Uncovering Table Storage Usage in Oracle Database: A Guide with dba_segments

For Oracle Database administrators (DBAs) and developers, managing storage efficiently is critical. Keeping track of table sizes helps identify potential space hogs and optimize resource allocation. This guide delves into a powerful SQL query that leverages the dba_segments view to effectively list all tables owned by a user, sorted by their size in megabytes (MB).

Sample SQL Command

 1List all tables owned by a user sorted by size
 2set lines 100 pages 999
 3col	segment_name	format a40
 4col 	mb 		format 999,999,999
 5select	segment_name
 6,	ceil(sum(bytes) / 1024 / 1024) "MB"
 7from	dba_segments
 8where	owner like '&user'
 9and	segment_type = 'TABLE'
10group	by segment_name
11order 	by ceil(sum(bytes) / 1024 / 1024) desc
12/

Understanding Table Storage in Oracle Database

Tables in Oracle Database store data and are comprised of segments that physically hold the data on disk. These segments can vary in size depending on the amount of data stored and the table's structure. Monitoring table size is essential for:

• Identifying Storage Bottlenecks: Large tables can consume significant disk space, impacting performance. Early detection helps in implementing storage optimization strategies.
• Optimizing Resource Allocation: Understanding table sizes allows for allocating storage resources efficiently across different database objects.
• Planning for Growth: Anticipating future data growth helps in proactively provisioning additional storage to accommodate it.

Unveiling Table Sizes with dba_segments

The dba_segments view provides comprehensive information about all database segments, including tables. By leveraging this view, we can construct a tailored SQL query to identify table sizes for a specific user.

Breakdown of the SQL Query:

 1set lines 100 pages 999
 2col segment_name format a40
 3col mb format 999,999,999
 4select segment_name
 5, ceil(sum(bytes) / 1024 / 1024) "MB"
 6from dba_segments
 7where owner like '&user'
 8and segment_type = 'TABLE'
 9group by segment_name
10order by ceil(sum(bytes) / 1024 / 1024) desc
11/

• set lines 100 pages 999: This line sets the default number of rows displayed per page (100) and allows for scrolling through a large result set (999 pages).
• col segment_name format a40&col mb format 999,999,999: These lines define column formatting for better readability. The segment_name is left-aligned with a width of 40 characters, and the "MB" column displays numbers with commas for easier visualization of large values.
• select segment_name, ceil(sum(bytes) / 1024 / 1024) "MB": This line selects the segment_name (table name) and calculates the size in MB. The ceil function rounds the calculated size up to the nearest whole number.
• from dba_segments: This clause specifies the dba_segments view as the data source.
• where owner like '&user': This clause filters the results to include only tables owned by the current user (replace&userwith your actual username).
• and segment_type = 'TABLE': This clause further refines the results to include only segments of type 'TABLE'.
• group by segment_name: This clause groups the results by the segment_name , ensuring each table appears only once with its total size.
• order by ceil(sum(bytes) / 1024 / 1024) desc: This clause sorts the results in descending order based on the calculated size in MB, allowing you to identify the largest tables first.
• /: This symbol terminates the SQL statement.

Key Points about the Query:

• This query retrieves information from the dba_segments view, which requires appropriate privileges to access.
• Replace &user with your actual username to filter results for your specific ownership.
• The query provides an estimate of table size based on segment data. For highly fragmented tables, additional considerations might be necessary for a more precise measurement.

Insights from Table Size Analysis

By running this query, you can gain valuable insights into your database's storage usage:

• Identify Storage Hogs: Large tables become immediately apparent, allowing you to prioritize optimization efforts.
• Plan for Future Growth: Understanding the current storage footprint helps in forecasting future needs and provisioning additional storage if required.
• Evaluate Schema Design: Analyzing table sizes can reveal opportunities for schema design improvements, such as partitioning large tables.

By regularly monitoring table sizes and implementing appropriate storage management strategies, you can optimize resource allocation within your Oracle Database.