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?
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:
INDEX RANGE SCAN DESCENDING
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
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
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