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CHAPTER 8 TRANSACTIONS
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In 11, Indexes, we ll see the salient effects of this feature of PL/SQL when measuring the performance of reverse key indexes. If you d like to see how PL/SQL performs in the manner described above, skip there for a moment to review the reverse key index benchmark.
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Integrity Constraints and Transactions
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It is interesting to note exactly when integrity constraints are checked. By default, integrity constraints are checked after the entire SQL statement has been processed. There are also deferrable constraints that permit the validation of integrity constraints to be postponed until either the application requests they be validated by issuing a SET CONSTRAINTS ALL IMMEDIATE command or upon issuing a COMMIT.
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IMMEDIATE Constraints
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For the first part of this discussion, we ll assume that constraints are in IMMEDIATE mode, which is the norm. In this case, the integrity constraints are checked immediately after the entire SQL statement has been processed. Note that I used the term SQL statement, not just statement. If I have many SQL statements in a PL/SQL stored procedure, each SQL statement will have its integrity constraints validated immediately after its individual execution, not after the stored procedure completes. So, why are constraints validated after the SQL statement executes Why not during This is because it is very natural for a single statement to make individual rows in a table momentarily inconsistent. Taking a look at the partial work by a statement would result in Oracle rejecting the results, even if the end result would be OK. For example, suppose we have a table like this: ops$tkyte%ORA11GR2> create table t ( x int unique ); Table created. ops$tkyte%ORA11GR2> insert into t values ( 1 ); 1 row created. ops$tkyte%ORA11GR2> insert into t values ( 2 ); 1 row created. ops$tkyte%ORA11GR2> commit; Commit complete. And we want to execute a multiple-row UPDATE: ops$tkyte%ORA11GR2> update t set x=x-1; 2 rows updated. If Oracle checked the constraint after each row was updated, on any given day we would stand a 5050 chance of having the UPDATE fail. The rows in T are accessed in some order, and if Oracle updated the X=1 row first, we would momentarily have a duplicate value for X and it would reject the UPDATE. Since Oracle waits patiently until the end of the statement, the statement succeeds because by the time it is done, there are no duplicates.
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Starting with Oracle 8.0, we also have the ability to defer constraint checking, which can be quite advantageous for various operations. The one that immediately jumps to mind is the requirement to cascade an UPDATE of a primary key to the child keys. Many people claim you should never need to do this that primary keys are immutable (I am one of those people), but many others persist in their desire to have a cascading UPDATE. Deferrable constraints make this possible.
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Note It is considered an extremely bad practice to perform update cascades to modify a primary key. It violates
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the intent of the primary key. If you have to do it once to correct bad information, that s one thing, but if you find you are constantly doing it as part of your application, you will want to go back and rethink that process you have chosen the wrong attributes to be the key!
In prior releases, it was actually possible to do a CASCADE UPDATE, but doing so involved a tremendous amount of work and had certain limitations. With deferrable constraints, it becomes almost trivial. The code could look like this: ops$tkyte%ORA11GR2> create table parent 2 ( pk int primary key ) 3 / Table created. ops$tkyte%ORA11GR2> create table child 2 ( fk constraint child_fk_parent 3 references parent(pk) 4 deferrable 5 initially immediate 6 ) 7 / Table created. ops$tkyte%ORA11GR2> insert into parent values ( 1 ); 1 row created. ops$tkyte%ORA11GR2> insert into child values ( 1 ); 1 row created. We have a parent table, PARENT, and a child table, CHILD. Table CHILD references table PARENT, and the constraint used to enforce that rule is called CHILD_FK_PARENT (child foreign key to parent). This constraint was created as DEFERRABLE, but it is set to INITIALLY IMMEDIATE. This means we can defer that constraint until COMMIT or to some other time. By default, however, it will be validated at the statement level. This is the most common use of the deferrable constraints. Most existing applications won t check for constraint violations on a COMMIT statement, and it is best not to surprise them with that. As defined, table CHILD behaves in the fashion tables always have, but it gives us the ability to explicitly change its behavior. Now let s try some DML on the tables and see what happens:
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