e possible plans
     are determined by the available indexes on each relation.
     There is always the possibility of performing a
     sequential scan on a relation, so a sequential scan plan is always
     created. Assume an index is defined on a
     relation (for example a B-tree index) and a query contains the
     restriction
     <code class="literal">relation.attribute OPR constant</code>. If
     <code class="literal">relation.attribute</code> happens to match the key of the B-tree
     index and