How to Coalesce Tablespace for improving space management by reducing fragmentation

In Oracle, tablespace coalescing is the process of merging adjacent free space chunks in a tablespace into a single larger chunk of free space. This process helps in improving space management by reducing fragmentation, making larger contiguous blocks of free space available for future use.

When and Why to Coalesce

• Fragmentation: Over time, as objects are created, modified, and dropped, the tablespace can become fragmented. Fragmentation results in many small non-contiguous free space chunks, which can make it difficult to allocate larger extents.
• Performance: Coalescing helps in improving performance by ensuring that larger chunks of space are available for allocation, which can reduce the time and resources needed to manage extents.

How to Coalesce Tablespace

Oracle provides the ALTER TABLESPACE ... COALESCE command to manually coalesce a tablespace. This command can be executed by a DBA to coalesce free space in the tablespace.

Syntax

ALTER TABLESPACE tablespace_name COALESCE;

• tablespace_name: The name of the tablespace to coalesce.

Example

Suppose you have a tablespace named USER_DATA, and you want to coalesce the free space within this tablespace. Here’s how you would do it:

ALTER TABLESPACE USER_DATA COALESCE;

Automatic Space Management

In modern Oracle versions, especially when using locally managed tablespaces with Automatic Segment Space Management (ASSM), the need to manually coalesce tablespaces is greatly reduced. Oracle automatically manages free space more efficiently, and coalescing is often performed automatically.

Checking Free Space and Fragmentation

You can query the DBA_FREE_SPACE view to check the free space and fragmentation in a tablespace.

SELECT tablespace_name, file_id, block_id, bytes
FROM dba_free_space
WHERE tablespace_name = 'USER_DATA'
ORDER BY file_id, block_id;

This query will show you the chunks of free space within the specified tablespace.

Example Scenario

Let’s say you notice that your USER_DATA tablespace is highly fragmented, and you want to coalesce it:

Check the Free Space:

           SELECT tablespace_name, file_id, block_id, bytes
           FROM dba_free_space
           WHERE tablespace_name = 'USER_DATA'
           ORDER BY file_id, block_id;

This gives you an idea of how fragmented your tablespace is.

Coalesce the Tablespace:

ALTER TABLESPACE USER_DATA COALESCE;

Recheck the Free Space:

SELECT tablespace_name, file_id, block_id, bytes

FROM dba_free_space

WHERE tablespace_name = 'USER_DATA'

ORDER BY file_id, block_id;

After coalescing, you should see fewer, larger chunks of free space, indicating reduced fragmentation.

Summary

• Tablespace Coalescing: Merges adjacent free space chunks to reduce fragmentation.
• Command: ALTER TABLESPACE tablespace_name COALESCE;
• Modern Oracle Versions: Automatic Segment Space Management (ASSM) reduces the need for manual coalescing.
• Benefits: Improves space management and performance by making larger contiguous free space available.

Using the ALTER TABLESPACE ... COALESCE command helps maintain optimal space utilization within Oracle tablespaces, especially in environments where manual space management is required.