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Oracle Column Default Value Tips

Expert Oracle Database Tips by Donald BurlesonMarch 25, 2015

 

Question:  Is there an overhead when a add a column default value to a table? 

I want to know the consequences of adding a DEFAULT value to an existing column in a table.

I understand that when you add a DEFAULT value to a column which is Nullable, Oracle updates all the NULL values for the column to the DEFAULT value, generating a lot of undo/redo data.

Is adding a DEFAULT value to a NOT NULL column a problem?

As the column is NOT NULL, an update would not be done, so no undo/redo data will be generated.

Answer:  I can think of only one case where modifying a table column default value could cause all rows to be changed.  

However, if you were to simultaneously alter the table to allow NOT NULL and a default, then yes, Oracle might perform a full-table scan to change all NULL rows.  

Try this, and see if it updates the table:

alter table
    fred
modify (mycol NOT NULL default ?EMPTY?);

However, I would prefer to use ordinary DML updates . . .

Obviously, this relationship cannot be enforced if you leave "orphan" appointments, which are not hooked to a patient.

Update fred set mycol = ?EMPTY? where mycol is NULL;

alter table
    fred
modify (mycol NOT NULL);

Otherwise, setting a column default would never be retroactive, only applying to new inserts and updates.  

Here are the table column default options, used to set column default values in CREATE TABLE and ALTER TABLE MODIFY commands:

CREATE|ALTER TABLE <table name>
( { <column name> <data type> [ DEFAULT <default> ]) . . . .

At table creation time, adding a column default creates no overhead at all:

create table t1 (id number default (0));

A column default is rarely changed at alter table time, but there are issues with NULL values and default values.  

When you add a DEFAULT value to a column which is Nullable, Oracle recognizes a NULL as a legitimate existing value and will not make any updates.  

Some people will deploy insert triggers to fill-in default values . . .  

Also, beware that Create Table As Select (CTAS) can have issues with default column values.

Steve Callan points out that Oracle 11g has a New Feature for the DEFAULT clause for a NOT NULL column:

"When you add a column, the initial value of each row for the new column is null. If you specify the DEFAULT clause for a NOT NULL column, then the default value is stored as metadata but the column itself is not populated with data. However, subsequent queries that specify the new column are rewritten so that the default value is returned in the result set.

This optimized behavior differs from earlier releases, when as part of the ALTER TABLE operation Oracle Database updated each row in the newly created column with the default value, and then fired any AFTER UPDATE triggers defined on the table."

 
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