Keywords: predicates, pushed down into views

# Predicates pushed into views or derived tables

An SQL statement that references a view can also include a predicate. Consider the view v2 (a,b):

    CREATE VIEW v2(a,b) AS
    SELECT sales_person, MAX(sales)
    FROM Sales
    GROUP BY sales_person

The following statement references the view and includes a predicate:

    SELECT *
    FROM v2
    WHERE a = 'LUCCHESSI'

When Derby transforms that statement by first transforming the view into a derived table, it places the predicate at the top level of the new query, outside the scope of the derived table:

    SELECT a, b
    FROM (SELECT sales_person, MAX(sales)
       FROM Sales
       WHERE sales_person = 'LUCCHESSI'
       GROUP BY sales_person)
       v1(a, b)

In the example in the preceding section (see View flattening), Derby was able to flatten the derived table into the main SELECT, so the predicate in the outer SELECT could be evaluated at a useful point in the query. This is not possible in this example, because the underlying view does not satisfy all the requirements of view flattening.

However, if the source of all of the column references in a predicate is a simple column reference in the underlying view or table, Derby is able to push the predicate down to the underlying view. Pushing down means that the qualification described by the predicate can be evaluated when the view is being evaluated. In our example, the column reference in the outer predicate, a, in the underlying view is a simple column reference to the underlying base table. So the final transformation of this statement after predicate pushdown is:

    SELECT a, b
    FROM (SELECT sales_person, MAX(sales) from Sales
    WHERE sales_person = 'LUCCHESSI'
    GROUP BY sales_person) v1(a, b)

Without the transformation, Derby would have to scan the entire table t1 to form all the groups, only to throw out all but one of the groups. With the transformation, Derby is able to make that qualification part of the derived table.

If there were a predicate that referenced column b, it could not be pushed down, because in the underlying view, column b is not a simple column reference.

Predicate pushdown transformation includes predicates that reference multiple tables from an underlying join.

Parent topic: View transformations
Related concepts
View flattening