9.22.Stored Procedures as Views & Derived Tables
Virtuoso allows using a stored procedure result set in place of a table. A view may also be defined as a stored procedure. This provides smooth integration to external procedural logic in queries.
When a procedure appears as a table, the procedure is called and its result set is inserted into a temporary space. Processing continues from that point on as if the data came from a table.
Queries involving procedure views or derived tables are subject to normal join order selection. For this purpose it is possible to associate a cost to a procedure used in a procedure view or derived table. If the option (order) clause is given at the end of the select, joins are done left to right. If a procedure is in the leftmost position in the from it will be called once for the query, if it is in the second position it will be called once for every row of the leftmost table that passes selection criteria applicable to it and so on.
Procedures used as tables can get parameters from the query. These parameters are expressed in the containing select's where clause as column = expression, where column is a parameter name of the procedure table.
A procedure derived table is of the form:
q_table_name '(' column_commalist ')' '(' column_def_commalist ')' alias
The first column commalist is the parameter list. The second column_def_list is a description of the result set, as in a CREATE TABLE statement. The correlation name alias is required if the procedure occurs as a derived table, with no view definition.
A procedure view is declared as follows:
CREATE procedure VIEW new_table_name AS q_table_name '(' column_commalist ')' '(' column_def_commalist ')'
The columns in the column definition list should correspond to the procedure's result set columns. The columns are explicit in the view so as to be able to interpret the view definition and to be able to compile procedures and queries using the view before the procedure itself is defined. Thus the procedure need be defined only at time of execution, not at time of definition.
The meta-data returned by ODBC catalog calls for a procedure view will show the columns as they were declared, just like a regular view. Procedure views are never updatable.