Merge hydrant runners flatly for realtime queries. #15757
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Prior to this patch, we have two layers of mergeRunners for realtime queries: one for each Sink (a logical segment) and one across all Sinks. This is done because to keep metrics and results grouped by Sink, given that each FireHydrant within a Sink has its own separate storage adapter. However, it costs extra memory usage due to the extra layer of materialization. This is especially pronounced for groupBy queries, which only use their merge buffers at the top layer of merging. The lower layer of merging materializes ResultRows directly into the heap, which can cause heap exhaustion if there are enough ResultRows. This patch changes to a single layer of merging when bySegment: false, just like Historicals. To accommodate that, segment metrics like query/segment/time are now per-FireHydrant instead of per-Sink. Two layers of merging are retained when bySegment: true. This isn't common, because it's typically only used when segment level caching is enabled on the Broker, which is off by default.
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The metric change should be called out in docs and release notes. |
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Prior to this patch, we have two layers of mergeRunners for realtime queries: one for each Sink (a logical segment) and one across all Sinks. This is to keep metrics and results grouped by Sink, given that each FireHydrant within a Sink has its own separate storage adapter.
However, it costs extra memory usage due to the extra layer of materialization. This is especially pronounced for groupBy queries, which only use their merge buffers at the top layer of merging. The lower layer of merging materializes ResultRows directly into the heap, which can cause heap exhaustion if there are enough ResultRows.
This patch changes to a single layer of merging when
bySegment: false, just like Historicals. To accommodate that, segment metrics likequery/segment/timeare now per-FireHydrant instead of per-Sink.Two layers of merging are retained when
bySegment: true. This isn't common, because it's typically only used when segment level caching is enabled on the Broker, which is off by default.