An Overview of Oracle database buffer cache behavior and keep blocks into memory solutions

An Overview of Oracle database buffer cache behavior and keep blocks into memory solutions.

Alireza Kamrani

18 Feb 2025

We can run some commands to cache  table blocks in Oracle buffer cache, but is there any guarantee to keep all blocks of mentioned table in buffer cache anytime?

In Oracle , if you want a table to always be cached in memory to improve performance, you can use the KEEP buffer pool or the Result Cache. However, BUFFER CACHE in the ALTER TABLE command is primarily a hint for caching table blocks in the default buffer cache.

Options to Keep a Table in Memory:

1. Using the KEEP Buffer Pool

Oracle allows you to keep frequently accessed tables in the KEEP buffer pool to avoid them being aged out of memory.

• Enable the KEEP pool in your SGA:

ALTER SYSTEM SET DB_KEEP_CACHE_SIZE = nG;

n: adjust it based on you memory.

• Assign the table to the KEEP pool:

ALTER TABLE my_table STORAGE (BUFFER_POOL KEEP);

Or also:

ALTER TABLE my_tabl CACHE;

2. Using the BUFFER CACHE Clause

You can specify that table blocks should be kept in the buffer cache:

ALTER TABLE my_table STORAGE (BUFFER CACHE);

• This helps in keeping the table’s data in the buffer cache for longer.

• Useful for small lookup tables that are frequently accessed.

3. Using the Result Cache (For Query Caching)

If the table is frequently queried and doesn't change often, you can enable query result caching:

ALTER TABLE my_table RESULT_CACHE (MODE FORCE);

• This keeps the query results in the Result Cache rather than just buffering the table blocks.


Which One Should You Use?

• Use KEEP Buffer Pool for frequently accessed small tables that should always stay in memory.

• Use BUFFER CACHE for tables where you want their data blocks to stay in the buffer cache longer.

• Use RESULT CACHE if the queries on the table return the same results frequently and the table doesn't change often.


Is there any solution to guarantee keeping all table blocks residents into memory?

No, Oracle does not force a full cache and guarantee that all blocks of a database will remain in memory.
After any of above solution about caching of a sample table in buffer , Oracle does not do anything immediately, and your command done easily!
So, running any mentioned solutions does not keeping blocks as soon as possible, when Oracle try to change your priorities about this table to keep in cache but these is not any guarantee about that and that is very dependent on pressure on memory and workloads.

While Oracle uses a sophisticated caching mechanism, the database will not guarantee that all blocks will stay in memory at all times. Oracle uses the System Global Area (SGA) and the Buffer Cache to cache frequently accessed data blocks, but the caching behavior is influenced by various factors, such as:

• Memory Availability: The SGA has a limited amount of memory, and the database will keep as many blocks in memory as possible, but once it exceeds its capacity, older or less frequently used blocks will be flushed to disk.

• Database Cache Management: Oracle dynamically manages the buffer cache to balance memory between different cache types (like the Database Buffer Cache, Shared Pool, etc.), based on usage patterns.

• LRU (Least Recently Used) Algorithm: Blocks that are not frequently accessed are more likely to be replaced by other blocks. This means that not all blocks are guaranteed to remain in memory.


• Memory Parameters: Settings like DB_CACHE_SIZE and PGA_AGGREGATE_TARGET determine how much memory is allocated to caching. However, the actual memory used can fluctuate based on system activity and load.

• Large Tables or Objects: Very large objects or tables might not fit entirely in memory, and Oracle will load and evict blocks based on access patterns.

In summary, Oracle will try to keep as much data in memory as it can based on available resources and usage patterns, but it does not guarantee that all blocks will remain cached in memory all the time.


What is usage of Force Full Caching mechanism?

If you are using the ALTER DATABASE FORCE FULL DATABASE CACHING command, the behavior changes slightly.

When you run this command, Oracle will indeed attempt to keep all data blocks in memory. Specifically:

• Force Full Database Caching: This command forces Oracle to cache all database blocks for tables and indexes into memory (specifically into the Database Buffer Cache). This means that Oracle will attempt to keep all blocks that are accessed in memory, regardless of the usual cache replacement mechanisms.

However, there are still some important nuances to understand:

• Memory Constraints: While Oracle will attempt to cache all blocks, it doesn't mean the entire database will fit into memory if the available memory is not enough. If the system doesn't have enough memory to store every block, the database will still perform evictions based on available resources. This command doesn't increase physical memory; it just prioritizes caching.

• Increased Memory Usage: This command will increase memory consumption because the cache size will grow, and more memory will be allocated to caching the blocks. You need to have sufficient memory for this to be effective.

• Specific to Tables and Indexes: This command will affect all user tables and indexes. It does not automatically apply to other structures like the redo log buffer or shared pool, which will still be managed according to normal memory rules.


• System Behavior: Even with the FORCE FULL DATABASE CACHING command, blocks are still subject to eviction when there are memory pressures, but the priority of eviction is adjusted. Oracle will avoid evicting blocks from the cache unless absolutely necessary.

In summary:

Yes, using ALTER DATABASE FORCE FULL DATABASE CACHING will attempt to keep all database blocks in memory, but it's still dependent on available system memory. It doesn't guarantee that all blocks will always stay in memory if there are memory limitations or if the system faces memory pressure, but Oracle will prioritize caching them more aggressively compared to the default behavior.


Alireza Kamrani.