ORA-01450: maximum key length (3215) exceeded

Today I faced very interesting thig and I want to share it.
I am just trying to create index online and getting this error:

SQL> create index owner.index_name_i1 on owner.table_name(column_name) tablespace tbl_name online;
 
ORA-00604: error occurred at recursive SQL level 1
ORA-01450: maximum key length (3215) exceeded
 
SQL> 

Problem:

When creating an index the total length of the index cannot exceed a certain value. Primarily this value depends on DB_BLOCK_SIZE.

If 2K block size then maximum index key length=758

If 4K block size then maximum index key length=1578

If 8K block size then maximum index key length=3218 

If 16K block size then maximum index key length=6498

How the maximum index key length is measured by?

Maximum index key length=Total index length (Sum of width of all indexed column + the number of indexed columns) + Length of the key (2 bytes) + ROWID (6 bytes) + the length of the rowid (1 byte)

The index key size is limited by the value of db_block_size, because a key value may not span multiple blocks. So, based on the size of the block size of index depends. In fact, it is required that any index block must contain at least TWO index entries per block.

So we can say that the maximum key length for an index will be less than half of the DB_BLOCK_SIZE. But we know that in a block there also needed space for PCTFREE, INITRANS and space for block overhead (Block Header, ROW Directory, Table Directory, etc.). After considering these bytes the actual space that can be used for the Index key is actually just over 1/3 of the DB_BLOCK_SIZE.

Solution:

1) Increase your database block size. Create a tablespace with bigger block size and create index on that tablespace.

2) If you have index on multiple columns then you can split index to single or 2 columns so that size does not extended over it can handle.

3) create/rebuild the index without online clause.

Because the online rebuild of the index creates a journal table and index. This internal journal IOT table contains more columns in its index. 

This is a feature of online rebuild. This time the error arises because that current value of the initialization parameter db_block_size is not large enough to create internal journal IOT.