LakshSingla commented on code in PR #15987: URL: https://github.com/apache/druid/pull/15987#discussion_r1512887840
########## 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 Review Comment: Left a stale comment, an oversight on my part. In the case of empty scanResultValues, it returns an empty sequence (and not a nil frame). This is a more optimal behavior, as compared to returning a nil frame, since both mean the same thing, but the former doesn't require the callers to do any processing on the empty frame. Tests are asserting this. -- 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. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
