rdblue commented on code in PR #11130:
URL: https://github.com/apache/iceberg/pull/11130#discussion_r1813684620
##########
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:
Review Comment:
Yes. `INSERT OVERWRITE` is an `INSERT` and the rows should be treated as new
rows.
I know there are cases here where users have historically built patterns
around `INSERT OVERWRITE` when `MERGE` was not available. For example, the
read-union-overwrite pattern was heavily used at Netflix to add new data to
existing partitions. The problem is that engines can't detect the intent and
carry row information through. These patterns also can't be optimized by
engines, so I think the best choice is to use the `INSERT` semantics here.
In addition, the Iceberg community has discouraged using `INSERT OVERWRITE`
for years because of the challenges with implicit data overwrites. Implicitly
overwriting a directory of data means that the physical layout needs to
implicitly align with writes. That's not a good pattern to use.
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