[GitHub] [spark] cloud-fan commented on pull request #21852: [SPARK-24893] [SQL] Remove the entire CaseWhen if all the outputs are semantic equivalence

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

cloud-fan commented on pull request #21852:
URL: https://github.com/apache/spark/pull/21852#issuecomment-722823667


   @wangyum this looks correct. After `SELECT *, rand(100) as rd FROM t`, the output column `rd` is deterministic, as the result of `rand(100)` is materialized. We can treat it like writing `SELECT *, rand(100) as rd FROM t` to a table and read back.
   
   However, I do see a problem:
   ```
   scala> sql("select 1 FROM (SELECT *, rand(100) as rd FROM t) t1").explain
   == Physical Plan ==
   *(1) Project [1 AS 1#4]
   +- FileScan json default.t[] Batched: false, DataFilters: [], Format: JSON, Location: InMemoryFileIndex[file:/Users/cloud0fan/dev/spark/assembly/spark-warehouse/t], PartitionFilters: [], PushedFilters: [], ReadSchema: struct<>
   ```
   
   Seems we shouldn't eliminate the project with `rand(100)`. But it's not related to this PR.


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