How Can I Check the Size of a Table in Oracle?
When managing databases in Oracle, understanding the size of your tables is crucial for optimizing performance, planning storage, and maintaining overall system health. Knowing how to check table size allows database administrators and developers to monitor space usage, identify potential bottlenecks, and make informed decisions about data management strategies. Whether you’re dealing with a small application or a large enterprise system, having quick access to this information can significantly streamline your workflow.
Oracle offers several methods to determine the size of a table, each suited to different needs and scenarios. From querying data dictionary views to using built-in functions, these approaches provide insights into how much space your tables occupy on disk. Grasping the fundamentals of these techniques not only helps in routine maintenance but also plays a vital role during database tuning and capacity planning.
In the following sections, we will explore the various ways to check table size in Oracle, highlighting their advantages and typical use cases. By the end, you’ll be equipped with practical knowledge to efficiently monitor and manage your Oracle tables’ storage footprint.
Using Data Dictionary Views to Determine Table Size
Oracle provides several data dictionary views that are instrumental in retrieving detailed information about the size of tables within a database. These views allow DBAs and developers to analyze storage usage at various granularities, including tables, partitions, and indexes.
One of the most commonly used views for this purpose is `DBA_SEGMENTS` (or `USER_SEGMENTS` when querying within a specific schema). This view contains segment-level storage information such as the amount of space allocated for a table or its associated objects.
To check the size of a table using `DBA_SEGMENTS`, you can execute a query like this:
“`sql
SELECT segment_name,
segment_type,
bytes / 1024 / 1024 AS size_in_mb
FROM dba_segments
WHERE segment_name = UPPER(‘your_table_name’)
AND owner = UPPER(‘your_schema_name’);
“`
This query returns the segment name, type (e.g., TABLE, INDEX, LOBSEGMENT), and size in megabytes. Note that a table might have multiple segments if it has partitions, LOBs, or indexes.
Other useful views include:
- `DBA_EXTENTS`: Displays the extents allocated to the segment, useful for more detailed space usage analysis.
- `DBA_TABLES`: Contains metadata about tables, including `NUM_ROWS` and other statistics but does not directly show size.
- `DBA_LOBS` and `DBA_LOB_SEGMENTS`: Provide details about LOB storage, which can significantly impact table size.
Calculating Table Size Including Indexes and LOBs
When assessing the total storage used by a table, it is important to consider not only the table data itself but also associated objects such as indexes and LOB segments. These components can contribute significantly to the overall disk space consumed.
To get a complete picture, you can aggregate the sizes from the `DBA_SEGMENTS` view for all segments related to the table:
“`sql
SELECT segment_type,
SUM(bytes) / 1024 / 1024 AS size_in_mb
FROM dba_segments
WHERE segment_name = UPPER(‘your_table_name’)
AND owner = UPPER(‘your_schema_name’)
GROUP BY segment_type;
“`
This query returns the size breakdown by segment type, helping you identify where the space is being used.
Using PL/SQL to Automate Table Size Retrieval
For repeated or programmatic retrieval of table sizes, you can encapsulate the logic within a PL/SQL procedure or function. This method allows for easy integration into maintenance scripts or monitoring tools.
Here is an example function that returns the total size in megabytes of a specified table including its indexes and LOB segments:
“`plsql
CREATE OR REPLACE FUNCTION get_table_size (
p_owner IN VARCHAR2,
p_table_name IN VARCHAR2
) RETURN NUMBER IS
v_total_size NUMBER;
BEGIN
SELECT NVL(SUM(bytes), 0) / 1024 / 1024
INTO v_total_size
FROM dba_segments
WHERE owner = UPPER(p_owner)
AND segment_name = UPPER(p_table_name);
RETURN v_total_size;
END;
/
“`
You can then call this function as follows:
“`sql
SELECT get_table_size(‘HR’, ‘EMPLOYEES’) AS size_mb FROM dual;
“`
This approach simplifies repeated checks and can be extended to include additional segment types or filters.
Understanding the Output and Limitations
When interpreting size data from Oracle’s data dictionary views, it’s important to keep in mind:
- The sizes reported correspond to allocated space, not necessarily used space. Free space within extents is included in the size calculations.
- Tables with partitions will have multiple segment entries; summing these provides the total allocated size.
- If table compression is enabled, the physical size may be less than the logical data size.
- Temporary segments and undo data are not included in these views.
| View | Description | Use Case |
|---|---|---|
| DBA_SEGMENTS | Contains segment allocation info for tables, indexes, and LOBs | Primary source for table size and related objects |
| DBA_EXTENTS | Details individual extents for each segment | Analyzing fragmentation and extent distribution |
| DBA_TABLES | Metadata and statistics about tables | Checking row counts and table properties |
| DBA_LOB_SEGMENTS | Information on LOB storage segments | Assessing size of large object data linked to tables |
Methods to Check Table Size in Oracle
Oracle provides several ways to determine the size of a table, depending on the level of detail and accuracy required. Common approaches involve querying data dictionary views and using built-in PL/SQL packages.
Below are some of the most effective methods to check the size of a table in Oracle:
- Querying DBA/USER_SEGMENTS View: This method retrieves the allocated space for a table segment, including data and indexes.
- Using DBMS_SPACE Package: Provides detailed space usage statistics, including allocated, used, and free space within a table segment.
- Estimating Table Size via Data Length: Summing up the lengths of all rows can provide approximate data size but excludes overhead and segment metadata.
Querying USER_SEGMENTS to Find Table Size
The USER_SEGMENTS view contains segment-level storage information. The size of the table is represented by the BYTES and BLOCKS columns, indicating the total allocated space.
“`sql
SELECT
segment_name,
segment_type,
bytes / 1024 / 1024 AS size_mb,
blocks
FROM
user_segments
WHERE
segment_name = UPPER(‘your_table_name’)
AND segment_type = ‘TABLE’;
“`
| Column | Description |
|---|---|
| segment_name | Name of the table segment |
| segment_type | Type of segment (TABLE, INDEX, etc.) |
| size_mb | Size of the table segment in megabytes |
| blocks | Number of data blocks allocated |
This query reports the total space allocated to the table on disk, which includes data and row overhead but excludes dependent objects like indexes.
Checking Table Size Including Indexes
To obtain a comprehensive size report that includes both the table and its associated indexes, you can query the USER_SEGMENTS view for all relevant segments.
“`sql
SELECT
segment_type,
SUM(bytes) / 1024 / 1024 AS size_mb
FROM
user_segments
WHERE
segment_name IN (
SELECT index_name FROM user_indexes WHERE table_name = UPPER(‘your_table_name’)
UNION ALL
SELECT UPPER(‘your_table_name’) FROM dual
)
GROUP BY
segment_type;
“`
| Segment Type | Size (MB) |
|---|---|
| TABLE | Data segment size in MB |
| INDEX | Total size of all indexes in MB |
This allows you to understand the total storage footprint of a table, including its indexes.
Using DBMS_SPACE to Get Detailed Space Usage
The Oracle built-in package DBMS_SPACE offers precise details about the space usage of segments. The procedure SPACE_USAGE can be used to retrieve allocated space, used space, and free space.
“`plsql
DECLARE
total_blocks NUMBER;
total_bytes NUMBER;
unused_blocks NUMBER;
unused_bytes NUMBER;
BEGIN
DBMS_SPACE.SPACE_USAGE(
segment_owner => USER,
segment_name => ‘YOUR_TABLE_NAME’,
segment_type => ‘TABLE’,
total_blocks => total_blocks,
total_bytes => total_bytes,
unused_blocks => unused_blocks,
unused_bytes => unused_bytes
);
DBMS_OUTPUT.PUT_LINE(‘Total Size (bytes): ‘ || total_bytes);
DBMS_OUTPUT.PUT_LINE(‘Used Size (bytes): ‘ || (total_bytes – unused_bytes));
DBMS_OUTPUT.PUT_LINE(‘Unused Size (bytes): ‘ || unused_bytes);
END;
/
“`
total_bytes: Total allocated space for the segment.unused_bytes: Space within the segment that is allocated but not currently used.- Used space can be calculated by subtracting unused space from the total.
This method is highly accurate but requires execution privileges on the DBMS_SPACE package and may not be available in all environments.
Estimating Table Size Using Data Length from ALL_TABLES
Oracle’s ALL_TABLES and USER_TABLES views provide an estimated average row length and number of rows, which can be used to approximate the table size.
“`sql
SELECT
table_name,
num_rows,
avg_row_len,
(num_rows * avg_row_len) / 1024 / 1024 AS approx_size_mb
FROM
user_tables
WHERE
table_name = UPPER(‘your_table_name’);
“`
| Column | Description |
|---|---|
| num_rows | Number of rows in the table (based on statistics) |