gianm commented on a change in pull request #8194: HllSketch Merge/Build
BufferAggregators: Speed up init with prebuilt sketch.
URL: https://github.com/apache/incubator-druid/pull/8194#discussion_r308977498
##########
File path:
extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/hll/HllSketchBuildBufferAggregator.java
##########
@@ -64,11 +74,27 @@ public HllSketchBuildBufferAggregator(
this.lgK = lgK;
this.tgtHllType = tgtHllType;
this.size = size;
+ this.emptySketch = new byte[size];
+
+ //noinspection ResultOfObjectAllocationIgnored (HllSketch writes to
"emptySketch" as a side effect of construction)
+ new HllSketch(lgK, tgtHllType, WritableMemory.wrap(emptySketch));
}
@Override
public void init(final ByteBuffer buf, final int position)
{
+ // Copy prebuilt empty sketch object.
+
+ final int oldPosition = buf.position();
+ try {
+ buf.position(position);
+ buf.put(emptySketch);
+ }
+ finally {
+ buf.position(oldPosition);
+ }
+
+ // Add an HllSketch for this chunk to our sketchCache.
Review comment:
Ah, thanks for the catch. I replaced it with:
```java
putSketchIntoCache(buf, position, HllSketch.writableWrap(mem));
```
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