view-procedure          A scheme thunk which, when called,
                                 produces a handle for the view.  coltab
                                 and bastab are specified if and only if
                                 view-procedure is not.

Descriptors for base tables (not views) are tables (pointed to by system catalog). Descriptor (base) tables have the fields:

     PRI column-number           sequential integers from 1
         primary-key?            boolean TRUE for primary key components
         column-name
         column-integrity-rule
         domain-name

A primary key is any column marked as primary-key? in the corresponding descriptor table. All the primary-key? columns must have lower column numbers than any non-primary-key? columns. Every table must have at least one primary key. Primary keys must be sufficient to distinguish all rows from each other in the table. All of the system defined tables have a single primary key.

A domain is a category describing the allowable values to occur in a column. It is described by a (base) table with the fields:

     PRI domain-name
         foreign-table
         domain-integrity-rule
         type-id
         type-param

The type-id field value is a symbol. This symbol may be used by the underlying base table implementation in storing that field.

If the foreign-table field is non-#f then that field names a table from the catalog. The values for that domain must match a primary key of the table referenced by the type-param (or #f, if allowed). This package currently does not support composite foreign-keys.

The types for which support is planned are:

         atom
         symbol
         string                  [<length>]
         number                  [<base>]
         money                   <currency>
         date-time
         boolean
     
         foreign-key             <table-name>
         expression
         virtual                 <expression>