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Oracle alter table change column datatype

Oracle Database Tips by Donald Burleson

Oracle alter table change column datatype

You can use the Oracle "alter table" syntax to change the data type for an existing column, as shown in this example:

alter table
   column_name    varchar2(30)

To alter a LONG datatype column into a CLOB datatype you simply enter this DDL:

create table mytable (pk number, blob_column long)
-- add lots of rows
alter table mytable modify (blob_column clob);

Of course, you must deal with existing column data.  When modifying a tables column datatype you may want to export the rows, redefine the table and then re-import you data.

In a traditional database you would need to follow these cumbersome steps to alter a column data type:

1 - Create the new column at the end of the table.

2 - Run an update to populate the new table column

3 - Drop the old table column

4 - Re-name the new column to the original column name

Also, starting in Oracle 11g, the new virtual column feature is useful for changing table column datatypes.

Implement Bitmap Indexes

I was called upon to troubleshoot and fix a State Police query system that was experiencing slow query performance. The system was read-only except for a 30-minute window at night for data loading. Upon inspection of the SQL, I noted complex combinational WHERE clauses:

WHERE color='BLU' and make='CHEVY' and year=1997 and doors=2;

The distinct values for each of these columns were less than 200, and concatenated indexes were employed. Replacing the b-tree indexes with bitmap indexes resulted in a stunning performance improvement for the entire system, taking queries from 3 seconds down to under one-tenth of a second.







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