discord9 commented on PR #21908:
URL: https://github.com/apache/datafusion/pull/21908#issuecomment-4436688712

   > Thanks for this contribution!
   > 
   > As @Jefffrey suggests, this PR would be easier to review if you clarified 
the behavior you want to change. For example, "forbid lossy cast" is quite 
vague about specifically which casts were previously allowed, why those casts 
are lossy, and which casts will now be disallowed as a result of this PR. It 
would also be nice to create a separate GitHub issue, as the PR template 
requests.
   > 
   > I wonder whether we want closed-by-default or open-by-default behavior. 
That is, instead of this approach, we could start by assuming that all casts 
are unsafe and then enumerate the specific casts that we want to allow. That 
would be safer, e.g., in the event of new types or new casts being added. 
Implementation might be simpler as well.
   
   sounds reasonable, how about for now only allow downcast like optimize 
`cast(ts_ms)=lit_ns` to `ts_ms==lit_ms`, does sound much safer and feels like 
the original intend


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