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Oracle In-Memory Area

The Large Pool is an optional component of the SGA.

If enabled, the In-Memory Area contains the In-Memory column store (IM column store).

The IM column store stores tables and partitions in a format that is optimized for rapid scans (especially for OLAP)

Dual format

The IM Column store simultaneously allows to store data in the buffer cache in (traditional) row-format and in the In-Memory Area in columnar-format

Although the data is stored in two formats, the memory requirements are not doubled.

INMEMORY attribute

Some Oracle objects can be given the inmemory attribute which adds the object to the IM column store.

This attribute can be set on

tablespaces
tables
partitions and subpartitions
materialized views
a subset of columns in that object

In-Memory Compression Unit (IMCU)

The IMCU is a compressed, read-only storage unit that contains data for one or more columns.

Snapshot Metadata Unit (SMU)

An SMU contains metadata and transactional information for an associated IMCU.

Expression Statistics Store (ESS)

The ESS stores statistics about expression evaluation.

The ESS is a permanent (not disabable) component of the database. Thus, it is independent of the IM column store and resides in the SGA and also persists on disk.

In-Memory Expression Unit (IMEU)

The IMEU is a storage container for materialized IM expressions and user-defined virtual columns.