rdblue commented on a change in pull request #2762:
URL: https://github.com/apache/iceberg/pull/2762#discussion_r662573474
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File path: site/docs/spec.md
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@@ -979,30 +979,88 @@ This serialization scheme is for storing single values as
individual binary valu
## Format version changes
+Some tables in this spec have columns that specify requirements for v1 and v2
tables. These requirements are intended for writers when adding metadata files
to a table with the given version.
+
+| Requirement | Write behavior |
+|-------------|----------------|
+| (blank) | The field should be omitted |
+| _optional_ | The field can be written |
+| _required_ | The field must be written |
+
+Readers should be more permissive because v1 metadata files are allowed in v2
tables so that tables can be upgraded to v2 without rewriting the metadata
tree. For manifest list and manifest files, this table shows the expected v2
read behavior:
+
+| v1 | v2 | v2 read behavior |
+|------------|------------|------------------|
+| | _optional_ | Read the field as _optional_ |
+| | _required_ | Read the field as _optional_; it may be missing in
v1 files |
Review comment:
In most cases, yes. But reading the field requires you to set it as
optional or else the reader will fail when it isn't present in the older file.
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