LakshSingla commented on code in PR #15987: URL: https://github.com/apache/druid/pull/15987#discussion_r1512993866
########## processing/src/main/java/org/apache/druid/query/scan/ScanResultValueFramesIterable.java: ########## @@ -0,0 +1,319 @@ +/* + * Licensed to the Apache Software Foundation (ASF) under one + * or more contributor license agreements. See the NOTICE file + * distributed with this work for additional information + * regarding copyright ownership. The ASF licenses this file + * to you under the Apache License, Version 2.0 (the + * "License"); you may not use this file except in compliance + * with the License. You may obtain a copy of the License at + * + * http://www.apache.org/licenses/LICENSE-2.0 + * + * Unless required by applicable law or agreed to in writing, + * software distributed under the License is distributed on an + * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY + * KIND, either express or implied. See the License for the + * specific language governing permissions and limitations + * under the License. + */ + +package org.apache.druid.query.scan; + +import com.google.common.base.Function; +import com.google.common.collect.Iterables; +import com.google.common.collect.Lists; +import org.apache.druid.error.DruidException; +import org.apache.druid.frame.Frame; +import org.apache.druid.frame.FrameType; +import org.apache.druid.frame.allocation.MemoryAllocatorFactory; +import org.apache.druid.frame.write.FrameWriter; +import org.apache.druid.frame.write.FrameWriterFactory; +import org.apache.druid.frame.write.FrameWriterUtils; +import org.apache.druid.frame.write.FrameWriters; +import org.apache.druid.java.util.common.Pair; +import org.apache.druid.java.util.common.guava.Sequence; +import org.apache.druid.java.util.common.io.Closer; +import org.apache.druid.query.FrameSignaturePair; +import org.apache.druid.query.IterableRowsCursorHelper; +import org.apache.druid.segment.Cursor; +import org.apache.druid.segment.column.RowSignature; + +import java.io.Closeable; +import java.util.Collections; +import java.util.Iterator; +import java.util.List; +import java.util.NoSuchElementException; + +/** + * Returns a thread-unsafe iterable, that converts a sequence of {@link ScanResultValue} to an iterable of {@link FrameSignaturePair}. + * ScanResultValues can have heterogenous row signatures, and the returned sequence would have batched + * them into frames appropriately. + * <p> + * The batching process greedily merges the values from the scan result values that have the same signature, while + * still maintaining the manageable frame sizes that is determined by the memory allocator by splitting the rows + * whenever necessary. + * <p> + * It is necessary that we don't batch and store the ScanResultValues somewhere (like a List) while we do this processing + * to prevent the heap from exhausting, without limit. It has to be done online - as the scan result values get materialized, + * we produce frames. A few ScanResultValues might be stored however (if the frame got cut off in the middle) + * <p> + * Assuming that we have a sequence of scan result values like: + * <p> + * ScanResultValue1 - RowSignatureA - 3 rows + * ScanResultValue2 - RowSignatureB - 2 rows + * ScanResultValue3 - RowSignatureA - 1 rows + * ScanResultValue4 - RowSignatureA - 4 rows + * ScanResultValue5 - RowSignatureB - 3 rows + * <p> + * Also, assume that each individual frame can hold two rows (in practice, it is determined by the row size and + * the memory block allocated by the memory allocator factory) + * <p> + * The output would be a sequence like: + * Frame1 - RowSignatureA - rows 1-2 from ScanResultValue1 + * Frame2 - RowSignatureA - row 3 from ScanResultValue1 + * Frame3 - RowSignatureB - rows 1-2 from ScanResultValue2 + * Frame4 - RowSignatureA - row 1 from ScanResultValue3, row 1 from ScanResultValue4 + * Frame5 - RowSignatureA - row 2-3 from ScanResultValue4 + * Frame6 - RowSignatureA - row 4 from ScanResultValue4 + * Frame7 - RowSignatureB - row 1-2 from ScanResultValue5 + * Frame8 - RowSignatureB - row 3 from ScanResultValue6 + * <p> + * TODO(laksh): What if single scan result value, and empty + */ + +public class ScanResultValueFramesIterable implements Iterable<FrameSignaturePair> +{ + + final Sequence<ScanResultValue> resultSequence; + final MemoryAllocatorFactory memoryAllocatorFactory; + final boolean useNestedForUnknownTypes; + final RowSignature defaultRowSignature; + final Function<RowSignature, Function<?, Object[]>> resultFormatMapper; + + public ScanResultValueFramesIterable( + Sequence<ScanResultValue> resultSequence, + MemoryAllocatorFactory memoryAllocatorFactory, + boolean useNestedForUnknownTypes, + RowSignature defaultRowSignature, + Function<RowSignature, Function<?, Object[]>> resultFormatMapper + ) + { + this.resultSequence = resultSequence; + this.memoryAllocatorFactory = memoryAllocatorFactory; + this.useNestedForUnknownTypes = useNestedForUnknownTypes; + this.defaultRowSignature = defaultRowSignature; + this.resultFormatMapper = resultFormatMapper; + } + + @Override + public Iterator<FrameSignaturePair> iterator() + { + return new ScanResultValueFramesIterator( + resultSequence, + memoryAllocatorFactory, + useNestedForUnknownTypes, + defaultRowSignature, + resultFormatMapper + ); + } + + private static class ScanResultValueFramesIterator implements Iterator<FrameSignaturePair> + { + + /** + * Memory allocator factory to use for frame writers + */ + final MemoryAllocatorFactory memoryAllocatorFactory; + + /** + * Replace unknown types in the row signature with {@code COMPLEX<JSON>} + */ + final boolean useNestedForUnknownTypes; + + /** + * Default row signature to use if the scan result value doesn't cantain any row signature. This will usually be + * the row signature of the scan query containing only the column names (and no types) + */ + final RowSignature defaultRowSignature; + + /** + * Mapper to convert the scan result value to rows + */ + final Function<RowSignature, Function<?, Object[]>> resultFormatMapper; + + /** + * Accumulating the closers for all the resources created so far + */ + final Closer closer = Closer.create(); + + /** + * Iterator from the scan result value's sequence, so that we can fetch the individual values. Closer registers the + * iterator so that we can clean up any resources held by the sequence's iterator, and prevent leaking + */ + final ScanResultValueIterator resultSequenceIterator; + + /** + * Either null, or points to the current non-empty cursor (and the cursor would point to the current row) + */ + Cursor currentCursor = null; + + /** + * Row signature of the current row + */ + RowSignature currentRowSignature = null; + + + public ScanResultValueFramesIterator( + Sequence<ScanResultValue> resultSequence, + MemoryAllocatorFactory memoryAllocatorFactory, + boolean useNestedForUnknownTypes, + RowSignature defaultRowSignature, + Function<RowSignature, Function<?, Object[]>> resultFormatMapper + ) + { + this.memoryAllocatorFactory = memoryAllocatorFactory; + this.useNestedForUnknownTypes = useNestedForUnknownTypes; + this.defaultRowSignature = defaultRowSignature; + this.resultFormatMapper = resultFormatMapper; + this.resultSequenceIterator = new ScanResultValueIterator(resultSequence); + + closer.register(resultSequenceIterator); + + // Makes sure that we run through all the empty scan result values at the beginning and are pointing to a valid + // row + populateCursor(); + } + + @Override + public boolean hasNext() + { + return !done(); + } + + @Override + public FrameSignaturePair next() + { + if (!hasNext()) { + throw new NoSuchElementException("No more frames to produce. Call `hasNext()` before calling `next()`"); + } + + // It would ensure that the cursor and the currentRowSignature is populated properly before we + // start all the processing + populateCursor(); + boolean firstRowWritten = false; + // While calling populateCursor() repeatedly, currentRowSignature might change. Therefore we store the signature + // with which we have written the frames + final RowSignature writtenSignature = currentRowSignature; + FrameWriterFactory frameWriterFactory = FrameWriters.makeFrameWriterFactory( + FrameType.COLUMNAR, + memoryAllocatorFactory, + currentRowSignature, + Collections.emptyList() + ); + Frame frame; + try (final FrameWriter frameWriter = frameWriterFactory.newFrameWriter(new SettableCursorColumnSelectorFactory( + () -> currentCursor, + currentRowSignature + ))) { + while (populateCursor()) { // Do till we don't have any more rows, or the next row isn't compatible with the current row Review Comment: That is done by the `frameWriter.addSelection()`. I'll add that comment in the appropriate line. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. 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