NAME Eval.pm - query executor. DESCRIPTION This module implements the query executor of Qios. The query executor comprises the computation of query execution plan and the query evaluation engine. Query execution plan The physical query execution plan is computed from the optimized query trees obtained using module *Optor.pm*. The selection of query execution plan is implemented by the procedure which computes sub-optimal physical execution plan by selecting physical operations for all logical operations (query nodes) forming the physical query tree in a bottom-up manner. The result of the logical query optimization are bushy query trees. The results of the inner sub-trees have to be materialized in order to avoid their repeating evaluation. Each of the logical operations (query nodes) can be implemented using one or more physical operations which are listed in the following section. The procedures for the selection of physical operations are integrated in the routines that open scan operator for query nodes (logical operators). The use of physical operators based on B+-trees are planned to be implemented. Physical operations After the selection of the physical operations, the same skeleton of the query tree is employed as the iterator tree for the evaluation of physical query. The physical operations are implemented as scans. The currently implemented physical operations are the following. *Access methods* phsqex - sequentially extension phixex - hash-index on extension phsqin - sequentially all instances phixin - hash-index on all instances *Select* phsqse - select loop phixse - hash access for equality *Project* phsqpr - project loop *Join* phsqjn - nested-loop join phixjn - hash-join for equality COMMENTS This module is part of the *qios* programming environment.