This blog discusses how to configure Oracle® Automatic In-Memory (AIM) to manage objects in the In-Memory (IM) Column Store.
Introduction
The IM column store feature allows you to populate segments in a columnar format. Unlike a buffer cache, IM column store populates the complete segment into memory. Automatic Data Optimization (ADO) and AIM manage the objects in the IM column store.
Oracle introduced AIM in Oracle Database version 18c, and it uses access tracking, column statistics, and other relevant statistics to manage objects in the IM column store. AIM ensures that active segments always remain populated by automatically evicting cold (infrequently accessed) segments.
An in-memory segment is only eligible for eviction if the INMEMORY priority
is NONE, and AIM evicts an in-memory segment when the IM column store space is
exhausted. The database uses the internal statistics of populated segments to
define the set of objects to evict. If any ADO policy is enabled to prevent
in-memory segment eviction, then the ADO policy overrides AIM and prevents
eviction.
Enable AIM
The system-level initialization parameter, INMEMORY_AUTOMATIC_LEVEL, enables AIM and has the following values:
- OFF (default): Disables AIM.
- LOW: The cold segments get evicted from the IM column in case of memory pressure.
- MEDIUM: This level ensures that any hot segment that was not populated because of memory pressure is populated first.
The following image shows the INMEMORY_AUTOMATIC_LEVEL parameter and how to modify its value:
Set the time interval for AIM
Use the DBMS_INMEMORY_ADMIN package to set the time interval for the usage statistics that AIM checks. The default value is 31 days. You can change this setting as shown in the following example:
Understand how AIM works
You can query the dynamic performance (V$) view, V$IM_SEGMENTS, to list
currently populated segments in an IM column store and check the tables with
the INMEMORY ENABLED status for a schema. In the following example, you can
see segment TAB1 already shows in the populated segments lists because its
priority is set to HIGH.
Now, you can populate other tables with INMEMORY ENABLED. Tables are populated
in an IM column store after you access the tables, as shown in the following
example:
Next, you can try to populate one more table, EXAMPLE3, with HIGH priority
to give it preference over segments with priority of NONE. Querying
V$IM_SEGMENTS reveals that tables with a priority of NONE do not show up.
This is because the IM column store is under memory pressure. You can see
EXAMPLE3 is partially populated, and BYTES_NOT_POPULATED is not 0.
That is why AIM evicted the other cold segments (or segments having priority of
NONE). Also note that if a segment is partially populated, queries accessing
those objects in the IM column store run fine—they get the remaining data
that they do not find in the IM column store from the database.
You can query DBA_INMEMORY_AIMTASKS to see the status of all the tasks
created by AIM. DBA_INMEMORY_AIMTASKDETAILS lists all the details of
actions taken by AIM on segments present in the IM column store. In the
following image, you can see that no action has been taken on TAB1 because
its priority is HIGH and as explained earlier while populating table EXAMPLE3,
the IM column store was under memory pressure so AIM evicted the other segments.
Conclusion
Though AIM is a helpful feature to automate objects in IM column store, it has restrictions because it can only be enabled on databases hosted on cloud-based systems and engineered systems. Databases hosted on other system types, such as premise systems, generate an error when you attempt to enable AIM.
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