emkornfield commented on code in PR #10835:
URL: https://github.com/apache/iceberg/pull/10835#discussion_r1702329275


##########
format/spec.md:
##########
@@ -241,7 +245,14 @@ Struct evolution requires the following rules for default 
values:
 
 #### Column Projection
 
-Columns in Iceberg data files are selected by field id. The table schema's 
column names and order may change after a data file is written, and projection 
must be done using field ids. If a field id is missing from a data file, its 
value for each row should be `null`.
+Columns in Iceberg data files are selected by field id. The table schema's 
column names and order may change after a data file is written, and projection 
must be done using field ids.
+
+Values for field ids which are not present in a data file must be resolved 
according the following rules:
+
+* Return the value from partition metadata if an [Identity 
Transform](#partition-transforms) exists for the field and the partition value 
is present in the `partition` struct on `data_file` object in the manifest. 

Review Comment:
   It seems for this case we probably want a subsection of the specification or 
an implementation note for projecting partition values?  Maybe that would be a 
good follow-up (I'm not sure which engines actually allow projecting partition 
values since they aren't technically part of the schema.)



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