AddFiles: tighten file schemas - #39975
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…clared schema Parquet writers commonly declare every column optional whatever the data holds. Taken at face value, a file with no nulls in a column the table has as required would ask the pre-pass to relax that column for no reason. Tightening uses the footer's null counts to drop such requests. The evidence travels beside the schema, not inside it. ReadFooterSchema emits (canonical declared-schema JSON, proven-null-free paths) and CollectDistinctSchemas folds the paths per distinct schema by intersection: a column stays proven only when every file proved it, equivalently a relaxation is requested when any file needs it. The commit side will rebuild the group's most conservative member with FileSchemas.markRequired before classifying, which produces the same table schema as tightening each file individually would. union adds new columns as optional whatever the file declares. Tightening only affects columns the table already has as required: the union adds new columns as optional whatever the file declares.
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AddFiles: tighten file schemas with footer null counts, folded per dclared schema
Parquet writers commonly declare every column optional whatever the data holds. Taken at face value, a file with no nulls in a column the table has as required would ask the pre-pass to relax that column for no reason. Tightening uses the footer's null counts to drop such requests.
The evidence travels beside the schema, not inside it. ReadFooterSchema emits (canonical declared-schema JSON, proven-null-free paths) and CollectDistinctSchemas folds the paths per distinct schema by intersection: a column stays proven only when every file proved it, equivalently a relaxation is requested when any file needs it. The commit side will rebuild the group's most conservative member with FileSchemas.markRequired before classifying, which produces the same table schema as tightening each file individually would. union adds new columns as optional whatever the file declares.
Tightening only affects columns the table already has as required: the union adds new columns as optional whatever the file declares.
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