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Don Burleson Blog 









Oracle reverse key index tips

Don Burleson


It has been suggested that using reverse-key indexes will speed-up Oracle INSERT statements, especially with an increasing key, like an index on an Oracle sequence (which is used for the primary key of the target table).  For large batch inserts, Oracle reverse key indexes will greatly speed-up data loads because the high-order index key has been reversed. 

Note: An Oracle reverse key index DOES NOT change the functionality of the index, and it's not the same as reversing the index key values.

In general, an Oracle reverse key index relieve data block contention (buffer busy waits) when inserting into any index where the index key is a monotonically increasing value which must be duplicated in the higher-level index nodes.

With the index key reversal, only the lowest-level index node is changed, and not all of the high-order index nodes, resulting in far faster insert speed.  For updates, Oracle updates the index nodes with each update statement.

Depending on the size of your update batch, it's often faster to drop, update and then re-build the index. If you have more than one CPU, you might consider doing a parallel index rebuild for faster speed. It splits-up the full-table scan: 

From the Oracle documentation:

Creating a reverse key index, compared to a standard index, reverses the bytes of each column indexed (except the rowid) while keeping the column order. Such an arrangement can help avoid performance degradation with Oracle9i Real Application Clusters where modifications to the index are concentrated on a small set of leaf blocks.

By reversing the keys of the index, the insertions become distributed across all leaf keys in the index.

Reverse key indexes are also used to reduce contention when using OPS, RAC and Grid:

Reverse-key indexes reduce "hot spots" in indexes, especially primary key indexes, by reversing the bytes of the leaf blocks and thus eliminating the contention for leaf blocks across instances.

Rich Niemeic also offers advice on buffer busy waits and reverse-key indexes:

If it's on an index block, you should rebuild the index, partition the index, or use a reverse key index.

Also,  see my notes on tuning to reduce index contention.



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