andygrove commented on PR #5407:
URL: 
https://github.com/apache/datafusion-comet/pull/5407#issuecomment-5444201255

   `spark.sql.variant.pushVariantIntoScan` defaults to `false` on Spark 4.0.x 
but `true` on 4.1.0, 4.1.3, and 4.2.0. On 4.1 I see `SELECT v FROM t` rewritten 
to `ReadSchema: struct<v:struct<0:variant>>` and the scan falls back, so 
nothing that projects `v` keeps a native scan out of the box. Every test here 
pins the conf to `false`, including the header in `variant.sql`, so CI doesn't 
cover the default, and 4.1 is our default profile.
   
   Could you add the required configuration to the description and the user 
docs, and note it next to the benchmark numbers? And is handling the pushed 
struct worth attempting here? `RequestedVariantField.fullVariant` uses path `$` 
with `targetType = VariantType`, so a whole-value projection under pushdown is 
exactly a one-field `VariantStruct`. If that's better as a follow-up, could you 
file an issue and link it so this doesn't sit unreachable by default on 4.1 and 
4.2?
   
   Separately, the reason users will see is `Unsupported v of type VariantType 
(shredded; not supported by native scan)`. That word "shredded" points at 
`allowReadingShredded`, which isn't the lever. Can it name 
`pushVariantIntoScan` instead?
   


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