amogh-jahagirdar opened a new pull request, #17320:
URL: https://github.com/apache/iceberg/pull/17320

   When a query projects only a nested field that has an initial default and 
doesn't include the struct's real fields, PruneColumns reduces the struct to an 
empty group. That discards the file leaf columns whose definition levels carry 
the struct's per-row null-ness, so a null struct was incorrectly materialized 
as a struct of default values.
   
   To address this we solve this in two parts:
   
   1. Retain 1 leaf that we probe for (even if it's not being projected) just 
so we have a definition level to work with
   
   2. The default value constant reader uses that real leafs definition level 
to derive if the parent struct is null or not so that we surface the spec 
compliant results.


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