NAME
    Cost.pm -- query cost computation.

DESCRIPTION
    Module includes the implementation of a cost function for the query
    optimizer of *Qios*. The standard relational cost functions are used to
    estimate the number of bytes processed by the physical query tree
    including the cost of materialization of the intermediate query
    evaluation results.

Statistics
    The statistics on the classes is generated dynamically triggered by the
    query optimizer and when the class is loaded into the database. The
    following data is gathered for a given class: the number of class
    objects, the size of objects and the cardinality of attributes.

Cost computation
    The computation of the estimation of the size of the result for the
    operations *select*, *project* and *join* is realized using standard
    relational formulas based on the selectivity of attributes and
    presupposing the normal distribution of attributes values.

COMMENTS
    This module is part of the *qios* programming environment.