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druid/docs/content/querying/groupbyquery.md

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groupBy Queries

These types of queries take a groupBy query object and return an array of JSON objects where each object represents a grouping asked for by the query.

Note: If you are doing aggregations with time as your only grouping, or an ordered groupBy over a single dimension, consider Timeseries and TopN queries as well as groupBy. Their performance may be better in some cases. See Alternatives below for more details.

An example groupBy query object is shown below:

{
  "queryType": "groupBy",
  "dataSource": "sample_datasource",
  "granularity": "day",
  "dimensions": ["country", "device"],
  "limitSpec": { "type": "default", "limit": 5000, "columns": ["country", "data_transfer"] },
  "filter": {
    "type": "and",
    "fields": [
      { "type": "selector", "dimension": "carrier", "value": "AT&T" },
      { "type": "or", 
        "fields": [
          { "type": "selector", "dimension": "make", "value": "Apple" },
          { "type": "selector", "dimension": "make", "value": "Samsung" }
        ]
      }
    ]
  },
  "aggregations": [
    { "type": "longSum", "name": "total_usage", "fieldName": "user_count" },
    { "type": "doubleSum", "name": "data_transfer", "fieldName": "data_transfer" }
  ],
  "postAggregations": [
    { "type": "arithmetic",
      "name": "avg_usage",
      "fn": "/",
      "fields": [
        { "type": "fieldAccess", "fieldName": "data_transfer" },
        { "type": "fieldAccess", "fieldName": "total_usage" }
      ]
    }
  ],
  "intervals": [ "2012-01-01T00:00:00.000/2012-01-03T00:00:00.000" ],
  "having": {
  	"type": "greaterThan",
  	"aggregation": "total_usage",
  	"value": 100
  }
}

There are 11 main parts to a groupBy query:

property description required?
queryType This String should always be "groupBy"; this is the first thing Druid looks at to figure out how to interpret the query yes
dataSource A String or Object defining the data source to query, very similar to a table in a relational database. See DataSource for more information. yes
dimensions A JSON list of dimensions to do the groupBy over; or see DimensionSpec for ways to extract dimensions. yes
limitSpec See LimitSpec. no
having See Having. no
granularity Defines the granularity of the query. See Granularities yes
filter See Filters no
aggregations See Aggregations no
postAggregations See Post Aggregations no
intervals A JSON Object representing ISO-8601 Intervals. This defines the time ranges to run the query over. yes
context An additional JSON Object which can be used to specify certain flags. no

To pull it all together, the above query would return n*m data points, up to a maximum of 5000 points, where n is the cardinality of the country dimension, m is the cardinality of the device dimension, each day between 2012-01-01 and 2012-01-03, from the sample_datasource table. Each data point contains the (long) sum of total_usage if the value of the data point is greater than 100, the (double) sum of data_transfer and the (double) result of total_usage divided by data_transfer for the filter set for a particular grouping of country and device. The output looks like this:

[ 
  {
    "version" : "v1",
    "timestamp" : "2012-01-01T00:00:00.000Z",
    "event" : {
      "country" : <some_dim_value_one>,
      "device" : <some_dim_value_two>,
      "total_usage" : <some_value_one>,
      "data_transfer" :<some_value_two>,
      "avg_usage" : <some_avg_usage_value>
    }
  }, 
  {
    "version" : "v1",
    "timestamp" : "2012-01-01T00:00:12.000Z",
    "event" : {
      "dim1" : <some_other_dim_value_one>,
      "dim2" : <some_other_dim_value_two>,
      "sample_name1" : <some_other_value_one>,
      "sample_name2" :<some_other_value_two>,
      "avg_usage" : <some_other_avg_usage_value>
    }
  },
...
]

Behavior on multi-value dimensions

groupBy queries can group on multi-value dimensions. When grouping on a multi-value dimension, all values from matching rows will be used to generate one group per value. It's possible for a query to return more groups than there are rows. For example, a groupBy on the dimension tags with filter "t1" AND "t3" would match only row1, and generate a result with three groups: t1, t2, and t3. If you only need to include values that match your filter, you can use a filtered dimensionSpec. This can also improve performance.

See Multi-value dimensions for more details.

Implementation details

Strategies

GroupBy queries can be executed using two different strategies. The default strategy for a cluster is determined by the "druid.query.groupBy.defaultStrategy" runtime property on the broker. This can be overridden using "groupByStrategy" in the query context. If neither the context field nor the property is set, the "v2" strategy will be used.

  • "v2", the default, is designed to offer better performance and memory management. This strategy generates per-segment results using a fully off-heap map. Data nodes merge the per-segment results using a fully off-heap concurrent facts map combined with an on-heap string dictionary. This may optionally involve spilling to disk. Data nodes return sorted results to the broker, which merges result streams using an N-way merge. The broker materializes the results if necessary (e.g. if the query sorts on columns other than its dimensions). Otherwise, it streams results back as they are merged.

  • "v1", a legacy engine, generates per-segment results on data nodes (historical, realtime, middleManager) using a map which is partially on-heap (dimension keys and the map itself) and partially off-heap (the aggregated values). Data nodes then merge the per-segment results using Druid's indexing mechanism. This merging is multi-threaded by default, but can optionally be single-threaded. The broker merges the final result set using Druid's indexing mechanism again. The broker merging is always single-threaded. Because the broker merges results using the indexing mechanism, it must materialize the full result set before returning any results. On both the data nodes and the broker, the merging index is fully on-heap by default, but it can optionally store aggregated values off-heap.

Differences between v1 and v2

Query API and results are compatible between the two engines; however, there are some differences from a cluster configuration perspective:

  • groupBy v1 controls resource usage using a row-based limit (maxResults) whereas groupBy v2 uses bytes-based limits. In addition, groupBy v1 merges results on-heap, whereas groupBy v2 merges results off-heap. These factors mean that memory tuning and resource limits behave differently between v1 and v2. In particular, due to this, some queries that can complete successfully in one engine may exceed resource limits and fail with the other engine. See the "Memory tuning and resource limits" section for more details.
  • groupBy v1 imposes no limit on the number of concurrently running queries, whereas groupBy v2 controls memory usage by using a finite-sized merge buffer pool. By default, the number of merge buffers is 1/4 the number of processing threads. You can adjust this as necessary to balance concurrency and memory usage.
  • groupBy v1 supports caching on either the broker or historical nodes, whereas groupBy v2 only supports caching on historical nodes.
  • groupBy v1 supports using chunkPeriod to parallelize merging on the broker, whereas groupBy v2 ignores chunkPeriod.
  • groupBy v2 supports both array-based aggregation and hash-based aggregation. The array-based aggregation is used only when the grouping key is a single indexed string column. In array-based aggregation, the dictionary-encoded value is used as the index, so the aggregated values in the array can be accessed directly without finding buckets based on hashing.

Memory tuning and resource limits

When using groupBy v2, three parameters control resource usage and limits:

  • druid.processing.buffer.sizeBytes: size of the off-heap hash table used for aggregation, per query, in bytes. At most druid.processing.numMergeBuffers of these will be created at once, which also serves as an upper limit on the number of concurrently running groupBy queries.
  • druid.query.groupBy.maxMergingDictionarySize: size of the on-heap dictionary used when grouping on strings, per query, in bytes. Note that this is based on a rough estimate of the dictionary size, not the actual size.
  • druid.query.groupBy.maxOnDiskStorage: amount of space on disk used for aggregation, per query, in bytes. By default, this is 0, which means aggregation will not use disk.

If maxOnDiskStorage is 0 (the default) then a query that exceeds either the on-heap dictionary limit, or the off-heap aggregation table limit, will fail with a "Resource limit exceeded" error describing the limit that was exceeded.

If maxOnDiskStorage is greater than 0, queries that exceed the in-memory limits will start using disk for aggregation. In this case, when either the on-heap dictionary or off-heap hash table fills up, partially aggregated records will be sorted and flushed to disk. Then, both in-memory structures will be cleared out for further aggregation. Queries that then go on to exceed maxOnDiskStorage will fail with a "Resource limit exceeded" error indicating that they ran out of disk space.

With groupBy v2, cluster operators should make sure that the off-heap hash tables and on-heap merging dictionaries will not exceed available memory for the maximum possible concurrent query load (given by druid.processing.numMergeBuffers).

When using groupBy v1, all aggregation is done on-heap, and resource limits are done through the parameter druid.query.groupBy.maxResults. This is a cap on the maximum number of results in a result set. Queries that exceed this limit will fail with a "Resource limit exceeded" error indicating they exceeded their row limit. Cluster operators should make sure that the on-heap aggregations will not exceed available JVM heap space for the expected concurrent query load.

Alternatives

There are some situations where other query types may be a better choice than groupBy.

  • For queries with no "dimensions" (i.e. grouping by time only) the Timeseries query will generally be faster than groupBy. The major differences are that it is implemented in a fully streaming manner (taking advantage of the fact that segments are already sorted on time) and does not need to use a hash table for merging.

  • For queries with a single "dimensions" element (i.e. grouping by one string dimension), the TopN query will sometimes be faster than groupBy. This is especially true if you are ordering by a metric and find approximate results acceptable.

Nested groupBys

Nested groupBys (dataSource of type "query") are performed differently for "v1" and "v2". The broker first runs the inner groupBy query in the usual way. "v1" strategy then materializes the inner query's results on-heap with Druid's indexing mechanism, and runs the outer query on these materialized results. "v2" strategy runs the outer query on the inner query's results stream with off-heap fact map and on-heap string dictionary that can spill to disk. Both strategy perform the outer query on the broker in a single-threaded fashion.

Server configuration

When using the "v2" strategy, the following runtime properties apply:

Property Description Default
druid.query.groupBy.defaultStrategy Default groupBy query strategy. v2
druid.query.groupBy.bufferGrouperInitialBuckets Initial number of buckets in the off-heap hash table used for grouping results. Set to 0 to use a reasonable default. 0
druid.query.groupBy.bufferGrouperMaxLoadFactor Maximum load factor of the off-heap hash table used for grouping results. When the load factor exceeds this size, the table will be grown or spilled to disk. Set to 0 to use a reasonable default. 0
druid.query.groupBy.maxMergingDictionarySize Maximum amount of heap space (approximately) to use for the string dictionary during merging. When the dictionary exceeds this size, a spill to disk will be triggered. 100000000
druid.query.groupBy.maxOnDiskStorage Maximum amount of disk space to use, per-query, for spilling result sets to disk when either the merging buffer or the dictionary fills up. Queries that exceed this limit will fail. Set to zero to disable disk spilling. 0 (disabled)
druid.query.groupBy.singleThreaded Merge results using a single thread. false

This may require allocating more direct memory. The amount of direct memory needed by Druid is at least druid.processing.buffer.sizeBytes * (druid.processing.numMergeBuffers + druid.processing.numThreads + 1). You can ensure at least this amount of direct memory is available by providing -XX:MaxDirectMemorySize=<VALUE> at the command line.

When using the "v1" strategy, the following runtime properties apply:

Property Description Default
druid.query.groupBy.defaultStrategy Default groupBy query strategy. v2
druid.query.groupBy.maxIntermediateRows Maximum number of intermediate rows for the per-segment grouping engine. This is a tuning parameter that does not impose a hard limit; rather, it potentially shifts merging work from the per-segment engine to the overall merging index. Queries that exceed this limit will not fail. 50000
druid.query.groupBy.maxResults Maximum number of results. Queries that exceed this limit will fail. 500000
druid.query.groupBy.singleThreaded Merge results using a single thread. false

Query context

When using the "v2" strategy, the following query context parameters apply:

Property Description
groupByStrategy Overrides the value of druid.query.groupBy.defaultStrategy for this query.
groupByIsSingleThreaded Overrides the value of druid.query.groupBy.singleThreaded for this query.
bufferGrouperInitialBuckets Overrides the value of druid.query.groupBy.bufferGrouperInitialBuckets for this query.
bufferGrouperMaxLoadFactor Overrides the value of druid.query.groupBy.bufferGrouperMaxLoadFactor for this query.
maxMergingDictionarySize Can be used to lower the value of druid.query.groupBy.maxMergingDictionarySize for this query.
maxOnDiskStorage Can be used to lower the value of druid.query.groupBy.maxOnDiskStorage for this query.
sortByDimsFirst Sort the results first by dimension values and then by timestamp.
forcePushDownLimit When all fields in the orderby are part of the grouping key, the broker will push limit application down to the historical nodes. When the sorting order uses fields that are not in the grouping key, applying this optimization can result in approximate results with unknown accuracy, so this optimization is disabled by default in that case. Enabling this context flag turns on limit push down for limit/orderbys that contain non-grouping key columns.
forceHashAggregation Force to use hash-based aggregation.

When using the "v1" strategy, the following query context parameters apply:

Property Description
groupByStrategy Overrides the value of druid.query.groupBy.defaultStrategy for this query.
groupByIsSingleThreaded Overrides the value of druid.query.groupBy.singleThreaded for this query.
maxIntermediateRows Can be used to lower the value of druid.query.groupBy.maxIntermediateRows for this query.
maxResults Can be used to lower the value of druid.query.groupBy.maxResults for this query.
useOffheap Set to true to store aggregations off-heap when merging results.