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Find unused columns with dba_unused_col_tabs

Oracle Database Tips by Donald Burleson


Question: How can I find unused columns within an Oracle table?


Answer: There are a variety of ways to see if a column is being used, the most common approach being the use of AWR.  You have several choices, and you can use the 10g AWR if the column is indexed by querying the invocation of the indexed column.

If you want to know about specific columns, consider creating an index on all suspected columns, wait awhile, and then run an AWR index usage report. If the column is unused, the index will not show as being invoked.


Invocation Counts for cust_index
Interval                             Invocation
time                 Search Columns       Count
-------------------- -------------- -----------
04-10-21 15                       1           3
04-10-10 16                       0           1
04-10-10 19                       1           1
04-10-11 02                       0           2
04-10-11 04                       2           1
04-10-11 06                       3           1
04-10-11 11                       0           1

Using dba_unused_col_tabs


Another less desirable approach is setting a column as unused and querying with the dba_unused_col_tabs view.  The downside is that the column becomes unavailable for your production SQL and it can cause an undesired interruption in production processing.  You can "mark" as column as logically deleted, meaning that it will disappear from the table for SQL, but it will continue to exist internally.


alter table mytab set unused column mycol;

You can then query the dba_unused_col_tabs view which displays a list of all tables with un-used columns, including counts of the number of columns within a table that are unused.


select * from dba_unused_col_tabs;


Once you have decided to physically drop the column you can issue this DDL:


alter table mytab drop unused columns;

If you like Oracle tuning, see the book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.



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