rdblue commented on a change in pull request #1141:
URL: https://github.com/apache/iceberg/pull/1141#discussion_r553058029
##########
File path: site/docs/spec.md
##########
@@ -218,10 +218,31 @@ Partition specs capture the transform from table data to
partition values. This
| **`month`** | Extract a date or timestamp month, as months from
1970-01-01 | `date`, `timestamp(tz)`
| `int` |
| **`day`** | Extract a date or timestamp day, as days from 1970-01-01
| `date`, `timestamp(tz)`
| `date` |
| **`hour`** | Extract a timestamp hour, as hours from 1970-01-01
00:00:00 | `timestamp(tz)`
| `int` |
+| **`void`** | Always produces `null` (the void transform)
| Any
| Source type |
All transforms must return `null` for a `null` input value.
+#### Partition Field ID handling
+
+A partition field ID is an integer used to identify a partition field in
Iceberg manifest files.
+Field IDs are required in v2 and optional in v1.
+
+About compatibility between v1 and v2 tables:
+
+* For backward compatibility, if field ids are missing in a table metadata,
the reference implementation will sequentially generate ids for each field
starting at `1000` based on its position in the list of fields.
+* For forward compatibility, if field ids are not supported but present in the
metadata, old versions of the reference implementation will ignore those field
ids and then regenerate an auto-increment field id starting at 1000 for every
partition field.
+
+While working with a v1 table, old versions of the reference implementation
did not keep track of field IDs.
Review comment:
The v1 spec did not require consistent field IDs, so they were assigned
when creating each manifest file.
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