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Oracle create assertion tips

Oracle Database Tips by Donald BurlesonMay 18, 2016

Question: What do I do to use the SQL99 "create assertion" statements in Oracle?

Answer:  As of 12c, Oracle does not support assertions, but there is some buzz that Oracle 13c may contain assertions.  Let's take a look at the "create assertion" syntax and see how it might be helpful in Oracle.

Formally, an assertion is defined as a way to implement transaction level, cross record constraints, a way to enforce complex data rules that span many tables. In a nutshell, the create assertion syntax would move these complex rules out of triggers (or application programs) and place them directly in the database as a check constraint.  Currently, there are only three ways of implementing cross-table row validation in Oracle:

  • Validate in the application code
  • Use an inert/update trigger
  • Implement a materialized view

There is a great concern about the performance of DML when using assertions because an application can pre-approve batches of rows for inserts, while the assertion applies to row-at-a-time DML.  However, if you use deferrable constraints, then there may be ways to circumvent the performance issue. 

First, the create assertion can be used to replace insert triggers that validate incoming data rows.  For example, we might see a create assertion that restricts rows in a table to a specific set of column values.  This is implemented by providing a SQL query.

The rules can range from very simple (each department can only have one DBA) to extremely complex, and there is no practical limit on the complexity since assertions are implemented with a SQL statement.

Oracle provides one example of such a create assertion query, checking column values in two tables and applying it to a check constraint:

"This SQL statement creates an assertion to disallow suppliers based in cities of Albany, Palo Alto, or Portland from supplying, in quantities higher than 50, all the parts that are red or cost $10.00 or more:"

create assertion AllPartSupp as CHECK
   (not exists
     (select 'an s shipping all parts'
        from SUPPLIER s
       where s.CITY in ('Albany', 'Palo Alto', 'Portland')
         and not exists
               (select 'a p not shipped'
                  from PART p
                 where (p.COLOR = 'red' or p.PRICE >= 10)
                   and not exists
                        (select 'a connecting sh'
                           from SHIPMENT sh
                          where sh.QUANTITY > 50
                            and sh.SNO = s.SNO
                            and sh.PNO = p.PNO)))

It appears that assertions may be expensive to implement in any database because the SQL statement must fire before every insert on the target table, but the assertion approach does remove the need to write triggers to enforce complex data relationships between row columns in a table.

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