io.druid.segment

Interface DimensionMerger<EncodedTypedArray>

All Known Subinterfaces:

DimensionMergerLegacy<EncodedTypeArray>, DimensionMergerV9<EncodedTypeArray>

All Known Implementing Classes:

StringDimensionMergerLegacy, StringDimensionMergerV9

public interface DimensionMerger<EncodedTypedArray>

Processing related interface A DimensionMerger is a per-dimension stateful object that encapsulates type-specific operations and data structures used during the segment merging process (i.e., work done by IndexMerger/IndexMergerV9). This object is responsible for: - merging any relevant structures from the segments (e.g., encoding dictionaries) - writing the merged column data and any merged indexing structures (e.g., dictionaries, bitmaps) to disk At a high level, the index merging process can be broken down into the following steps: - Merge any value representation metadata across segments E.g. for dictionary encoded Strings, each segment has its own unique space of id->value mappings. These need to be merged across segments into a shared space of dictionary mappings. - Merge the rows across segments into a common sequence of rows - After constructing the merged sequence of rows, build any applicable index structures (e.g, bitmap indexes) - Write the value representation metadata (e.g. dictionary), the sequence of row values, and index structures to a merged segment. A class implementing this interface is expected to be highly stateful, updating its internal state as these functions are called.

Method Summary

Modifier and Type	Method and Description
boolean	canSkip() Return true if this dimension's data does not need to be written to the segment.
EncodedTypedArray	convertSegmentRowValuesToMergedRowValues(EncodedTypedArray segmentRow, int segmentIndexNumber) Convert a row from a single segment to its equivalent representation in the merged set of rows.
void	processMergedRow(EncodedTypedArray rowValues) Process a row from the merged sequence of rows and update the DimensionMerger's internal state.
void	writeIndexes(List<IntBuffer> segmentRowNumConversions, com.google.common.io.Closer closer) Internally construct any index structures relevant to this DimensionMerger.
void	writeMergedValueMetadata(List<IndexableAdapter> adapters) Given a list of segment adapters: - Read any value metadata (e.g., dictionary encoding information) from the adapters - Merge this value metadata and update the internal state of the implementing class.

Method Detail

writeMergedValueMetadata

void writeMergedValueMetadata(List<IndexableAdapter> adapters)
                       throws IOException

Given a list of segment adapters: - Read any value metadata (e.g., dictionary encoding information) from the adapters - Merge this value metadata and update the internal state of the implementing class. The implementer should maintain knowledge of the "index number" of the adapters in the input list, i.e., the position of each adapter in the input list. This "index number" will be used to refer to specific segments later in convertSegmentRowValuesToMergedRowValues(). Otherwise, the details of how this merging occurs and how to store the merged data is left to the implementer.

Parameters:

adapters - List of adapters to be merged.

Throws:

IOException

convertSegmentRowValuesToMergedRowValues

EncodedTypedArray convertSegmentRowValuesToMergedRowValues(EncodedTypedArray segmentRow,
                                                           int segmentIndexNumber)

Convert a row from a single segment to its equivalent representation in the merged set of rows. This function is used by the index merging process to build the merged sequence of rows. The implementing class is expected to use the merged value metadata constructed during writeMergedValueMetadata, if applicable. For example, an implementation of this function for a dictionary-encoded String column would convert the segment-specific dictionary values within the row to the common merged dictionary values determined during writeMergedValueMetadata().

Parameters:

segmentRow - A row from a segment to be converted to its representation within the merged sequence of rows.

segmentIndexNumber - Integer indicating which segment the row originated from.

processMergedRow

void processMergedRow(EncodedTypedArray rowValues)
               throws IOException

Process a row from the merged sequence of rows and update the DimensionMerger's internal state. After constructing a merged sequence of rows across segments, the index merging process will iterate through these rows and pass row values from each dimension to their correspodning DimensionMergers. This allows each DimensionMerger to build its internal view of the sequence of merged rows, to be written out to a segment later.

Parameters:

rowValues - The row values to be added.

Throws:

IOException

writeIndexes

void writeIndexes(List<IntBuffer> segmentRowNumConversions,
                  com.google.common.io.Closer closer)
           throws IOException

Internally construct any index structures relevant to this DimensionMerger. After receiving the sequence of merged rows via iterated processMergedRow() calls, the DimensionMerger can now build any index structures it needs. For example, a dictionary encoded String implementation would create its bitmap indexes for the merged segment during this step. The index merger will provide a list of row number conversion IntBuffer objects. Each IntBuffer is associated with one of the segments being merged; the position of the IntBuffer in the list corresponds to the position of segment adapters within the input list of writeMergedValueMetadata(). For example, suppose there are two segments A and B. Row 24 from segment A maps to row 99 in the merged sequence of rows, The IntBuffer for segment A would have a mapping of 24 -> 99.

Parameters:

segmentRowNumConversions - A list of row number conversion IntBuffer objects.

closer - Add Closeables for resource cleanup to this Closer if needed

Throws:

IOException

canSkip

boolean canSkip()

Return true if this dimension's data does not need to be written to the segment. For example, if a dictionary-encoded String dimension had only null values, it can be skipped.

Returns:

true if this dimension can be excluded from the merged segment.