Improve String Last/First Storage Efficiency

-Add classes for writing cell values in LZ4 block compressed format.
Payloads are indexed by element number for efficient random lookup
-update SerializablePairLongStringComplexMetricSerde to use block
compression
-SerializablePairLongStringComplexMetricSerde also uses delta encoding
of the Long by doing 2-pass encoding: buffers first to find min/max
numbers and delta-encodes as integers if possible

Entry points for doing block-compressed storage of byte[] payloads
are the CellWriter and CellReader class. See
SerializablePairLongStringComplexMetricSerde for how these are used
along with how to do full column-based storage (delta encoding here)
which includes 2-pass encoding to compute a column header

processing/src/main/java/org/apache/druid/jackson/AggregatorsModule.java

@@ -39,7 +39,7 @@
 import org.apache.druid.query.aggregation.LongMinAggregatorFactory;
 import org.apache.druid.query.aggregation.LongSumAggregatorFactory;
 import org.apache.druid.query.aggregation.PostAggregator;
-import org.apache.druid.query.aggregation.SerializablePairLongStringSerde;
+import org.apache.druid.query.aggregation.SerializablePairLongStringComplexMetricSerde;
 import org.apache.druid.query.aggregation.any.DoubleAnyAggregatorFactory;
 import org.apache.druid.query.aggregation.any.FloatAnyAggregatorFactory;
 import org.apache.druid.query.aggregation.any.LongAnyAggregatorFactory;
@@ -81,7 +81,7 @@ public AggregatorsModule()
 
     ComplexMetrics.registerSerde("hyperUnique", new HyperUniquesSerde());
     ComplexMetrics.registerSerde("preComputedHyperUnique", new PreComputedHyperUniquesSerde());
-    ComplexMetrics.registerSerde("serializablePairLongString", new SerializablePairLongStringSerde());
+    ComplexMetrics.registerSerde("serializablePairLongString", new SerializablePairLongStringComplexMetricSerde());
 
     setMixInAnnotation(AggregatorFactory.class, AggregatorFactoryMixin.class);
     setMixInAnnotation(PostAggregator.class, PostAggregatorMixin.class);

processing/src/main/java/org/apache/druid/query/aggregation/SerializablePairLongStringBufferStore.java

@@ -0,0 +1,174 @@
+/*
+ * 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.aggregation;
+
+import org.apache.druid.java.util.common.io.smoosh.FileSmoosher;
+import org.apache.druid.segment.serde.Serializer;
+import org.apache.druid.segment.serde.cell.CellWriter;
+import org.apache.druid.segment.serde.cell.DeserializingIOIterator;
+import org.apache.druid.segment.serde.cell.IOIterator;
+import org.apache.druid.segment.serde.cell.IntSerializer;
+import org.apache.druid.segment.serde.cell.NativeClearedByteBufferProvider;
+import org.apache.druid.segment.writeout.SegmentWriteOutMedium;
+import org.apache.druid.segment.writeout.WriteOutBytes;
+
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+import java.io.IOException;
+import java.nio.channels.WritableByteChannel;
+
+public class SerializablePairLongStringBufferStore
+{
+  private static final SerializablePairLongStringSimpleStagedSerde SERDE =
+      new SerializablePairLongStringSimpleStagedSerde();
+
+  private final WriteOutBytes writeOutBytes;
+  private final IntSerializer intSerializer = new IntSerializer();
+
+  private long minValue = Long.MAX_VALUE;
+  private long maxValue = Long.MIN_VALUE;
+
+  public SerializablePairLongStringBufferStore(WriteOutBytes writeOutBytes)
+  {
+    this.writeOutBytes = writeOutBytes;
+  }
+
+  public void store(@Nullable SerializablePairLongString pairLongString) throws IOException
+  {
+    if (pairLongString != null && pairLongString.lhs != null) {
+      minValue = Math.min(minValue, pairLongString.lhs);
+      maxValue = Math.max(maxValue, pairLongString.lhs);
+    }
+
+    byte[] bytes = SERDE.serialize(pairLongString);
+
+    writeOutBytes.write(intSerializer.serialize(bytes.length));
+    writeOutBytes.write(bytes);
+  }
+
+  /**
+   * each call transfers the temporary buffer into an encoded, block-compessed buffer of the segment. It is ready to be
+   * transferred to a {@link WritableByteChannel}
+   *
+   * @param byteBufferProvider    - provides a ByteBuffer used for block compressed encoding
+   * @param segmentWriteOutMedium - used to create temporary storage
+   * @return encoded buffer ready to be stored
+   * @throws IOException
+   */
+  public TransferredBuffer transferToRowWriter(
+      NativeClearedByteBufferProvider byteBufferProvider,
+      SegmentWriteOutMedium segmentWriteOutMedium
+  ) throws IOException
+  {
+    SerializablePairLongStringColumnHeader columnHeader = createColumnHeader();
+    SerializablePairLongStringDeltaEncodedStagedSerde serde =
+        new SerializablePairLongStringDeltaEncodedStagedSerde(
+            columnHeader.getMinValue(),
+            columnHeader.isUseIntegerDeltas()
+        );
+
+    // try-with-resources will call cellWriter.close() an extra time in the normal case, but it protects against
+    // buffer leaking in the case of an exception. In the normal path, close() performs some finalization of
+    // the CellWriter object. We want that object state finalized before creating the TransferredBuffer as a point of
+    // good style (though strictly speaking, it works fine to pass it in before calling close since TransferredBuffer
+    // does not do anything in the constructor with the object)
+    try (CellWriter cellWriter = new CellWriter.Builder(byteBufferProvider, segmentWriteOutMedium).build()) {
+      IOIterator<SerializablePairLongString> bufferIterator = new DeserializingIOIterator<>(
+          writeOutBytes.asInputStream(),
+          SERDE
+      );
+
+      while (bufferIterator.hasNext()) {
+        SerializablePairLongString pairLongString = bufferIterator.next();
+        byte[] serialized = serde.serialize(pairLongString);
+
+        cellWriter.write(serialized);
+      }
+
+      cellWriter.close();
+
+      return new TransferredBuffer(cellWriter, columnHeader);
+    }
+  }
+
+  @Nonnull
+  public SerializablePairLongStringColumnHeader createColumnHeader()
+  {
+    long maxDelta = maxValue - minValue;
+    SerializablePairLongStringColumnHeader columnHeader;
+
+    if (minValue < maxValue && maxDelta < 0 || minValue > maxValue) {
+      // true iff
+      // 1. we have overflow in our range || 2. we have only seen null values
+      // in this case, effectively disable delta encoding by using longs and a min value of 0
+      maxDelta = Long.MAX_VALUE;
+      minValue = 0;
+    }
+
+    if (maxDelta <= Integer.MAX_VALUE) {
+      columnHeader = new SerializablePairLongStringColumnHeader(
+          SerializablePairLongStringComplexMetricSerde.EXPECTED_VERSION,
+          true,
+          minValue
+      );
+    } else {
+      columnHeader = new SerializablePairLongStringColumnHeader(
+          SerializablePairLongStringComplexMetricSerde.EXPECTED_VERSION,
+          false,
+          minValue
+      );
+    }
+    return columnHeader;
+  }
+
+  public IOIterator<SerializablePairLongString> iterator() throws IOException
+  {
+    return new DeserializingIOIterator<>(writeOutBytes.asInputStream(), SERDE);
+  }
+
+  /**
+   * contains serialized data that is compressed and delta-encoded (Long)
+   * It's ready to be transferred to a {@link WritableByteChannel}
+   */
+  public static class TransferredBuffer implements Serializer
+  {
+    private final CellWriter cellWriter;
+    private final SerializablePairLongStringColumnHeader columnHeader;
+
+    public TransferredBuffer(CellWriter cellWriter, SerializablePairLongStringColumnHeader columnHeader)
+    {
+      this.cellWriter = cellWriter;
+      this.columnHeader = columnHeader;
+    }
+
+    @Override
+    public void writeTo(WritableByteChannel channel, FileSmoosher smoosher) throws IOException
+    {
+      columnHeader.transferTo(channel);
+      cellWriter.writeTo(channel, smoosher);
+    }
+
+    @Override
+    public long getSerializedSize()
+    {
+      return columnHeader.getSerializedSize() + cellWriter.getSerializedSize();
+    }
+  }
+}

processing/src/main/java/org/apache/druid/query/aggregation/SerializablePairLongStringColumnHeader.java

@@ -0,0 +1,108 @@
+/*
+ * 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.aggregation;
+
+import com.google.common.base.Objects;
+import org.apache.druid.segment.serde.cell.LongSerializer;
+
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.channels.WritableByteChannel;
+
+public class SerializablePairLongStringColumnHeader
+{
+  // header size is 4 bytes for word alignment for LZ4 (minmatch) compression
+  private static final int HEADER_SIZE_BYTES = 4;
+  private static final int USE_INTEGER_MASK = 0x80;
+  private static final int VERSION_INDEX = 0;
+  private static final int ENCODING_INDEX = 1;
+
+  private final LongSerializer longSerializer = new LongSerializer();
+  private final byte[] bytes;
+  private final long minValue;
+
+  private SerializablePairLongStringColumnHeader(byte[] bytes, long minTimestamp)
+  {
+    this.bytes = bytes;
+    this.minValue = minTimestamp;
+  }
+
+  public SerializablePairLongStringColumnHeader(byte version, boolean useIntegerDeltas, long minTimestamp)
+  {
+    this.minValue = minTimestamp;
+    bytes = new byte[HEADER_SIZE_BYTES];
+    bytes[VERSION_INDEX] = version;
+
+    if (useIntegerDeltas) {
+      bytes[ENCODING_INDEX] |= USE_INTEGER_MASK;
+    }
+  }
+
+  public static SerializablePairLongStringColumnHeader fromBuffer(ByteBuffer byteBuffer)
+  {
+    byte[] bytes = new byte[HEADER_SIZE_BYTES];
+
+    byteBuffer.get(bytes);
+
+    long minTimestamp = byteBuffer.getLong();
+
+    return new SerializablePairLongStringColumnHeader(bytes, minTimestamp);
+  }
+
+  public SerializablePairLongStringDeltaEncodedStagedSerde createSerde()
+  {
+    return new SerializablePairLongStringDeltaEncodedStagedSerde(minValue, isUseIntegerDeltas());
+  }
+
+  public void transferTo(WritableByteChannel channel) throws IOException
+  {
+    channel.write(ByteBuffer.wrap(bytes));
+    channel.write(longSerializer.serialize(minValue));
+  }
+
+  public byte getVersion()
+  {
+    return bytes[VERSION_INDEX];
+  }
+
+  public boolean isUseIntegerDeltas()
+  {
+    return (bytes[ENCODING_INDEX] & USE_INTEGER_MASK) != 0;
+  }
+
+  public long getMinValue()
+  {
+    return minValue;
+  }
+
+  public int getSerializedSize()
+  {
+    return HEADER_SIZE_BYTES + Long.BYTES;
+  }
+
+  @Override
+  public String toString()
+  {
+    return Objects.toStringHelper(this)
+                  .add("bytes", bytes)
+                  .add("minValue", minValue)
+                  .toString();
+  }
+}