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Oracle Partitioning Pruning Tips

Oracle Database Tips by Donald BurlesonMay 14, 2015

Question:  What is Oracle partition pruning and how does it make my SQL run faster?

Answer:  Oracle partition pruning is a divided and conquer approach where Oracle is Partition Aware, excluding any partitions that do not contain blocks that are needed to satisfy the query result set.

Some DBA's report that partition pruning can take a SQL query down from hours to minutes.

Partition pruning is a very important because the CBO eliminates un-needed partitions when building the partition access list. You only choose what you need.

Be cause you only access the piece of the table that contains the data needed by the query, partition pruning can also dramatically reduce the amount of data retrieved from disk and reduce processing time, especially for I/O bound databases with small ram data buffers (db_cache_size).

Partition Pruning Internals

Partition pruning occurs when a SQL statement involving a partitioned object is executed and the database recognizes that the selection criteria are only specific partitions.  This allows the database to access only the relevant partitions and ignore all partitions that are not necessary for the SQL statement.  In other words, partition pruning is the act of eliminating, or ignoring, partitions that are irrelevant to the SQL statement's selection criteria.

Beginning with Oracle 11g, partition pruning uses bloom filtering instead of subquery pruning.  A bloom filter essentially tests if an element is a member of a set or not.  In deciding whether to use partition pruning, a bloom filter uses partition pruning whenever a partitioned object is detected in the SQL statement.  This enhances the performance of partition pruning because bloom filtering does not require additional resources. Also, it is not evaluated as a cost based decision.  Instead, bloom filtering is constantly active as well as automatically activated for any join with a partitioned object.  This transparent enhancement improves the performance of partition pruning.

 
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