Initial extent size in 11.2.0.2

Initial Extent Size of a Partition Changed To 8MB From 64KB After Upgrade to 11.2.0.2 [ID 1295484.1]

Applies to:

Oracle Server - Enterprise Edition - Version: 11.2.0.2 and later   [Release: 11.2 and later ]
Information in this document applies to any platform.

Symptoms

Table partition created in 11.2.0.2 has default initial extent size 8MB vs 64KB in previous versions.
Table created in in 11.2.0.2 having many partitions with few rows in each, consumes more space than in earlier versions.

Changes

Upgrade to 11.2.0.2.

Cause

Hidden parameter _partition_large_extents was introduced in 11.2.0.2 to enable large extent allocation for partitioned tables if created in autoallocate locally managed tablespaces with default extent size.
The default value for the parameter is TRUE. This hidden parameter supersedes parameter cell_partition_large_extents which was introduced in 11.2.0.1.

Solution

Explanation of the default behaviour in 11.2.0.2

The hidden parameter _partition_large_extents, introduced in 11.2.0.2, affects exadata and non-exadata databases, it has the two values TRUE, FALSE. The default value is TRUE which means all partitioned objects in the db will start with 8MB extents if created in autoallocate locally managed tablespaces.

Similarly parameter _index_partition_large_extents was introduced for partitioned indexes, the default value for this one is FALSE.

Smaller size extents impacts performance of operations like load and scan, typically full table scan (FTS, multi-block IO). For partitioned objects with many partitions, the fact of having many partitions leads to more small extents with autoallocation than for a nonpartitioned table; more smaller extents lead to more expensive space allocation/deallocation operations, and the I/O performance of scanning can be impacted due to more extents with smaller extent size.

The change of the default allocation size for partitioned tables was introduced together with the enhanced  deferred segment creation support for partitioned tables in 11.2.0.2 (deferred_segment_creation parameter, default TRUE).
Using both new defaults ensures that a negative impact from pre-created partitioned tables could be ruled out since empty partitions will no longer allocate any space.

How to achieve the pre-11.2.0.2 behaviour

If the user forcefully sets the parameter _partition_large_extents to FALSE (either at session or at instance level), then the pre-11.2.0.2 behaviour (of starting from 64k extent size) will be restored.

The user can also override the default extent allocation on a per statement base by specifying INITIAL extent in the CREATE TABLE ... / ALTER TABLE ... ADD PARTITION ... command as well as by changing the default storage initial for the tablespace involved e.g. ALTER TABLESPACE ... DEFAULT STORAGE (INITIAL 65536).