jun-he commented on a change in pull request #1141:
URL: https://github.com/apache/iceberg/pull/1141#discussion_r567309330
##########
File path: site/docs/spec.md
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@@ -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, the v1 partition spec does not require
consistent field IDs and then they are assigned when creating each manifest
file.
+When creating a manifest, each field of the partition spec will be assigned an
ID starting at `1000`, and there is no guarantees about ID reuse across files.
+But as long as the partition spec will not be evolved, IDs will be consistent.
+
+This has a few implications:
+* Older writers may erase partition field IDs when writing to a v1 table. This
does not happen to v2 tables because writers will fail to read or write a v2
table.
+* Metadata tables need consistent field IDs across manifest files. To achieve
it, for v1 tables, please evolve the partition spec according to the
recommendations,
+i.e. don't reorder or delete partition fields; replace fields with with `void`
transform; add new fields to the end. Note that renames are OK and also note
that this does not apply for v2 tables.
Review comment:
Updated it with the reason (i.e. `v2 table tracks and persists the last
assigned partition field id`).
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