rdblue commented on code in PR #11130:
URL: https://github.com/apache/iceberg/pull/11130#discussion_r1813660233
##########
format/spec.md:
##########
@@ -298,16 +298,101 @@ Iceberg tables must not use field ids greater than
2147483447 (`Integer.MAX_VALU
The set of metadata columns is:
-| Field id, name | Type | Description |
-|-----------------------------|---------------|-------------|
-| **`2147483646 _file`** | `string` | Path of the file in which a
row is stored |
-| **`2147483645 _pos`** | `long` | Ordinal position of a row in
the source data file |
-| **`2147483644 _deleted`** | `boolean` | Whether the row has been
deleted |
-| **`2147483643 _spec_id`** | `int` | Spec ID used to track the file
containing a row |
-| **`2147483642 _partition`** | `struct` | Partition to which a row
belongs |
-| **`2147483546 file_path`** | `string` | Path of a file, used in
position-based delete files |
-| **`2147483545 pos`** | `long` | Ordinal position of a row,
used in position-based delete files |
-| **`2147483544 row`** | `struct<...>` | Deleted row values, used in
position-based delete files |
+| Field id, name | Type | Description
|
+|----------------------------------|---------------|--------------------------------------------------------------------------------------------------------|
+| **`2147483646 _file`** | `string` | Path of the file in which
a row is stored |
+| **`2147483645 _pos`** | `long` | Ordinal position of a row
in the source data file, starting at `0` |
+| **`2147483644 _deleted`** | `boolean` | Whether the row has been
deleted |
+| **`2147483643 _spec_id`** | `int` | Spec ID used to track the
file containing a row |
+| **`2147483642 _partition`** | `struct` | Partition to which a row
belongs |
+| **`2147483546 file_path`** | `string` | Path of a file, used in
position-based delete files |
+| **`2147483545 pos`** | `long` | Ordinal position of a
row, used in position-based delete files
|
+| **`2147483544 row`** | `struct<...>` | Deleted row values, used
in position-based delete files |
+| **`2147483543 _row_id`** | `long` | A unique long assigned
when row-lineage is enabled, see [Row Lineage](#row-lineage)
|
+| **`2147483542 _last_updated_sequence_number`** | `long` | The
sequence number which last updated this row when row-lineage is enabled [Row
Lineage](#row-lineage) |
+
+### Row Lineage
+
+In v3 and later, an Iceberg table can track row lineage fields for all newly
created rows. Row lineage is enabled by setting the field `row-lineage` to
true in the table's metadata. When enabled, engines must maintain the
`next-row-id` table field and the following row-level fields when writing data
files:
+
+* `_row_id` a unique long identifier for every row within the table. The value
is assigned via inheritance when a row is first added to the table and the
existing value is explicitly written when the row is copied into a new file.
+* `_last_updated_sequence_number` the sequence number of the commit that last
updated a row. The value is inherited when a row is first added or modified and
the existing value is explicitly written when the row is written to a different
data file but not modified.
+
+These fields are assigned and updated by inheritance because the commit
sequence number and starting row ID are not assigned until the snapshot is
successfully committed. Inheritance is used to allow writing data and manifest
files before values are known so that it is not necessary to rewrite data and
manifest files when an optimistic commit is retried.
+
+When row lineage is enabled, new snapshots cannot include [Equality
Deletes](#equality-delete-files). Row lineage is incompatible with equality
deletes because lineage values must be maintained, but equality deletes are
used to avoid reading existing data before writing changes.
+
+
+#### Row lineage assignment
+
+Row lineage fields are written when row lineage is enabled. When not enabled,
row lineage fields (`_row_id` and `_last_updated_sequence_number`) must not be
written to data files. The rest of this section applies when row lineage is
enabled.
+
+When a row is added or modified, the `_last_updated_sequence_number` field is
set to `null` so that it is inherited when reading. Similarly, the `_row_id`
field for an added row is set to `null` and assigned when reading.
+
+A data file with only new rows for the table may omit the
`_last_updated_sequence_number` and `_row_id`. If the columns are missing,
readers should treat both columns as if they exist and are set to null for all
rows.
+
+On read, if `_last_updated_sequence_number` is `null` it is assigned the
`sequence_number` of the data file's manifest entry. The data sequence number
of a data file is documented in [Sequence Number
Inheritance](#sequence-number-inheritance).
+
+When `null`, a row's `_row_id` field is assigned to the `first_row_id` from
its containing data file plus the row position in that data file (`_pos`). A
data file's `first_row_id` field is assigned using inheritance and is
documented in [First Row ID Inheritance](#first-row-id-inheritance). A
manifest's `first_row_id` is assigned when writing the manifest list for a
snapshot and is documented in [First Row ID
Assignment](#first-row-id-assignment). A snapshot's `first-row-id` is set to
the table's `next-row-id` and is documented in [Snapshot Row
IDs](#snapshot-row-ids).
+
+Values for `_row_id` and `_last_updated_sequence_number` are either read from
the data file or assigned at read time. As a result on read, rows in a table
always have non-null values for these fields when lineage is enabled.
+
+When an existing row is moved to a different data file for any reason, writers
are required to write `_row_id` and `_last_updated_sequence_number` according
to the following rules:
+
+1. The row's existing non-null `_row_id` must be copied into the new data file
+2. If the write has modified the row, the `_last_updated_sequence_number`
field must be set to `null` (so that the modification's sequence number
replaces the current value)
+3. If the write has not modified the row, the existing non-null
`_last_updated_sequence_number` value must be copied to the new data file
+
+
+#### Row lineage example
+
+This example demonstrates how `_row_id` and `_last_updated_sequence_number`
are assigned for a snapshot when row lineage is enabled. This starts with a
table with row lineage enabled and a `next-row-id` of 1000.
+
+Writing a new append snapshot would create snapshot metadata with
`first-row-id` assigned to the table's `next-row-id`:
+
+```json
+{
+ "operation": "append",
+ "first-row-id": 1000,
+ ...
+}
+```
+
+The snapshot's manifest list would contain existing manifests, plus new
manifests with an assigned `first_row_id` based on the `added_rows_count` of
previously listed added manifests:
+
+| `manifest_path` | `added_rows_count` | `existing_rows_count` |
`first_row_id` |
+|-----------------|--------------------|-----------------------|--------------------|
+| ... | ... | ... | ...
|
+| existing | 75 | 0 | 925
|
+| added1 | 100 | 25 | 1000
|
+| added2 | 0 | 100 | 1100
|
+| added3 | 125 | 25 | 1100
|
+
+The first added file, `added1`, is assigned the same `first_row_id` as the
snapshot and the following manifests are assigned `first_row_id` based on the
number of rows added by the previously listed manifests. The second file,
`added2`, does not change the `first_row_id` of the next manifest because it
contains no added data files.
+
+Within `added1`, the first added manifest, each data file's `first_row_id`
follows a similar pattern:
+
+| `status` | `file_path` | `record_count` | `first_row_id` |
+|----------|-------------|----------------|----------------|
+| EXISTING | data1 | 25 | 800 |
+| ADDED | data2 | 50 | null (1000) |
+| ADDED | data3 | 50 | null (1050) |
+
+The `first_row_id` of the EXISTING file `data1` was already assigned, so the
file metadata was copied into manifest `added1`.
+
+Files `data2` and `data3` are written with `null` for `first_row_id` and are
assigned `first_row_id` at read time based on the manifest's `first_row_id` and
the `record_count` of previously listed ADDED files in this manifest: (1,000 +
0) and (1,000 + 50).
+
+When the new snapshot is committed, the table's `next-row-id` must also be
updated (even if the new snapshot is not in the main branch). Because 225 rows
were added (`added1`: 100 + `added2`: 0 + `added3`: 125), the new value is
1,000 + 225 = 1,225:
+
+
+### Enabling Row Lineage for Non-empty Tables
+
+Any snapshot without the field `first-row-id` does not have any lineage
information and values for `_row_id` and `_last_updated_sequence_number` cannot
be assigned accurately.
+
+All files that were added before `row-lineage` was enabled should propagate
null for all of the `row-lineage` related
+fields. The values for `_row_id` and `_last_updated_sequence_number` should
always return null and when these rows are copied,
+null should be explicitly written. After this point, rows are treated as if
they were just created
+and assigned `row_id` and `_last_updated_sequence_number` as if they were new
rows.
Review Comment:
It is not possible to disable row lineage after enabling it.
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