HeartSaVioR commented on issue #24173: [SPARK-27237][SS] Introduce State schema 
validation among query restart
URL: https://github.com/apache/spark/pull/24173#issuecomment-475610980
 
 
   > Does this cover parameter optional/required changes?
   
   The schemas are exactly same as how states are stored, so there's no 
optional thing. If you meant nullable the code includes it in comparison.
   
   > Just wondering whether an opt-out param not needed?
   
   I'm not sure what you meant. If you meant toggling this option, I guess 
that's not necessary. Preventing query to be run would be always better than 
exposing possibility to get indeterministic behaviors.
   
   > Do you have what exactly has to be modified?
   
   The schema of state is determined in stateful operator, or even in state 
implementation (please refer implementation of state in symmetric join) which 
we don't have any contract/interface for that. Moreover, if we want to deal 
with state schema at only one place, that should be done in driver side, 
whereas states are being initialized in executor side which would require 
additional work/trick to get the information from driver side.
   
   > Number of partitions may change over time.
   
   For state, Spark disallows changing the number of partitions - that's why 
Spark retains `spark.sql.shuffle.partitions` when query is running for the 
first time, and ignore new values.

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