A filter is an element in a predicate that reduces the size of the
query result by eliminating a category of result rows.
Consider the following join query fragment:
... WHERE table1.col1 = table2.col2
AND table1.col1 <= 200
|
Note that, logically, table2.col2 has to be <= 200 as well; that is, it
is subject to the same filter as table1.col1. ALLBASE/SQL
propagates such filters from one joined table to
the other joined tables without your explicitly doing so.
Thus the above query is translated internally into:
... WHERE table1.col1 = table2.col2
AND table1.col1 <= 200
AND table2.col2 <= 200
|
This internal translation
is done for all equal-joins on multiple tables. Both range
predicates (<, <=, >, >=) and the equals predicate (=) are propagated.
The result is a performance improvement on equal-joined queries when
there are one or more range or equal predicate filters on one or more
of the tables.
Since the optimizer does not check for duplicates
in filters, you should not explicitly propagate filters
in writing the queries. If you do, SQLCore
processes the same filter twice. This causes performance
to deteriorate without adding anything significant.
