Package com.apple.foundationdb.record.provider.foundationdb.cursors

Class IntersectionCursor<T>

  • Type Parameters:
    T - the type of elements returned by the cursor
    All Implemented Interfaces:
    RecordCursor<T>, AutoCloseable, Iterator<T>
    @API(MAINTAINED)
public class IntersectionCursor<T>
extends Object
    A cursor that implements an intersection of matching elements from a set of cursors all of whom are ordered compatibly. For every matching set, this will return the element from the first child cursor. If each cursor returns something slightly different and the use case requires accessing the different results from each child, then one should use the IntersectionMultiCursor which returns a list of the child cursor values rather than just the first one.
    See Also:
    IntersectionMultiCursor

    • Method Detail

      • create

        @Nonnull
public static <M extends Message,​S extends FDBRecord<M>> IntersectionCursor<S> create​(@Nonnull
                                                                                            FDBRecordStoreBase<M> store,
                                                                                            @Nonnull
                                                                                            KeyExpression comparisonKey,
                                                                                            boolean reverse,
                                                                                            @Nonnull
                                                                                            Function<byte[],​RecordCursor<S>> left,
                                                                                            @Nonnull
                                                                                            Function<byte[],​RecordCursor<S>> right,
                                                                                            @Nullable
                                                                                            byte[] continuation)
        Create an intersection cursor from two compatibly-ordered cursors. As its comparison key function, it will evaluate the provided comparison key on each record from each cursor. This otherwise behaves exactly the same way as the overload of this function that takes a function to extract a comparison key.
        Type Parameters:
        M - the type of the Protobuf record elements of the cursor
        S - the type of record wrapping a record of type M
        Parameters:
        store - record store from which records will be fetched
        comparisonKey - the key expression used to compare records from different cursors
        reverse - whether records are returned in descending or ascending order by the comparison key
        left - a function to produce the first RecordCursor from a continuation
        right - a function to produce the second RecordCursor from a continuation
        continuation - any continuation from a previous scan
        Returns:
        a cursor containing all records in both child cursors
        See Also:
        create(Function, boolean, Function, Function, byte[], FDBStoreTimer)

      • create

        @Nonnull
public static <T> IntersectionCursor<T> create​(@Nonnull
                                               Function<? super T,​? extends List<Object>> comparisonKeyFunction,
                                               boolean reverse,
                                               @Nonnull
                                               Function<byte[],​RecordCursor<T>> left,
                                               @Nonnull
                                               Function<byte[],​RecordCursor<T>> right,
                                               @Nullable
                                               byte[] continuation,
                                               @Nullable
                                               FDBStoreTimer timer)
        Create an intersection cursor from two compatibly-ordered cursors. This cursor is identical to the cursor that would be produced by calling the overload of create() that takes a list of cursors.
        Type Parameters:
        T - the type of elements returned by the cursor
        Parameters:
        comparisonKeyFunction - the function expression used to compare elements from different cursors
        reverse - whether records are returned in descending or ascending order by the comparison key
        left - a function to produce the first RecordCursor from a continuation
        right - a function to produce the second RecordCursor from a continuation
        continuation - any continuation from a previous scan
        timer - the timer used to instrument events
        Returns:
        a cursor containing all elements in both child cursors
        See Also:
        create(Function, boolean, List, byte[], FDBStoreTimer)

      • create

        @Nonnull
public static <M extends Message,​S extends FDBRecord<M>> IntersectionCursor<S> create​(@Nonnull
                                                                                            FDBRecordStoreBase<M> store,
                                                                                            @Nonnull
                                                                                            KeyExpression comparisonKey,
                                                                                            boolean reverse,
                                                                                            @Nonnull
                                                                                            List<Function<byte[],​RecordCursor<S>>> cursorFunctions,
                                                                                            @Nullable
                                                                                            byte[] continuation)
        Create an intersection cursor from two or more compatibly-ordered cursors. As its comparison key function, it will evaluate the provided comparison key on each record from each cursor. This otherwise behaves exactly the same way as the overload of this function that takes a function to extract a comparison key.
        Type Parameters:
        M - the type of the Protobuf record elements of the cursor
        S - the type of record wrapping a record of type M
        Parameters:
        store - record store from which records will be fetched
        comparisonKey - the key expression used to compare records from different cursors
        reverse - whether records are returned in descending or ascending order by the comparison key
        cursorFunctions - a list of functions to produce RecordCursors from a continuation
        continuation - any continuation from a previous scan
        Returns:
        a cursor containing all records in all child cursors
        See Also:
        create(Function, boolean, List, byte[], FDBStoreTimer)

      • create

        @Nonnull
public static <T> IntersectionCursor<T> create​(@Nonnull
                                               Function<? super T,​? extends List<Object>> comparisonKeyFunction,
                                               boolean reverse,
                                               @Nonnull
                                               List<Function<byte[],​RecordCursor<T>>> cursorFunctions,
                                               @Nullable
                                               byte[] continuation,
                                               @Nullable
                                               FDBStoreTimer timer)
        Create an intersection cursor from two or more compatibly-ordered cursors. Note that this will throw an error if the list of cursors does not have at least two elements. The returned cursor will return all elements that appear in all of the provided cursors, preserving order. All cursors must return elements in the same order, and that order should be determined by the comparison key function, i.e., if reverse is false, then the elements should be returned in ascending order by that key, and if reverse is true, they should be returned in descending order. Additionally, if the comparison key function evaluates to the same value when applied to two elements (possibly from different cursors), then those two elements should be equal. In other words, the value of the comparison key should be the only necessary data that need to be extracted from each element returned by the child cursors to perform the intersection. Additionally, the provided comparison key function should not have any side-effects and should produce the same output every time it is applied to the same input.

        The cursors are provided as a list of functions rather than a list of RecordCursors. These functions should create a new RecordCursor instance with a given continuation appropriate for that cursor type. The value of that continuation will be determined by this function from the continuation parameter for the intersection cursor as a whole.

        Type Parameters:
        T - the type of elements returned by this cursor
        Parameters:
        comparisonKeyFunction - the function evaluated to compare elements from different cursors
        reverse - whether records are returned in descending or ascending order by the comparison key
        cursorFunctions - a list of functions to produce RecordCursors from a continuation
        continuation - any continuation from a previous scan
        timer - the timer used to instrument events
        Returns:
        a cursor containing all records in all child cursors

      • computeNextResultStates

        @Nonnull
protected CompletableFuture<List<com.apple.foundationdb.record.provider.foundationdb.cursors.KeyedMergeCursorState<T>>> computeNextResultStates()
        Compute the next result states for the cursor based on the status of the existing states. This should return a (not necessarily proper) sublist of this cursor's cursor states. By default, this assumes the cursors return results in a compatible order based on their comparison key and loops until they all have matching values. Extenders of this class may choose an alternative approach depending on the semantics of that cursor.

        To indicate that the intersection cursor should stop, this function should either return an empty list of states or a list containing at least one state that does not have a next item.

        Returns:
        the list of states included in the next result

      • mergeNoNextReasons

        @Nonnull
protected RecordCursor.NoNextReason mergeNoNextReasons()
        Merges all of the cursors and whether they have stopped and returns the "weakest" reason for the result to stop. It will return RecordCursor.NoNextReason.SOURCE_EXHAUSTED if any of the cursors are exhausted. If any of the cursors have stopped due to an in-band limit, it will return an in-band limit as well. Finally, if all of the stopped cursors have done so due to hitting an out-of-band limit, it will return an out-of-band limit as well. Note that, in practice, because an intersection cursor will return false from onHasNext if any of its child cursors have stopped, it is likely that there are only a small number (maybe one or two) cursors that have actually stopped when this method is called (e.g., the first cursor to exhaust its source or the first cursor to hit a limit imposed by the element scan limiter).
        Returns:
        the weakest reason for stopping

      • onNext

        @Nonnull
public CompletableFuture<RecordCursorResult<U>> onNext()
        Description copied from interface: RecordCursor
        Asynchronously return the next result from this cursor. When complete, the future will contain a RecordCursorResult, which represents exactly one of the following:
        1. The next object of type T produced by the cursor. In addition to the next record, this result includes a RecordCursorContinuation that can be used to continue the cursor after the last record returned. The returned continuation is guaranteed not to be an "end continuation" representing the end of the cursor: specifically, RecordCursorContinuation.isEnd() is always false on the returned continuation.
        2. The fact that the cursor is stopped and cannot produce another record and a RecordCursor.NoNextReason that explains why no record could be produced. The result include a continuation that can be used to continue the cursor after the last record returned. If the result's NoNextReason is anything other than RecordCursor.NoNextReason.SOURCE_EXHAUSTED, the returned continuation must not be an end continuation. Conversely, if the result's NoNextReason is SOURCE_EXHAUSTED, then the returned continuation must be an an "end continuation".
        In either case, the returned RecordCursorContinuation can be serialized to an opaque byte array using RecordCursorContinuation.toBytes(). This can be passed back into a new cursor of the same type, with all other parameters remaining the same.
        Specified by:
        onNext in interface RecordCursor<T>
        Returns:
        a future for the next result from this cursor representing either the next record or an indication of why the cursor stopped
        See Also:
        RecordCursorResult, RecordCursorContinuation

      • getContinuation

        @Nullable
@Deprecated
public byte[] getContinuation()
        Deprecated.
        Description copied from interface: RecordCursor
        Get a byte string that can be used to continue a query after the last record returned.
        Specified by:
        getContinuation in interface RecordCursor<T>
        Returns:
        opaque byte array denoting where the cursor should pick up. This can be passed back into a new cursor of the same type, with all other parameters remaining the same. Returns null if the underlying source is completely exhausted, independent of any limit passed to the cursor creator. Since such creators generally accept null to mean no continuation, that is, start from the beginning, one must check for null from getContinuation to keep from starting over. Result is not always defined if called before onHasNext or before next after onHasNext has returned true. That is, a continuation is only guaranteed when called "between" records from a while (hasNext) next loop or after its end.

      • accept

        public boolean accept​(@Nonnull
                      RecordCursorVisitor visitor)
        Description copied from interface: RecordCursor
        Accept a visit from hierarchical visitor, which implements RecordCursorVisitor. By contract, implementations of this method must return the value of visitor.visitLeave(this), which determines whether or not subsequent siblings of this cursor should be visited.
        Specified by:
        accept in interface RecordCursor<T>
        Parameters:
        visitor - a hierarchical visitor
        Returns:
        true if the subsequent siblings of the cursor should be visited, and false otherwise