Re-order columns of table in Oracle

It's sad that Oracle doesn't allow this, I get asked to do this by developers all the time..

Here's a slightly dangerous, somewhat quick and dirty method:

  1. Ensure you have enough space to copy the Table
  2. Note any Constraints, Grants, Indexes, Synonyms, Triggers, um.. maybe some other stuff - that belongs to a Table - that I haven't thought about?
  3. CREATE TABLE table_right_columns AS SELECT column1 column3, column2 FROM table_wrong_columns; -- Notice how we correct the position of the columns :)
  4. DROP TABLE table_wrong_columns;
  5. 'ALTER TABLE table_right_columns RENAME TO table_wrong_columns;`
  6. Now the yucky part: recreate all those items you noted in step 2 above
  7. Check what code is now invalid, and recompile to check for errors

And next time you create a table, please consider the future requirements! ;)


Look at the package DBMS_Redefinition. It will rebuild the table with the new ordering. It can be done with the table online.

As Phil Brown noted, think carefully before doing this. However there is overhead in scanning the row for columns and moving data on update. Column ordering rules I use (in no particular order):

  • Group related columns together.
  • Not NULL columns before null-able columns.
  • Frequently searched un-indexed columns first.
  • Rarely filled null-able columns last.
  • Static columns first.
  • Updateable varchar columns later.
  • Indexed columns after other searchable columns.

These rules conflict and have not all been tested for performance on the latest release. Most have been tested in practice, but I didn't document the results. Placement options target one of three conflicting goals: easy to understand column placement; fast data retrieval; and minimal data movement on updates.


I followed the solution above from Jonas and it worked well until I needed to add a second column. What I found is that when making the columns visible again Oracle does not necessarily set them visible in the order listed in the statement.

To demonstrate this follow Jonas' example above. As he showed, once the steps are complete the table is in the order that you'd expect. Things then break down when you add another column as shown below:

Example (continued from Jonas'):

Add another column which is to be inserted before column C.

ALTER TABLE t ADD (b2 INT);

Use the technique demonstrated above to move the newly added B2 column before column C.

ALTER TABLE t MODIFY (c INVISIBLE, d INVISIBLE, e INVISIBLE);
ALTER TABLE t MODIFY (c VISIBLE, d VISIBLE, e VISIBLE);

DESCRIBE t;

Name
----
A
B
B2
D
E
C

As shown above column C has moved to the end. It seems that the ALTER TABLE statement above processed the columns in the order D, E, C rather than in the order specified in the statement (perhaps in physical table order). To ensure that the column is placed where desired it is necessary to make the columns visible one by one in the desired order.

ALTER TABLE t MODIFY (c INVISIBLE, d INVISIBLE, e INVISIBLE);
ALTER TABLE t MODIFY c VISIBLE;
ALTER TABLE t MODIFY d VISIBLE;
ALTER TABLE t MODIFY e VISIBLE;

DESCRIBE t;

Name
----
A
B
B2
C
D
E

Since the release of Oracle 12c it is now easier to rearrange columns logically.

Oracle 12c added support for making columns invisible and that feature can be used to rearrange columns logically.

Quote from the documentation on invisible columns:

When you make an invisible column visible, the column is included in the table's column order as the last column.

Example

Create a table:

CREATE TABLE t (
    a INT,
    b INT,
    d INT,
    e INT
);

Add a column:

ALTER TABLE t ADD (c INT);

Move the column to the middle:

ALTER TABLE t MODIFY (d INVISIBLE, e INVISIBLE);
ALTER TABLE t MODIFY (d VISIBLE, e VISIBLE);

DESCRIBE t;

Name
----
A
B
C
D
E

Credits

I learned about this from an article by Tom Kyte on new features in Oracle 12c.

Tags:

Sql

Oracle

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