Package org.apache.druid.query

Class QueryToolChest<ResultType,​QueryType extends Query<ResultType>>

    • Constructor Detail

      • QueryToolChest

        protected QueryToolChest()

    • Method Detail

      • getBaseResultType

        public final com.fasterxml.jackson.databind.JavaType getBaseResultType()

      • getBySegmentResultType

        public final com.fasterxml.jackson.databind.JavaType getBySegmentResultType()

      • decorateObjectMapper

        public com.fasterxml.jackson.databind.ObjectMapper decorateObjectMapper​(com.fasterxml.jackson.databind.ObjectMapper objectMapper,
                                                                        QueryType query)
        Perform any per-query decoration of an ObjectMapper that enables it to read and write objects of the query's QueryToolChest. It is used by QueryResource on the write side, and DirectDruidClient on the read side.

        For most queries, this is a no-op, but it can be useful for query types that support more than one result serialization format. Queries that implement this method must not modify the provided ObjectMapper, but instead must return a copy.

        Jackson's default implementation of deserialization is usually optimised and this method should be overriden only if there is a functional requirement of so. The method must be benchmarked in isolation, without other portions of the query engine executing as modifying this method can alter the performance of queries where deserializing is a major portion of the execution.

      • mergeResults

        public QueryRunner<ResultType> mergeResults​(QueryRunner<ResultType> runner)
        This method wraps a QueryRunner. The input QueryRunner, by contract, will provide a series of ResultType objects in time order (ascending or descending). This method should return a new QueryRunner that merges the stream of ordered ResultType objects.

        A default implementation constructs a ResultMergeQueryRunner which creates a CombiningSequence using the supplied QueryRunner with createResultComparator(Query) and createMergeFn(Query)} supplied by this toolchest.

        Generally speaking, the logic that exists in makePostComputeManipulatorFn should actually exist in this method. Additionally, if a query supports PostAggregations, this method should take steps to ensure that it computes PostAggregations a minimum number of times. This is most commonly achieved by computing the PostAgg results during merge and also rewriting the query such that it has the minimum number of PostAggs (most often zero).

        Parameters:
        runner - A QueryRunner that provides a series of ResultType objects in time order (ascending or descending)
        Returns:
        a QueryRunner that merges the stream of ordered ResultType objects

      • makeMetrics

        public abstract QueryMetrics<? super QueryType> makeMetrics​(QueryType query)
        Creates a QueryMetrics object that is used to generate metrics for this specific query type. This exists to allow for query-specific dimensions and metrics. That is, the ToolChest is expected to set some meaningful dimensions for metrics given this query type. Examples might be the topN threshold for a TopN query or the number of dimensions included for a groupBy query.

        QueryToolChests for query types in core (druid-processing) and public extensions (belonging to the Druid source tree) should use delegate this method to GenericQueryMetricsFactory.makeMetrics(Query) on an injected instance of GenericQueryMetricsFactory, as long as they don't need to emit custom dimensions and/or metrics.

        If some custom dimensions and/or metrics should be emitted for a query type, a plan described in "Making subinterfaces of QueryMetrics" section in QueryMetrics's class-level Javadocs should be followed.

        One way or another, this method should ensure that QueryMetrics.query(Query) is called with the given query passed on the created QueryMetrics object before returning.

        Parameters:
        query - The query that is being processed
        Returns:
        A QueryMetrics that can be used to make metrics for the provided query

      • makePreComputeManipulatorFn

        public abstract com.google.common.base.Function<ResultType,​ResultType> makePreComputeManipulatorFn​(QueryType query,
                                                                                                         MetricManipulationFn fn)
        Creates a Function that can take in a ResultType and return a new ResultType having applied the MetricManipulatorFn to each of the metrics.

        This function's primary purpose is to help work around some challenges that exist around deserializing results across the wire. Specifically, different aggregators will generate different object types in a result set, if we wanted jackson to be able to deserialize these directly, we'd need to generate a response class for each query that jackson could use to deserialize things. That is not what we do. Instead, we have jackson deserialize Object instances and then use a MetricManipulatorFn to convert from those object instances to the actual object that the aggregator expects. As such, this would be more effectively named "makeObjectDeserializingFn".

        It is safe and acceptable for implementations of this method to first validate that the MetricManipulationFn is MetricManipulatorFns.DESERIALIZING_INSTANCE and throw an exception if it is not. If such an exception is ever thrown, it is indicative of a bug in the caller which should be fixed by not calling this method with anything other than the deserializing manipulator function.

        There are some implementations where this was also tasked with computing PostAggregators, but this is actually not a good place to compute those as this function can be called in a number of cases when PostAggs are not really meaningful to compute. Instead, PostAggs should be computed in the mergeResults call and the mergeResults implementation should take care to ensure that PostAggs are only computed the minimum number of times necessary.

        This function is called very early in the processing pipeline on the Broker.

        Parameters:
        query - The Query that is currently being processed
        fn - The function that should be applied to all metrics in the results
        Returns:
        A function that will apply the provided fn to all metrics in the input ResultType object

      • makePostComputeManipulatorFn

        public com.google.common.base.Function<ResultType,​ResultType> makePostComputeManipulatorFn​(QueryType query,
                                                                                                 MetricManipulationFn fn)
        This manipulator functions primary purpose is to conduct finalization of aggregator values. It would be better named "makeFinalizingManipulatorFn", even that should really be done as part of mergeResults(org.apache.druid.query.QueryRunner<ResultType>) instead of with this separate method.

        It is safe and acceptable for implementations of this method to first validate that the MetricManipulationFn is either MetricManipulatorFns.FINALIZING_INSTANCE or MetricManipulatorFns.IDENTITY_INSTANCE and throw an exception if it is not. If such an exception is ever thrown, it is indicative of a bug in the caller which should be fixed by not calling this method with unsupported manipulator functions.

        Parameters:
        query - The Query that is currently being processed
        fn - The function that should be applied to all metrics in the results
        Returns:
        A function that will apply the provided fn to all metrics in the input ResultType object

      • getResultTypeReference

        public abstract com.fasterxml.jackson.core.type.TypeReference<ResultType> getResultTypeReference()
        Returns a TypeReference object that is just passed through to Jackson in order to deserialize the results of this type of query.
        Returns:
        A TypeReference to indicate to Jackson what type of data will exist for this query

      • getCacheStrategy

        @Nullable
public <T> CacheStrategy<ResultType,​T,​QueryType> getCacheStrategy​(QueryType query,
                                                                              @Nullable
                                                                              com.fasterxml.jackson.databind.ObjectMapper mapper)
        Returns a CacheStrategy to be used to load data into the cache and remove it from the cache.

        This is optional. If it returns null, caching is effectively disabled for the query.

        Type Parameters:
        T - The type of object that will be stored in the cache
        Parameters:
        query - The query whose results might be cached
        mapper - Object mapper to convert the deserialized generic java objects to desired types. It can be nullable to preserve backward compatibility.
        Returns:
        A CacheStrategy that can be used to populate and read from the Cache

      • preMergeQueryDecoration

        public QueryRunner<ResultType> preMergeQueryDecoration​(QueryRunner<ResultType> runner)
        Wraps a QueryRunner. The input QueryRunner is the QueryRunner as it exists *before* being passed to mergeResults().

        In fact, the return value of this method is always passed to mergeResults, so it is equivalent to just implement this functionality as extra decoration on the QueryRunner during mergeResults().

        In the interests of potentially simplifying these interfaces, the recommendation is to actually not override this method and instead apply anything that might be needed here in the mergeResults() call.

        Parameters:
        runner - The runner to be wrapped
        Returns:
        The wrapped runner

      • postMergeQueryDecoration

        public QueryRunner<ResultType> postMergeQueryDecoration​(QueryRunner<ResultType> runner)
        Wraps a QueryRunner. The input QueryRunner is the QueryRunner as it exists coming out of mergeResults()

        In fact, the input value of this method is always the return value from mergeResults, so it is equivalent to just implement this functionality as extra decoration on the QueryRunner during mergeResults().

        In the interests of potentially simplifying these interfaces, the recommendation is to actually not override this method and instead apply anything that might be needed here in the mergeResults() call.

        Parameters:
        runner - The runner to be wrapped
        Returns:
        The wrapped runner

      • filterSegments

        public <T extends LogicalSegment> List<T> filterSegments​(QueryType query,
                                                         List<T> segments)
        This method is called to allow the query to prune segments that it does not believe need to actually be queried. It can use whatever criteria it wants in order to do the pruning, it just needs to return the list of Segments it actually wants to see queried.
        Type Parameters:
        T - A Generic parameter because Java is cool
        Parameters:
        query - The query being processed
        segments - The list of candidate segments to be queried
        Returns:
        The list of segments to actually query

      • canPerformSubquery

        public boolean canPerformSubquery​(Query<?> subquery)
        Returns whether this toolchest is able to handle the provided subquery.

        When this method returns true, the core query stack will pass subquery datasources over to the toolchest and will assume they are properly handled.

        When this method returns false, the core query stack will throw an error if subqueries are present. In the future, instead of throwing an error, the core query stack will handle the subqueries on its own.

      • resultsAsArrays

        public Sequence<Object[]> resultsAsArrays​(QueryType query,
                                          Sequence<ResultType> resultSequence)
        Converts a sequence of this query's ResultType into arrays. The array signature is given by resultArraySignature(QueryType). This functionality is useful because it allows higher-level processors to operate on the results of any query in a consistent way. This is useful for the SQL layer and for any algorithm that might operate on the results of an inner query.

        Not all query types support this method. They will throw UnsupportedOperationException, and they cannot be used by the SQL layer or by generic higher-level algorithms.

        Some query types return less information after translating their results into arrays, especially in situations where there is no clear way to translate fully rich results into flat arrays. For example, the scan query does not include the segmentId in its array-based results, because it could potentially conflict with a 'segmentId' field in the actual datasource being scanned.

        It is possible that there will be multiple arrays returned for a single result object. For example, in the topN query, each TopNResultValue will generate a separate array for each of its values.

        By convention, the array form should include the __time column, if present, as a long (milliseconds since epoch).

        Parameters:
        resultSequence - results of the form returned by mergeResults(org.apache.druid.query.QueryRunner<ResultType>)
        Returns:
        results in array form
        Throws:
        UnsupportedOperationException - if this query type does not support returning results as arrays

      • resultsAsFrames

        public Optional<Sequence<FrameSignaturePair>> resultsAsFrames​(QueryType query,
                                                              Sequence<ResultType> resultSequence,
                                                              MemoryAllocatorFactory memoryAllocatorFactory,
                                                              boolean useNestedForUnknownTypes)
        Converts a sequence of this query's ResultType into a sequence of FrameSignaturePair. The array signature is the one give by resultArraySignature(Query). If the toolchest doesn't support this method, then it can return an empty optional. It is the duty of the callees to throw an appropriate exception in that case or use an alternative fallback approach

        Check documentation of resultsAsArrays(Query, Sequence) as the behaviour of the rows represented by the frame sequence is identical.

        Each Frame has a separate RowSignature because for some query types like the Scan query, every column in the final result might not be present in the individual ResultType (and subsequently Frame). Therefore, this is done to preserve the space by not populating the column in that particular Frame and omitting it from its signature

        Parameters:
        query - Query being executed by the toolchest. Used to determine the rowSignature of the Frames
        resultSequence - results of the form returned by mergeResults(QueryRunner)
        memoryAllocatorFactory -
        useNestedForUnknownTypes - true if the unknown types in the results can be serded using complex types

      • canExecuteFully

        public <T> boolean canExecuteFully​(Query<T> query)