[GitHub] [druid] vtlim commented on a diff in pull request #12415: Add docs to metric spec for auto compaction

[GitBox <gi...@apache.org>]

vtlim commented on code in PR #12415:
URL: https://github.com/apache/druid/pull/12415#discussion_r847616165


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docs/configuration/index.md:
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@@ -970,6 +970,7 @@ A description of the compaction config is:
 |`granularitySpec`|Custom `granularitySpec`. See [Automatic compaction granularitySpec](#automatic-compaction-granularityspec)|No|
 |`dimensionsSpec`|Custom `dimensionsSpec`. See [Automatic compaction dimensionsSpec](#automatic-compaction-dimensions-spec)|No|
 |`transformSpec`|Custom `transformSpec`. See [Automatic compaction transformSpec](#automatic-compaction-transform-spec)|No|
+|`metricsSpec`|Custom `metricsSpec`. The compaction task uses the specified `metricsSpec` rather than generating one. Note that if `metricsSpec` is specified then the compaction task will preserve existing metrics of the original data. This means that for any row that already has metric (with the same name defined in metricSpec), the metric aggregator in metricSpec is skipped and the existing metric is preserved. If the row does not already have the metric, then the metric aggregator is applied on the source column as usual. Metrics is then combined and rollup across segments as usual.|No|

Review Comment:
   ```suggestion
   |`metricsSpec`|Custom [`metricsSpec`](../ingestion/ingestion-spec.md#metricsspec). The compaction task preserves any existing metrics regardless of whether `metricsSpec` is specified. If `metricsSpec` is specified, Druid does not reapply any aggregators matching the metric names specified in `metricsSpec` to rows that already have the associated metrics. For rows that do not already have the metric specified in `metricsSpec`, Druid applies the metric aggregator on the source column, then proceeds to combine the metrics across segments as usual. If `metricsSpec` is not specified, Druid automatically discovers the metrics in the existing and compacted segments and combines existing metrics with the same metric name across segments.|No|
   ```
   



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