[GitHub] [incubator-druid] leventov commented on a change in pull request #7185: Avoid many unnecessary materializations of collections of 'all segments in cluster' cardinality

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leventov commented on a change in pull request #7185: Avoid many unnecessary materializations of collections of 'all segments in cluster' cardinality
URL: https://github.com/apache/incubator-druid/pull/7185#discussion_r263142135
 
 

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 File path: server/src/main/java/org/apache/druid/metadata/MetadataSegmentManager.java
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 @@ -59,6 +60,8 @@
 
   Collection<ImmutableDruidDataSource> getDataSources();
 
+  Iterable<DataSegment> iterateAllSegments();
 
 Review comment:
   I intentionally renamed the method to not start with "get". Although formally it returns a lambda whose creation is cheap, iteration itself involves `flatMap()`, creation of many streams, and therefore is not so cheap. 
   
   See item 16 in [checklist](https://docs.google.com/document/d/17EEKT6fih9Dd5NfXjBoECcKbVp1eOB2vb3jKqTF9pPc/edit).
   
   I will add Javadoc to this method to additionally highlight that the iteration is not super cheap (however, far cheaper than materialization to collections that was done before.)

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