yihua commented on PR #9876:
URL: https://github.com/apache/hudi/pull/9876#issuecomment-1778076328

   I discussed the comments with @danny0405 offline.  Two things to address in 
this PR:
   
   (1) Instead of putting both partial and full schemas in the log block 
header, when partial updates are enabled, only the partial schema is added to 
the log block header in the same `SCHEMA` header and the full schema for 
snapshot reads is always going to be passed in from the table schema.  To 
indicate the schema is partial, a new log block header `IS_PARTIAL` should be 
added.
   (2) We should let users in the MERGE INTO statement to specify if they want 
partial updates in the log files in MOR tables, e.g., using sth like `col = 
EXISTING` to indicate that the column values should be kept as is.  We may not 
support this in the PR, but instead we should have an interim write config to 
control this behavior.


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