Package org.apache.druid.segment

Interface CursorHolder

All Superinterfaces:

AutoCloseable, Closeable

All Known Implementing Classes:

IncrementalIndexCursorHolder, QueryableIndexCursorHolder

public interface CursorHolder
extends Closeable

Provides Cursor and if available, VectorCursor which readers can use to scan a set of rows defined originally from a CursorBuildSpec which describes how data is to be scanned, transformed, filter, grouped and aggregated, and/or ordered.

If canVectorize() then asVectorCursor() will return a non-null value allowing for processing rows in batches instead of individually.

If isPreAggregated() is true, readers which are aggregating must call getAggregatorsForPreAggregated() to get the updated set of AggregatorFactory to correctly process results

getOrdering() defines how the data is ordered in the Cursor or VectorCursor, allowing readers to compare to their own desired ordering and potentially skip ordering their own results if it is pre-ordered.

Method Summary

Modifier and Type	Method	Description
Cursor	asCursor()	Create a Cursor for use with non-vectorized query engines.
default VectorCursor	asVectorCursor()	Create a VectorCursor for use with vectorized query engines.
default boolean	canVectorize()	Returns true if this CursorHolder supports calling asVectorCursor().
default void	close()	Release any resources acquired by cursors.
default List<AggregatorFactory>	getAggregatorsForPreAggregated()	Returns a set of replacement AggregatorFactory if and only if isPreAggregated() is true.
default List<OrderBy>	getOrdering()	Returns cursor ordering, which may or may not match CursorBuildSpec.getPreferredOrdering().
default Order	getTimeOrder()	If getOrdering() starts with ColumnHolder.TIME_COLUMN_NAME, returns the time ordering; otherwise returns Order.NONE.
default boolean	isPreAggregated()	Returns true if the Cursor or VectorCursor contains pre-aggregated columns for all grouping columns specified in CursorBuildSpec.getGroupingColumns() and all AggregatorFactory specified in CursorBuildSpec.getAggregators().

Method Details

asCursor

@Nullable
Cursor asCursor()

Create a Cursor for use with non-vectorized query engines.

asVectorCursor

@Nullable
default VectorCursor asVectorCursor()

Create a VectorCursor for use with vectorized query engines.

canVectorize

default boolean canVectorize()

Returns true if this CursorHolder supports calling asVectorCursor().

isPreAggregated

default boolean isPreAggregated()

Returns true if the Cursor or VectorCursor contains pre-aggregated columns for all grouping columns specified in CursorBuildSpec.getGroupingColumns() and all AggregatorFactory specified in CursorBuildSpec.getAggregators().

If this method returns true, ColumnSelectorFactory and VectorColumnSelectorFactory created from Cursor and VectorCursor respectively will provide selectors for AggregatorFactory.getName(), and engines should rewrite the query using AggregatorFactory.getCombiningFactory(), since the values returned from these selectors will be of type AggregatorFactory.getIntermediateType(), so the cursor becomes a "fold" operation rather than a "build" operation.

getAggregatorsForPreAggregated

@Nullable
default List<AggregatorFactory> getAggregatorsForPreAggregated()

Returns a set of replacement AggregatorFactory if and only if isPreAggregated() is true. The query engine should replace the query aggregators with these aggregators, which are combining aggregators derived from the CursorBuildSpec passed into CursorFactory.makeCursorHolder(CursorBuildSpec). If isPreAggregated() is not true, this method returns null

getOrdering

default List<OrderBy> getOrdering()

Returns cursor ordering, which may or may not match CursorBuildSpec.getPreferredOrdering(). If returns an empty list then the cursor has no defined ordering.

Cursors associated with this holder return rows in this ordering, using the natural comparator for the column type. Includes ColumnHolder.TIME_COLUMN_NAME if appropriate.

getTimeOrder

default Order getTimeOrder()

If getOrdering() starts with ColumnHolder.TIME_COLUMN_NAME, returns the time ordering; otherwise returns Order.NONE.

close

default void close()

Release any resources acquired by cursors.

Specified by:

close in interface AutoCloseable

Specified by:

close in interface Closeable