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Tutorial: Compacting segments
This tutorial demonstrates how to compact existing segments into fewer but larger segments.
Because there is some per-segment memory and processing overhead, it can sometimes be beneficial to reduce the total number of segments.
For this tutorial, we'll assume you've already downloaded Druid as described in the single-machine quickstart and have it running on your local machine.
It will also be helpful to have finished Tutorial: Loading a file and Tutorial: Querying data.
Load the initial data
For this tutorial, we'll be using the Wikipedia edits sample data, with an ingestion task spec that will create a separate segment for each hour in the input data.
The ingestion spec can be found at quickstart/tutorial/compaction-init-index.json. Let's submit that spec, which will create a datasource called compaction-tutorial:
bin/post-index-task --file quickstart/tutorial/compaction-init-index.json
After the ingestion completes, go to http://localhost:8081/#/datasources/compaction-tutorial in a browser to view information about the new datasource in the Coordinator console.
There will be 24 segments for this datasource, one segment per hour in the input data:
Running a COUNT(*) query on this datasource shows that there are 39,244 rows:
dsql> select count(*) from "compaction-tutorial";
┌────────┐
│ EXPR$0 │
├────────┤
│ 39244 │
└────────┘
Retrieved 1 row in 1.38s.
Compact the data
Let's now combine these 24 segments into one segment.
We have included a compaction task spec for this tutorial datasource at quickstart/tutorial/compaction-final-index.json:
{
"type": "compact",
"dataSource": "compaction-tutorial",
"interval": "2015-09-12/2015-09-13",
"tuningConfig" : {
"type" : "index",
"targetPartitionSize" : 5000000,
"maxRowsInMemory" : 25000,
"forceExtendableShardSpecs" : true
}
}
This will compact all segments for the interval 2015-09-12/2015-09-13 in the compaction-tutorial datasource.
The parameters in the tuningConfig control how many segments will be present in the compacted set of segments.
In this tutorial example, only one compacted segment will be created, as the 39244 rows in the input is less than the 5000000 targetPartitionSize.
Let's submit this task now:
bin/post-index-task --file quickstart/tutorial/compaction-final-index.json
After the task finishes, refresh the http://localhost:8081/#/datasources/compaction-tutorial page.
The original 24 segments will eventually be marked as "unused" by the Coordinator and removed, with the new compacted segment remaining.
By default, the Druid coordinator will not mark segments as unused until the coordinator process has been up for at least 15 minutes, so you may see the old segment set and the new compacted set at the same time in the coordinator, e.g.:
The new compacted segment has a more recent version than the original segments, so even when both sets of segments are shown by the coordinator, queries will only read from the new compacted segment.
Let's try running a COUNT(*) on compaction-tutorial again, where the row count should still be 39,244:
dsql> select count(*) from "compaction-tutorial";
┌────────┐
│ EXPR$0 │
├────────┤
│ 39244 │
└────────┘
Retrieved 1 row in 1.30s.
After the coordinator has been running for at least 15 minutes, the http://localhost:8081/#/datasources/compaction-tutorial page should show there is only 1 segment:
