I want to add a new column in to my production database (11g) that has millions of records but need default value only those existing records, meaning no new records should populate this default value.
Is it possible?
as per my understanding you want to add column to existing table and update it with default value.
alter table table_name add(col_name data_type);
Update table_name set col_name=some_default_value;
Related
I need to add a new row in existing table using Oracle plsql.which doesn't have unique identity column. For example table contain id_no,name of 40 rows.i need to add 40 more rows in same table. I want to add next continues number of max(id_no). For name I am going to insert it manually.without using auto increment.
Try an insert statement with subquery select max(id_no)+1 from table.
I have a table in which I want to add a new char type column
Before adding the column took the export , then dropped the table and then recreated the table and then did the import using the same dump file. When I checked the value of new added column in table , it doesn't holds any value.
How can I default the value to Spaces in this newly added column.
You don't need to drop and recreate the table to achieve this. You can simply alter the table and add the new column. You can also specify what the default value is for the existing values for the column.
alter table mytab add ( col2 CHAR2 DEFAULT ' ' );
However, are you really sure you want the column to have spaces?
I have an existing table with millions of records. Now I want to add a new column with default value. But for performances reasons I don't want to apply the new value immediately to existing records. Is there one handy way to do it?
So far, what in my mind is to work around this issue is to:
Add the new column, but do not specify a default (DDL)
Update by chunk the old rows to the default value (to avoid locking the table) (DML)
COMMIT
Alter the column to add a default (DDL)
Another option would be
Add the new column without any default value but make it accept
NULL
While inserting new record, insert NULL for this column
CREATE a AFTER INSERT trigger and update the column with it's
default value.
Something like
CREATE TRIGGER trg_update
AFTER INSERT
ON table_name
BEGIN
UPDATE table_name
SET new_column = (default_value)
WHERE Id_Column = :new.Id_Column;
END;
How can I change DATA TYPE of a column from number to varchar2 without deleting the table data?
You can't.
You can, however, create a new column with the new data type, migrate the data, drop the old column, and rename the new column. Something like
ALTER TABLE table_name
ADD( new_column_name varchar2(10) );
UPDATE table_name
SET new_column_name = to_char(old_column_name, <<some format>>);
ALTER TABLE table_name
DROP COLUMN old_column_name;
ALTER TABLE table_name
RENAME COLUMN new_column_name TO old_coulumn_name;
If you have code that depends on the position of the column in the table (which you really shouldn't have), you could rename the table and create a view on the table with the original name of the table that exposes the columns in the order your code expects until you can fix that buggy code.
You have to first deal with the existing rows before you modify the column DATA TYPE.
You could do the following steps:
Add the new column with a new name.
Update the new column from old column.
Drop the old column.
Rename the new column with the old column name.
For example,
alter table t add (col_new varchar2(50));
update t set col_new = to_char(col_old);
alter table t drop column col_old cascade constraints;
alter table t rename column col_new to col_old;
Make sure you re-create any required indexes which you had.
You could also try the CTAS approach, i.e. create table as select. But, the above is safe and preferrable.
The most efficient way is probably to do a CREATE TABLE ... AS SELECT
(CTAS)
alter table table_name modify (column_name VARCHAR2(255));
Since we can't change data type of a column with values, the approach that I was followed as below,
Say the column name you want to change type is 'A' and this can be achieved with SQL developer.
First sort table data by other column (ex: datetime).
Next copy the values of column 'A' and paste to excel file.
Delete values of the column 'A' an commit.
Change the data type and commit.
Again sort table data by previously used column (ex: datetime).
Then paste copied data from excel and commit.
I have a table A (3 columns) in production which is around 10 million records. I wanted to add one more column to that table and also I want to make default value to 1. Is it going to impact production DB performance If add a column with default value 1 or something else. What would be best approach to this to avoid any kind of performance impact on DB? your thoughts are much appreciated!!
In Oracle 11g the process of adding a new column with a default value has been considerably optimized. If a newly added column is specified as NOT NULL, default value for that column is maintained in the data dictionary and it's no longer required for a default value of a column to be stored for all records in a table, so it's no longer required to update each record with a default value. Such an optimization considerably reduces amount of time the table is exclusively locked during the operation.
alter table <tab_name> add(<col_name> <data_type> default <def_val> not null)
Moreover, column with a default value added that way will not consume space, until you deliberately start to update that column or insert a record with a non default value for that column. So the operation of adding a new column with a default value and not null constraint specified completes pretty quick.
i think that it is better that you create a table as backup table with this syntax:
create table BackUpTable as SELECT * FROM YourTable;
alter table BackUpTable add (newColumn number(5,0)default 1);