jiacai2050 commented on issue #4809:
URL: 
https://github.com/apache/arrow-datafusion/issues/4809#issuecomment-1371825520

   @alamb Thanks for bring me in.
   
   I have one concern when implement this feature in datafusion.
   
   Support time range of one query is `[2022-12-02, 2022-12-04]`, and the real 
dataset is
   - 2022-12-01, 10
   - 2022-12-03, 20
   - 2022-12-04, 30
   
   Then what is the result of `locf(avg(c0)` for `2022-12-02`, `None` or `10`?
   
   In Prometheus, it will be 10 when lookback-delta is `1d`, and this require 
to rewrite time range of the query to `[start-delta, end]`, and trim data to 
`[start, end]` after fill in gaps.
   
   I don't know how timescale deal with this case, IMO rewrite time query of 
one query may not suitable for datafusion since it's a generic SQL engine.
   
   Any ideas about this first value issue?


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