Onine DDL operations are much more polite than offline DDL operations. They usually wait patiently for transactions that hold resources they need until these transactions end, and they do not block new DML statements.

As I wrote in the past, adding a constraint as Enabled and Validated (which is the default for new constrtaints) is an offline operation, but if we split it into two DDL statements – one for adding the constraint as Enabled and Not Validated and the second for making the constraint Validated – then each of these two separate statements is an online operation.

In this post I’d like to show that the first step – creating the constraint as Enabled and Not Validated – is even “more online” than it seems.

Let’s create some table t and insert one record into it:

One> create table t (
  2    a number,
  3    b number
  4  );

Table created.

One> insert into t(a,b) values (111,-1);

1 row created.

I did not commit or rollback this transaction, so it is still open and it’s locking the table in RX mode.

Now, from another session (note the SQL Prompts “One” and “Two”), I’ll add an Enabled and Not Validated check constraint to the table:

Two> alter table t
  2    add constraint t_b_chk
  3    check (b>0) enable novalidate;

Session Two is blocked now by session One (the wait event is “enq: TX – row lock contention”). Since it is an online operation it just waits, without throwing an ORA-54 error as an offline operation would have done.

But actually, it seems that this wait is unnecessary. The operation has already happened.

Using session One, we can see that the constraint already appears in the data dictionary:

One> select constraint_name,
  2         search_condition,
  3         status,
  4         validated
  5  from user_constraints
  6  where table_name = 'T';
--------------- ---------------- ---------- -------------
T_B_CHK         B>0              ENABLED    NOT VALIDATED

And if we try now to perform a DML that violates the constraint, we’ll get an error message, because the constraint is already enabled:

One> insert into t(a,b) values (222,-2);
insert into t(a,b) values (222,-2)
ERROR at line 1:
ORA-02290: check constraint (DEMO.T_B_CHK) violated

Session Two will be released as soon as session One either commits or rolls back, but it seems that it could have been released before that.
Even if we kill session Two before the transaction in session One ends, it doesn’t really matter, because the constraint has already been created and enabled.

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