deemoliu opened a new pull request #7906:
URL: https://github.com/apache/pinot/pull/7906


   ## Description
   
   Recently we got interesting use cases from industry about partial upsert. 
   
   Users have two event as follows, t is the timestamp column and t1<t2
   
   {t1, a1, b1, c1, d1}
   {t2, a2, nil, nil, nil}
   
   user specified field "a" as Overwrite field, and "b", "c", "d" field are 
empty in the second event.
   she expected merge result to be {a2, b1, c1, d1} 
   However the merge result was {a2, nil, nil, nil} which is the same as full 
upsert.
   
   The reason of this issue is because she didn't specify the mergers for "b", 
"c", "d" fields. Thus these fields will use the default behavior, "Overwrite 
regardless null".
   ```
   {
     "upsertConfig": {
       "mode": "PARTIAL",
       "partialUpsertStrategies":{
         "a": "OVERWRITE"
       }
     }
   }
   ```
   Her issue can be fixed with the following config, since the "overwrite" 
merger behavior is "Overwrite unless null".
   ```
   {
     "upsertConfig": {
       "mode": "PARTIAL",
       "partialUpsertStrategies":{
         "a": "OVERWRITE",
         "b": "OVERWRITE",
         "c": "OVERWRITE",
         "d": "OVERWRITE"
       }
     }
   }
   ```
   
   In the PR, I added a global strategy. so that user can use "globalStrategy", 
user will not need to set partialUpsertStategy for fields "b", "c", "d" fields.
   ```
   {
     "upsertConfig": {
       "mode": "PARTIAL",
       "globalUpsertStrategy": "OVERWRITE",
       "partialUpsertStrategies":{
         "a": "OVERWRITE"
       }
     }
   }
   ```
   NOTE: if we don't specify the overwrite. the overwrite behavior is  
"Overwrite regardless null".
   
   ## Upgrade Notes
   Does this PR prevent a zero down-time upgrade? (Assume upgrade order: 
Controller, Broker, Server, Minion)
   * [ ] Yes (Please label as **<code>backward-incompat</code>**, and complete 
the section below on Release Notes)
   
   Does this PR fix a zero-downtime upgrade introduced earlier?
   * [ ] Yes (Please label this as **<code>backward-incompat</code>**, and 
complete the section below on Release Notes)
   
   Does this PR otherwise need attention when creating release notes? Things to 
consider:
   - New configuration options
   - Deprecation of configurations
   - Signature changes to public methods/interfaces
   - New plugins added or old plugins removed
   * [ ] Yes (Please label this PR as **<code>release-notes</code>** and 
complete the section on Release Notes)
   ## Release Notes
   <!-- If you have tagged this as either backward-incompat or release-notes,
   you MUST add text here that you would like to see appear in release notes of 
the
   next release. -->
   
   <!-- If you have a series of commits adding or enabling a feature, then
   add this section only in final commit that marks the feature completed.
   Refer to earlier release notes to see examples of text.
   -->
   ## Documentation
   <!-- If you have introduced a new feature or configuration, please add it to 
the documentation as well.
   See 
https://docs.pinot.apache.org/developers/developers-and-contributors/update-document
   -->
   


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