[GitHub] [arrow-datafusion] adsharma edited a comment on issue #533: Add extension plugin to parse SQL into logical plan

[GitBox <gi...@apache.org>]

adsharma edited a comment on issue #533:
URL: https://github.com/apache/arrow-datafusion/issues/533#issuecomment-860999601


   I don't have much context about the proposal. Trying to understand things better. Please bear with me.
   
   The reason why SQL doesn't have `select * from t version as of n` is that it violates the isolation property in ACID, by allowing a query to read data from two different points in time (or logical sequence numbers if you prefer).
   
   Instead, they prefer a flow such as:
   
   ```
   BEGIN // implicitly selects a version and all queries following would read from that version
   select * from t1
   union all
   select * from t2;
   COMMIT
   ```
   
   I can also imagine a variant such as:
   
   ```
   BEGIN TRANSACTION @n1
   ...
   COMMIT
   ```
   
   which has the same effect. The benefit of these variants is that it makes it harder to write SQL that violates isolation properties.


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