ORA-01450 in Oracle Databases: Causes, Limits, and Resolution

While designing or modifying indexes in Oracle databases, DBAs may encounter the error ORA-01450: maximum key length exceeded. This error occurs when the combined length of indexed columns exceeds the maximum allowable index key size defined by Oracle. Although the error message appears straightforward, its root cause is closely related to Oracle block size architecture and tablespace configuration, which are often overlooked during index design. 

 

For example, attempting to create a composite index may result in the following error:  

 

SQL> create index idx_emp on employee(emp_no,emp_name,dept_no)
*
ERROR at line 1:
ORA-01450: maximum key length (6398) exceeded

Further investigation, including reference to Oracle Support documentation (ORA-01450 When Creating an Index – Doc ID 293599.1), reveals that Oracle enforces a strict limitation on index key length. This limit is determined by the actual data block size of the tablespace where the index is stored. For an 8 KB block size, the maximum index key length is approximately 6398 bytes.

It is important to note that this restriction depends on the tablespace block size, not solely on the DB_BLOCK_SIZE initialization parameter. When indexes are created in tablespaces with nonstandard block sizes, the allowable index key length is recalculated based on that block size.

Resolution Options

There are two supported approaches to resolving ORA-01450:

Solution 1: Reduce the Index Key Size 

Redesign the index by removing unnecessary columns or reducing column widths to ensure the total key length remains within the supported limit. Refer to ORA-01450 and Maximum Key Length – How It Is Calculated (Doc ID 136158.1) for calculation details.

Solution 2: Use a Nonstandard Block Size Tablespace

Create a tablespace with a larger block size and store the index in that tablespace. 

 

First, verify the current database block size:

SQL> show parameter db_block_

NAME                                     TYPE                              VALUE
------------------------------------ --------------------------------- ------------------------------
db_block_buffers                     integer                             0
db_block_checking                  string                               FALSE
db_block_checksum                string                               TYPICAL
db_block_size                          integer                             16384

Check the database cache configuration:

 

SQL> show parameter db_cache

NAME TYPE VALUE
----------------- -------------- ------------
db_cache_advice string ON
db_cache_size big integer 0

Configure the required cache for the nonstandard block size:

SQL> alter system set db_32K_cache_size=500M scope=both;

System altered.

Create a tablespace with nonstandard block size 

 

SQL> create tablespace TMP_TEST32K datafile '+DATA' size 1G autoextend on blocksize 32k;

Tablespace created.

Finally, create the index in the newly created tablespace:

 

SQL>create index idx_emp on employee(emp_no,emp_name,dept_no) tablespace TMP_TEST32K;

Index created.

In conclusion, ORA-01450 is caused by index key length limits tied to tablespace block size. Proper index design or the controlled use of larger block size tablespaces can prevent and resolve this error.