A model's rules define the logical and
physical search space --
the space of query trees and plans
from which the lowest cost plan will be chosen.
The first source
for transformation rules
was
standard database textbooks [Elmasri 94][Ullman 87].
These texts contain
standard relational algebra transformations.
Assuming
that it is almost always beneficial
to push down selects,
the parser pushes
the SELECT operator
down as far as possible
in the initial query.
That is,
in the initial query tree,
the SELECT logical operator
lies directly over
the GET logical operator, if possible.
As a result,
SELECT push-down rules are not required
in Model D.
(This demonstrates how
the transformation rules that
are required are dependent on the
structure of the initial query tree.)
Some of the standard transformation rules
which could be applied
were
not implemented.
Most of the Pull up SELECT rules such as
AGG_LIST through SELECT
(equivalent to Pull Up Select through an AGG_LIST)
are not part of Model D.
We did not anticipate a large reduction in cost
based on these rules.
As a result,
the model has only a few
rules to move the SELECT logical
operator
(except for PROJECT_THRU_SELECT, discussed below).
Model D does have rules
to push down the PROJECT logical operator.
Here are the rules to push project through four
of the other logical operators:
These rules
allow a project
that starts out
above the AGG_LIST
logical operator
in the initial logical expression
to be pushed down
to lie just over the GET logical operator.
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