gruuya opened a new pull request, #7981:
URL: https://github.com/apache/arrow-datafusion/pull/7981

   ## Which issue does this PR close?
   
   Closes #7827.
   
   ## Rationale for this change
   
   While in principle the output of `DISTINCT ON` can be emulated by combining 
several aggregation, projection and sorting queries/operators, in practice this 
is very verbose and impractical for users, unlike `DISTINCT ON` which is quite 
succinct. On the other hand that does mean that `DISTINCT ON` can be optimized 
into the equivalent representation under the hood during logical planning, 
while at the same time providing the compact syntactical interface.
   
   ## What changes are included in this PR?
   
   - A new logical plan sub-variety for handling the `DISTINCT ON` from 
Postgres.
   - Parsing and planning for it
   - Optimization which converts it into the equivalent representation using 
the more common logical plan primitives
   - Proto serialization
   - Tests
   
   ## Are these changes tested?
   
   Yes, there are also new SLTs.
   
   ## Are there any user-facing changes?
   
   SQL support extends to encompass `DISTINCT ON` queries.


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