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Don Burleson Blog 







  Oracle descending index tips

Oracle Database Tips by Donald BurlesonMarch 25, 2015

Question:  If Oracle can read indexes in both directions, what is the purpose of the Oracle descending index?  How to I implement an Oracle descending index?  Are there special execution plans which use descending indexes?

Answer:  The Oracle database supports descending indexes and they have special min/max execution plans associated with their usage.  With a descending index you will see the execution plan reference the index:


It is interesting to note that when you create an index with the "desc" option, it creates a index as a composite function based index.  The choice of ascending or descending index when defining an index would depend on the kind of SQL queries that you want to answer using that index.

For example, a query that asks for the key values in descending order would benefit from a descending index. Any query with an ORDER BY DESC would use a descending index to remove the need for a back-end sort operation.

Note that in some releases the descending index is buggy and causes sub-optimal execution plans.  See MOSC bug 11072246 (Oracle 11gr2) and MOSC Note titled ""non optimal index is used when index column is in desc order".

Also, remember that an ascending index can be read in descending order, so there are VERY few benefits to creating a index with the DESC clause.  In most cases a "vanilla" ascending index will be able to handle requests for data in descending order, using the index and sorting the result set, much faster and more efficiently than the descending index.

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