Abacn commented on issue #19817:
URL: https://github.com/apache/beam/issues/19817#issuecomment-1204222164

   There is a problem of replace all DATETIME fieldtype to logical type, per 
@TheNeuralBit:
   > One thing that I realized as I wrote up my PR: If we make DATETIME a 
logical type backed by INT64, then it will have to use VarLongCoder, rather 
than InstantCoder as it currently does.
   >
   > It seems like that would be a problem since InstantCoder is designed to 
make lexicographic order correspond to chronological order.
   Note that InstantCoder is not only used in processing schemas but also in 
coding windows: 
https://github.com/apache/beam/blob/bf39489b2a1fd45e6798483d083e4ad240f66891/sdks/java/core/src/main/java/org/apache/beam/sdk/util/WindowedValue.java#L618
   Tried to replace the InstantCoder implementation using either a VarInt or a 
ByteArray it breaks window decoding here. Seems like there is some predefined 
stream not using InstantCoder.encode to pack a timestamp.
   
   Thus I decided not to change InstantCoder at this moment, instead extend the 
support MicrosInstant logical type in Java sdk and DateTime logical type in 
python sdk, seems to be the simplest way to make xlang read/write records 
containing timestamp type work.


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