Updating oracle table sequence
If you are using exclusive mode, Oracle sequences numbers are always generated in order.
Here I demonstrate this by inserting the value 1 and then two more rows, enabling identity to generate the default values: , you can now create a default column value not only when you use the DEFAULT keyword or leave the column entirely out of the INSERT statement but also when you set the column value explicitly to NULL.
In the past, if a column used a default value, you either had to use the DEFAULT keyword in the INSERT/UPDATE statement or leave the column entirely out of the INSERT/UPDATE statement.
That meant that in order to use a default value at certain times and not others, you needed at least two INSERT/UPDATE statements with complicated if/then/else constructs.
Table T has more than 87,000 records, so adding a column would typically take a significant amount of time, but as you can see, the addition is practically instantaneous in Oracle Database 12, almost every row in the table was a migrated row, because the row grew by orders of magnitude.
That table probably would be due for a reorganization in the prior releases but not in Oracle Database 12 release increases the size from 4,000 bytes to 32 K, bringing the SQL VARCHAR2, NVARCHAR2, and RAW datatypes in line with their PL/SQL counterparts.