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| <h2> |
| <a id="segment-management" class="anchor" href="#segment-management" aria-hidden="true"><span aria-hidden="true" class="octicon octicon-link"></span></a>SEGMENT MANAGEMENT</h2> |
| <p>Each load into CarbonData is written into a separate folder called Segment.Segments is a powerful |
| concept which helps to maintain consistency of data and easy transaction management.CarbonData provides DML (Data Manipulation Language) commands to maintain the segments.</p> |
| <ul> |
| <li><a href="#show-segment">Show Segments</a></li> |
| <li><a href="#delete-segment-by-id">Delete Segment by ID</a></li> |
| <li><a href="#delete-segment-by-date">Delete Segment by Date</a></li> |
| <li><a href="#query-data-with-specified-segments">Query Data with Specified Segments</a></li> |
| </ul> |
| <h3> |
| <a id="show-segment" class="anchor" href="#show-segment" aria-hidden="true"><span aria-hidden="true" class="octicon octicon-link"></span></a>SHOW SEGMENT</h3> |
| <p>This command is used to list the segments of CarbonData table.</p> |
| <pre><code>SHOW [HISTORY] SEGMENTS |
| [FOR TABLE | ON] [db_name.]table_name |
| [AS (select query from table_name_segments)] |
| </code></pre> |
| <p>By default, SHOW SEGMENT command will return following fields:</p> |
| <ul> |
| <li> |
| <p>Segment ID</p> |
| </li> |
| <li> |
| <p>Segment Status</p> |
| </li> |
| <li> |
| <p>Load Start Time</p> |
| </li> |
| <li> |
| <p>Load Time Taken</p> |
| </li> |
| <li> |
| <p>Partition</p> |
| </li> |
| <li> |
| <p>Data Size</p> |
| </li> |
| <li> |
| <p>Index Size</p> |
| <p>Example: |
| Show visible segments</p> |
| <pre><code>SHOW SEGMENTS ON CarbonDatabase.CarbonTable |
| </code></pre> |
| <p>Show all segments, include invisible segments</p> |
| <pre><code>SHOW HISTORY SEGMENTS ON CarbonDatabase.CarbonTable |
| </code></pre> |
| <p>When more detail of the segment is required, user can issue SHOW SEGMENT by query.</p> |
| <p>The query should be against table name with '_segments' appended and select from following fields:</p> |
| </li> |
| <li> |
| <p>id: String, the id of the segment</p> |
| </li> |
| <li> |
| <p>status: String, status of the segment</p> |
| </li> |
| <li> |
| <p>loadStartTime: String, loading start time</p> |
| </li> |
| <li> |
| <p>loadEndTime: String, loading end time</p> |
| </li> |
| <li> |
| <p>timeTakenMs: Long, time spent in loading of the segment in milliseconds</p> |
| </li> |
| <li> |
| <p>partitions: String array, partition key and values</p> |
| </li> |
| <li> |
| <p>dataSize: Long, data size in bytes</p> |
| </li> |
| <li> |
| <p>indexSize: Long, index size in bytes</p> |
| </li> |
| <li> |
| <p>mergedToId: String, the target segment that this segment has been compacted</p> |
| </li> |
| <li> |
| <p>format: String, data format of the segment</p> |
| </li> |
| <li> |
| <p>path: String, in case of external segment this will be the path of the segment, otherwise it is null</p> |
| </li> |
| <li> |
| <p>segmentFileName: String, name of the segment file</p> |
| <p>Example:</p> |
| <pre><code>SHOW SEGMENTS ON CarbonTable AS |
| SELECT * FROM CarbonTable_segments |
| |
| SHOW SEGMENTS ON CarbonTable AS |
| SELECT id, dataSize FROM CarbonTable_segments |
| WHERE status='Success' |
| ORDER BY dataSize |
| |
| SHOW SEGMENTS ON CarbonTable AS |
| SELECT avg(timeTakenMs) FROM CarbonTable_segments |
| </code></pre> |
| </li> |
| </ul> |
| <h3> |
| <a id="delete-segment-by-id" class="anchor" href="#delete-segment-by-id" aria-hidden="true"><span aria-hidden="true" class="octicon octicon-link"></span></a>DELETE SEGMENT BY ID</h3> |
| <p>This command is used to delete segment by using the segment ID. Each segment has a unique segment ID associated with it. |
| Using this segment ID, you can remove the segment.</p> |
| <p>The following command will get the segmentID.</p> |
| <pre><code>SHOW SEGMENTS FOR TABLE [db_name.]table_name LIMIT number_of_segments |
| </code></pre> |
| <p>After you retrieve the segment ID of the segment that you want to delete, execute the following command to delete the selected segment.</p> |
| <pre><code>DELETE FROM TABLE [db_name.]table_name WHERE SEGMENT.ID IN (segment_id1, segments_id2, ...) |
| </code></pre> |
| <p>Example:</p> |
| <pre><code>DELETE FROM TABLE CarbonDatabase.CarbonTable WHERE SEGMENT.ID IN (0) |
| DELETE FROM TABLE CarbonDatabase.CarbonTable WHERE SEGMENT.ID IN (0,5,8) |
| </code></pre> |
| <h3> |
| <a id="delete-segment-by-date" class="anchor" href="#delete-segment-by-date" aria-hidden="true"><span aria-hidden="true" class="octicon octicon-link"></span></a>DELETE SEGMENT BY DATE</h3> |
| <p>This command will allow to delete the CarbonData segment(s) from the store based on the date provided by the user in the DML command. |
| The segment created before the particular date will be removed from the specific stores.</p> |
| <pre><code>DELETE FROM TABLE [db_name.]table_name WHERE SEGMENT.STARTTIME BEFORE DATE_VALUE |
| </code></pre> |
| <p>Example:</p> |
| <pre><code>DELETE FROM TABLE CarbonDatabase.CarbonTable WHERE SEGMENT.STARTTIME BEFORE '2017-06-01 12:05:06' |
| </code></pre> |
| <h3> |
| <a id="query-data-with-specified-segments" class="anchor" href="#query-data-with-specified-segments" aria-hidden="true"><span aria-hidden="true" class="octicon octicon-link"></span></a>QUERY DATA WITH SPECIFIED SEGMENTS</h3> |
| <p>This command is used to read data from specified segments during CarbonScan.</p> |
| <p>Get the Segment ID:</p> |
| <pre><code>SHOW SEGMENTS FOR TABLE [db_name.]table_name LIMIT number_of_segments |
| </code></pre> |
| <p>Set the segment IDs for table</p> |
| <pre><code>SET carbon.input.segments.<database_name>.<table_name> = <list of segment IDs> |
| </code></pre> |
| <p><strong>NOTE:</strong> |
| carbon.input.segments: Specifies the segment IDs to be queried. This property allows you to query specified segments of the specified table. The CarbonScan will read data from specified segments only.</p> |
| <p>If user wants to query with segments reading in multi-threading mode, then CarbonSession.threadSet can be used instead of SET query.</p> |
| <pre><code>CarbonSession.threadSet ("carbon.input.segments.<database_name>.<table_name>","<list of segment IDs>"); |
| </code></pre> |
| <p>Reset the segment IDs</p> |
| <pre><code>SET carbon.input.segments.<database_name>.<table_name> = *; |
| </code></pre> |
| <p>If user wants to query with segments reading in multi-threading mode, then CarbonSession.threadSet can be used instead of SET query.</p> |
| <pre><code>CarbonSession.threadSet ("carbon.input.segments.<database_name>.<table_name>","*"); |
| </code></pre> |
| <p><strong>Examples:</strong></p> |
| <ul> |
| <li>Example to show the list of segment IDs, segment status, and other required details and then specify the list of segments to be read.</li> |
| </ul> |
| <pre><code>SHOW SEGMENTS FOR carbontable1; |
| |
| SET carbon.input.segments.db.carbontable1 = 1,3,9; |
| </code></pre> |
| <ul> |
| <li>Example to query with segments reading in multi-threading mode:</li> |
| </ul> |
| <pre><code>CarbonSession.threadSet ("carbon.input.segments.db.carbontable_Multi_Thread","1,3"); |
| </code></pre> |
| <ul> |
| <li>Example for threadset in multi-thread environment (following shows how it is used in Scala code):</li> |
| </ul> |
| <pre><code>def main(args: Array[String]) { |
| Future { |
| CarbonSession.threadSet ("carbon.input.segments.db.carbontable_Multi_Thread","1") |
| spark.sql("select count(empno) from carbon.input.segments.db.carbontable_Multi_Thread").show(); |
| } |
| } |
| </code></pre> |
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