Help Center/ MapReduce Service/ Component Operation Guide (Paris Region)/ Using CarbonData (for MRS 3.x or Later)/ CarbonData Syntax Reference/ DDL/ ALTER TABLE COMPACTION
Updated on 2022-02-22 GMT+08:00
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ALTER TABLE COMPACTION

Function

The ALTER TABLE COMPACTION command is used to merge a specified number of segments into a single segment. This improves the query performance of a table.

Syntax

ALTER TABLE[db_name.]table_name COMPACT 'MINOR/MAJOR/SEGMENT_INDEX';

ALTER TABLE[db_name.]table_name COMPACT 'CUSTOM' WHERE SEGMENT.ID IN (id1, id2, ...);

Parameter Description

Table 1 ALTER TABLE COMPACTION parameters

Parameter

Description

db_name

Database name. If this parameter is not specified, the current database is selected.

table_name

Table name.

MINOR

Minor compaction. For details, see Combining Segments.

MAJOR

Major compaction. For details, see Combining Segments.

SEGMENT_INDEX

This configuration enables you to merge all the CarbonData index files (.carbonindex) inside a segment to a single CarbonData index merge file (.carbonindexmerge). This enhances the first query performance. For more information, see Table 1.

CUSTOM

Custom compaction. For details, see Combining Segments.

Precautions

N/A

Examples

ALTER TABLE ProductDatabase COMPACT 'MINOR';

ALTER TABLE ProductDatabase COMPACT 'MAJOR';

ALTER TABLE ProductDatabase COMPACT 'SEGMENT_INDEX';

ALTER TABLE ProductDatabase COMPACT 'CUSTOM' WHERE SEGMENT.ID IN (0, 1);

System Response

ALTER TABLE COMPACTION does not show the response of the compaction because it is run in the background.

If you want to view the response of minor and major compactions, you can check the logs or run the SHOW SEGMENTS command.

Example:

+------+------------+--------------------------+------------------+------------+------------+-------------+--------------+--+
|  ID  |   Status   |     Load Start Time      | Load Time Taken  | Partition  | Data Size  | Index Size  | File Format  |
+------+------------+--------------------------+------------------+------------+------------+-------------+--------------+--+
| 3    | Success    | 2020-09-28 22:53:26.336  | 3.726S           | {}         | 6.47KB     | 3.30KB      | columnar_v3  |
| 2    | Success    | 2020-09-28 22:53:01.702  | 6.688S           | {}         | 6.47KB     | 3.30KB      | columnar_v3  |
| 1    | Compacted  | 2020-09-28 22:51:15.242  | 5.82S            | {}         | 6.50KB     | 3.43KB      | columnar_v3  |
| 0.1  | Success    | 2020-10-30 20:49:24.561  | 16.66S           | {}         | 12.87KB    | 6.91KB      | columnar_v3  |
| 0    | Compacted  | 2020-09-28 22:51:02.6    | 6.819S           | {}         | 6.50KB     | 3.43KB      | columnar_v3  |
+------+------------+--------------------------+------------------+------------+------------+-------------+--------------+--+

In the preceding information:

  • Compacted indicates that data has been compacted.
  • 0.1 indicates the compacting result of segment 0 and segment 1.

The compact operation does not incur any change to other operations.

Compacted segments, such as segment 0 and segment 1, become useless. To save space, before you perform other operations, run the CLEAN FILES command to delete compacted segments. For more information about the CLEAN FILES command, see CLEAN FILES.

Parent topic: DDL