Table of Contents

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). See How much direct memory does Druid use? for more details.

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.

Performance tuning for groupBy v2

Limit pushdown optimization

Druid pushes down the limit spec in groupBy queries to the segments on historicals wherever possible to early prune unnecessary intermediate results and minimize the amount of data transferred to brokers. By default, this technique is applied only when all fields in the orderBy spec is a subset of the grouping keys. This is because the limitPushDown doesn't guarantee the exact results if the orderBy spec includes any fields that are not in the grouping keys. However, you can enable this technique even in such cases if you can sacrifice some accuracy for fast query processing like in topN queries. See forceLimitPushDown in advanced groupBy v2 configurations.

Optimizing hash table

The groupBy v2 engine uses an open addressing hash table for aggregation. The hash table is initalized with a given initial bucket number and gradually grows on buffer full. On hash collisions, the linear probing technique is used.

The default number of initial buckets is 1024 and the default max load factor of the hash table is 0.7. If you can see too many collisions in the hash table, you can adjust these numbers. See bufferGrouperInitialBuckets and bufferGrouperMaxLoadFactor in Advanced groupBy v2 configurations.

Parallel combine

Once a historical finishes aggregation using the hash table, it sorts aggregates and merge them before sending to the broker for N-way merge aggregation in the broker. By default, historicals use all their available processing threads (configured by druid.processing.numThreads) for aggregation, but use a single thread for sorting and merging aggregates which is an http thread to send data to brokers.

This is to prevent some heavy groupBy queries from blocking other queries. In Druid, the processing threads are shared between all submitted queries and they are not interruptible. It means, if a heavy query takes all available processing threads, all other queries might be blocked until the heavy query is finished. GroupBy queries usually take longer time than timeseries or topN queries, they should release processing threads as soon as possible.

However, you might care about the performance of some really heavy groupBy queries. Usually, the performance bottleneck of heavy groupBy queries is merging sorted aggregates. In such cases, you can use processing threads for it as well. This is called parallel combine. To enable parallel combine, see numParallelCombineThreads in Advanced groupBy v2 configurations. Note that parallel combine can be enabled only when data is actually spilled (see Memory tuning and resource limits).

Once parallel combine is enabled, the groupBy v2 engine can create a combining tree for merging sorted aggregates. Each intermediate node of the tree is a thread merging aggregates from the child nodes. The leaf node threads read and merge aggregates from hash tables including spilled ones. Usually, leaf nodes are slower than intermediate nodes because they need to read data from disk. As a result, less threads are used for intermediate nodes by default. You can change the degree of intermeidate nodes. See intermediateCombineDegree in Advanced groupBy v2 configurations.

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.

Configurations

This section describes the configurations for groupBy queries. You can set system-wide configurations by adding them to runtime properties or query-specific configurations by adding them to query contexts. All runtime properties are prefixed by druid.query.groupBy.

Commonly tuned configurations

Configurations for groupBy v2

Supported runtime properties:

Property Description Default
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)

Supported query contexts:

Key Description
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.

Advanced configurations

Common configuragions for all groupBy strategies

Supported runtime properties:

Property Description Default
druid.query.groupBy.defaultStrategy Default groupBy query strategy. v2
druid.query.groupBy.singleThreaded Merge results using a single thread. false

Supported query contexts:

Key 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.
GroupBy v2 configurations

Supported runtime properties:

Property Description Default
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 (1024). 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 (0.7). 0
druid.query.groupBy.forceHashAggregation Force to use hash-based aggregation. false
druid.query.groupBy.intermediateCombineDegree Number of intermediate nodes combined together in the combining tree. Higher degrees will need less threads which might be helpful to improve the query performance by reducing the overhead of too many threads if the server has sufficiently powerful cpu cores. 8
druid.query.groupBy.numParallelCombineThreads Hint for the number of parallel combining threads. This should be larger than 1 to turn on the parallel combining feature. The actual number of threads used for parallel combining is min(druid.query.groupBy.numParallelCombineThreads, druid.processing.numThreads). 1 (disabled)

Supported query contexts:

Key Description Default
bufferGrouperInitialBuckets Overrides the value of druid.query.groupBy.bufferGrouperInitialBuckets for this query. None
bufferGrouperMaxLoadFactor Overrides the value of druid.query.groupBy.bufferGrouperMaxLoadFactor for this query. None
forceHashAggregation Overrides the value of druid.query.groupBy.forceHashAggregation None
intermediateCombineDegree Overrides the value of druid.query.groupBy.intermediateCombineDegree None
numParallelCombineThreads Overrides the value of druid.query.groupBy.numParallelCombineThreads None
sortByDimsFirst Sort the results first by dimension values and then by timestamp. false
forceLimitPushDown 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. false
GroupBy v1 configurations

Supported runtime properties:

Property Description Default
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

Supported query contexts:

Key Description Default
maxIntermediateRows Can be used to lower the value of druid.query.groupBy.maxIntermediateRows for this query. None
maxResults Can be used to lower the value of druid.query.groupBy.maxResults for this query. None
useOffheap Set to true to store aggregations off-heap when merging results. false