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Avoid reading unnecessary dimension values when downsampling (#124451)

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Read dimension values once per tsid/bucket docid range instead of for each document being processed.
The dimension value within a bucket-interval docid range is always to same and this avoids unnecessary reads.

Latency of downsampling the tsdb track index into a 1 hour interval downsample index drop by ~16% (running on my local machine).
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Martijn van Groningen 2025-03-10 12:12:42 +01:00 • committed by GitHub
parent a5f186bb5d
commit 6afd3ecc58
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2 changed files with 32 additions and 9 deletions
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36
x-pack/plugin/downsample/src/main/java/org/elasticsearch/xpack/downsample/DimensionFieldProducer.java
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@ -44,16 +44,29 @@ public class DimensionFieldProducer extends AbstractDownsampleFieldProducer {
isEmpty = true;
}
void collect(final Object value) {
void collectOnce(final Object value) {
assert isEmpty;
Objects.requireNonNull(value);
if (isEmpty) {
this.value = value;
this.isEmpty = false;
return;
}
if (value.equals(this.value) == false) {
throw new IllegalArgumentException("Dimension value changed without tsid change [" + value + "] != [" + this.value + "]");
this.value = value;
this.isEmpty = false;
}
/**
* This is an expensive check, that slows down downsampling significantly.
* Given that index is sorted by tsid as primary key, this shouldn't really happen.
*/
boolean validate(FormattedDocValues docValues, int docId) throws IOException {
if (docValues.advanceExact(docId)) {
int docValueCount = docValues.docValueCount();
for (int i = 0; i < docValueCount; i++) {
var value = docValues.nextValue();
if (value.equals(this.value) == false) {
assert false : "Dimension value changed without tsid change [" + value + "] != [" + this.value + "]";
}
}
}
return true;
}
}
@ -69,12 +82,17 @@ public class DimensionFieldProducer extends AbstractDownsampleFieldProducer {
@Override
public void collect(FormattedDocValues docValues, int docId) throws IOException {
if (dimension.isEmpty == false) {
assert dimension.validate(docValues, docId);
return;
}
if (docValues.advanceExact(docId) == false) {
return;
}
int docValueCount = docValues.docValueCount();
for (int i = 0; i < docValueCount; i++) {
this.dimension.collect(docValues.nextValue());
this.dimension.collectOnce(docValues.nextValue());
}
}