Small time series agg improvement (elastic#106288)

After tsid hashing was introduced (elastic#98023), the time series aggregator generates the tsid (from all dimension fields) instead of using the value from the _tsid field directly. This generation of the tsid happens for every time serie, parent bucket and segment combination.

This changes alters that by only generating the tsid once per time serie and segment. This is done by just locally recording the current tsid.

Author: martijnvg

docs/changelog/106288.yaml

@@ -0,0 +1,5 @@
+pr: 106288
+summary: Small time series agg improvement
+area: TSDB
+type: enhancement
+issues: []

modules/aggregations/src/main/java/org/elasticsearch/aggregations/bucket/timeseries/TimeSeriesAggregator.java

@@ -144,6 +144,7 @@ protected LeafBucketCollector getLeafCollector(AggregationExecutionContext aggCt
             long currentTsidOrd = -1;
             long currentBucket = -1;
             long currentBucketOrdinal;
+            BytesRef currentTsid;
 
             @Override
             public void collect(int doc, long bucket) throws IOException {
@@ -157,12 +158,16 @@ public void collect(int doc, long bucket) throws IOException {
                     return;
                 }
 
-                TimeSeriesIdFieldMapper.TimeSeriesIdBuilder tsidBuilder = new TimeSeriesIdFieldMapper.TimeSeriesIdBuilder(null);
-                for (TsidConsumer consumer : dimensionConsumers.values()) {
-                    consumer.accept(doc, tsidBuilder);
+                BytesRef tsid;
+                if (currentTsidOrd == aggCtx.getTsidHashOrd()) {
+                    tsid = currentTsid;
+                } else {
+                    TimeSeriesIdFieldMapper.TimeSeriesIdBuilder tsidBuilder = new TimeSeriesIdFieldMapper.TimeSeriesIdBuilder(null);
+                    for (TsidConsumer consumer : dimensionConsumers.values()) {
+                        consumer.accept(doc, tsidBuilder);
+                    }
+                    currentTsid = tsid = tsidBuilder.buildLegacyTsid().toBytesRef();
                 }
-
-                BytesRef tsid = tsidBuilder.buildLegacyTsid().toBytesRef();
                 long bucketOrdinal = bucketOrds.add(bucket, tsid);
                 if (bucketOrdinal < 0) { // already seen
                     bucketOrdinal = -1 - bucketOrdinal;