druid/docs/querying/query-context.md

id	title
query-context	Query context

The query context is used for various query configuration parameters. The following parameters apply to all queries.

property	default	description
timeout	druid.server.http.defaultQueryTimeout	Query timeout in millis, beyond which unfinished queries will be cancelled. 0 timeout means no timeout. To set the default timeout, see Broker configuration
priority	0	Query Priority. Queries with higher priority get precedence for computational resources.
lane	null	Query lane, used to control usage limits on classes of queries. See Broker configuration for more details.
queryId	auto-generated	Unique identifier given to this query. If a query ID is set or known, this can be used to cancel the query
useCache	true	Flag indicating whether to leverage the query cache for this query. When set to false, it disables reading from the query cache for this query. When set to true, Apache Druid uses druid.broker.cache.useCache or druid.historical.cache.useCache to determine whether or not to read from the query cache
populateCache	true	Flag indicating whether to save the results of the query to the query cache. Primarily used for debugging. When set to false, it disables saving the results of this query to the query cache. When set to true, Druid uses druid.broker.cache.populateCache or druid.historical.cache.populateCache to determine whether or not to save the results of this query to the query cache
useResultLevelCache	true	Flag indicating whether to leverage the result level cache for this query. When set to false, it disables reading from the query cache for this query. When set to true, Druid uses druid.broker.cache.useResultLevelCache to determine whether or not to read from the result-level query cache
populateResultLevelCache	true	Flag indicating whether to save the results of the query to the result level cache. Primarily used for debugging. When set to false, it disables saving the results of this query to the query cache. When set to true, Druid uses druid.broker.cache.populateResultLevelCache to determine whether or not to save the results of this query to the result-level query cache
bySegment	false	Return "by segment" results. Primarily used for debugging, setting it to true returns results associated with the data segment they came from
finalize	true	Flag indicating whether to "finalize" aggregation results. Primarily used for debugging. For instance, the hyperUnique aggregator will return the full HyperLogLog sketch instead of the estimated cardinality when this flag is set to false
maxScatterGatherBytes	druid.server.http.maxScatterGatherBytes	Maximum number of bytes gathered from data processes such as Historicals and realtime processes to execute a query. This parameter can be used to further reduce maxScatterGatherBytes limit at query time. See Broker configuration for more details.
maxQueuedBytes	druid.broker.http.maxQueuedBytes	Maximum number of bytes queued per query before exerting backpressure on the channel to the data server. Similar to maxScatterGatherBytes, except unlike that configuration, this one will trigger backpressure rather than query failure. Zero means disabled.
serializeDateTimeAsLong	false	If true, DateTime is serialized as long in the result returned by Broker and the data transportation between Broker and compute process
serializeDateTimeAsLongInner	false	If true, DateTime is serialized as long in the data transportation between Broker and compute process
enableParallelMerge	true	Enable parallel result merging on the Broker. Note that druid.processing.merge.useParallelMergePool must be enabled for this setting to be set to true. See Broker configuration for more details.
parallelMergeParallelism	druid.processing.merge.pool.parallelism	Maximum number of parallel threads to use for parallel result merging on the Broker. See Broker configuration for more details.
parallelMergeInitialYieldRows	druid.processing.merge.task.initialYieldNumRows	Number of rows to yield per ForkJoinPool merge task for parallel result merging on the Broker, before forking off a new task to continue merging sequences. See Broker configuration for more details.
parallelMergeSmallBatchRows	druid.processing.merge.task.smallBatchNumRows	Size of result batches to operate on in ForkJoinPool merge tasks for parallel result merging on the Broker. See Broker configuration for more details.

In addition, some query types offer context parameters specific to that query type.

TopN queries

property	default	description
minTopNThreshold	1000	The top minTopNThreshold local results from each segment are returned for merging to determine the global topN.

Timeseries queries

property	default	description
skipEmptyBuckets	false	Disable timeseries zero-filling behavior, so only buckets with results will be returned.

GroupBy queries

See GroupBy query context.

Vectorizable queries

The GroupBy and Timeseries query types can run in vectorized mode, which speeds up query execution by processing batches of rows at a time. Not all queries can be vectorized. In particular, vectorization currently has the following requirements:

• All query-level filters must either be able to run on bitmap indexes or must offer vectorized row-matchers. These include "selector", "bound", "in", "like", "regex", "search", "and", "or", and "not".
• All filters in filtered aggregators must offer vectorized row-matchers.
• All aggregators must offer vectorized implementations. These include "count", "doubleSum", "floatSum", "longSum", "hyperUnique", and "filtered".
• No virtual columns.
• For GroupBy: All dimension specs must be "default" (no extraction functions or filtered dimension specs).
• For GroupBy: No multi-value dimensions.
• For Timeseries: No "descending" order.
• Only immutable segments (not real-time).

Other query types (like TopN, Scan, Select, and Search) ignore the "vectorize" parameter, and will execute without vectorization. These query types will ignore the "vectorize" parameter even if it is set to "force".

Vectorization is an alpha-quality feature as of Druid {{DRUIDVERSION}}. We heartily welcome any feedback and testing from the community as we work to battle-test it.

property	default	description
vectorize	false	Enables or disables vectorized query execution. Possible values are false (disabled), true (enabled if possible, disabled otherwise, on a per-segment basis), and force (enabled, and groupBy or timeseries queries that cannot be vectorized will fail). The "force" setting is meant to aid in testing, and is not generally useful in production (since real-time segments can never be processed with vectorized execution, any queries on real-time data will fail). This will override druid.query.vectorize if it's set.
vectorSize	512	Sets the row batching size for a particular query. This will override druid.query.vectorSize if it's set.