Clustering Factor is a measure of the orderedness of an index in comparison to the table that it is based upon. It is used as an indicator for computing the estimated cost of the table
lookup foilowing an index access.

Clustering Factor is a measure of the orderedness of an index in comparison to the table that it is based upon. It is used as an indicator for computing the estimated cost of the table lookup foilowing an index access.

The Clustering Factor records the number of data blocks that will be accessed when scanning an index.

Clustering factor values can be seen in the following views: ALL|DBA|USER_INDEXES. It can be set also in the ALL|DBA|USER_IND_PARTITIONS and ALL|DBA|USER_IND_SUBPARTITIONS when the indexes are partitioned.

It is calculated as follows:

o The index is scanned in order.

o The block portion of the rowid pointed at by the current indexed value is compared with that pointed at by the previous indexed value. This is achieved by comparing adjacent rowids in the index leaf block (the blocks themselves do not need to be visited).

o If these rowids are pointing at different blocks then a counter is incremented.

o This is continued throughout the whole index.

o The resultant count is then stored.

If the count is close to the number of blocks in the table then the index is well ordered. This is true because the counter only gets incremented when the actual data is found in a different block from the last piece of row data.

If the count is close to the number of rows in the table then the index is less well ordered. In this case adjacent index entries do not tend to point to the same block in the table, thus more block reads are likely to be required. Data blocks are more likely to have to be revistited with a higher clustering factor.

The clustering factor can be used to adjust the potential numberof blocks that will be accessed by a particular predicate or query. This is useful for determining the number of base table blocks that will be visited when accessed via the index.

The clustering factor is effectively a count of the number of data blocks visited as the result of an index lookup. Multiplying the clustering factor by the selectivity will give the cost of the operation. It is predominately used to calculate costs for index range scans.