huaxingao commented on code in PR #17822:
URL: https://github.com/apache/iceberg/pull/17822#discussion_r4291895014
##########
format/spec.md:
##########
@@ -655,6 +658,125 @@ Sorting floating-point numbers should produce the
following behavior: `-NaN` < `
A data or delete file is associated with a sort order by the sort order's id
within [a manifest](#manifests). Therefore, the table must declare all the sort
orders for lookup. A table could also be configured with a default sort order
id, indicating how the new data should be sorted by default. Writers should use
this default sort order to sort the data on write, but are not required to if
the default order is prohibitively expensive, as it would be for streaming
writes.
+### Constraints
+
+Constraints are added in v4 and are not supported in v3 or earlier.
+
+A **constraint** declares a condition that a table's rows are expected to
satisfy. Its definition is stored in table metadata. Each snapshot contains the
writer-reported status of each constraint that existed when the snapshot was
created; see [Constraint Validation](#constraint-validation).
+
+Three constraint types are defined:
+
+* `check` -- every row must satisfy a predicate
+* `unique` -- the values of a set of fields must be distinct across the rows
in which every field in the set is non-null; a row in which any field in the
set is null is not compared, so more than one such row may exist
+* `primary-key` -- the values of a set of fields must be distinct across all
rows and must not be null
+
+A constraint is stored separately from the table schema and must refer to
fields by field ID. Constraints may evolve independently from the schema, being
added, modified, or removed.
+
+#### Constraint Fields
+
+A constraint consists of the following fields:
+
+| Requirement | Field name | Type | Description |
+|-------------|---------------------------|-----------|-------------|
+| _required_ | **`constraint-id`** | `int` | ID of the constraint;
unique within the table |
+| _required_ | **`type`** | `string` | The constraint type:
`check`, `unique`, or `primary-key` |
+| _required_ | **`name`** | `string` | A name for the
constraint that is unique within the table. Names are for human consumption and
must not be used to identify a constraint in metadata |
+| _required_ | **`enforced`** | `boolean` | Whether writers must
only add rows that satisfy the constraint |
+| _required_ | **`timestamp-ms`** | `long` | Timestamp in
milliseconds from the unix epoch when the constraint was created or last
modified. The timestamp is informational and must not be used to determine
whether a constraint applies to a snapshot or whether it holds |
+| _optional_ | **`expression`** | `expression` | The predicate that
every row must satisfy, see [Check Constraint
Expressions](#check-constraint-expressions). Required for a `check` constraint
and must not be set for other types |
+| _optional_ | **`field-ids`** | `list<int>` | The set of field IDs
that the constraint applies to. Required for a `unique` or `primary-key`
constraint and must not be set for a `check` constraint |
+
+The fields that define what a constraint requires are embedded directly in the
constraint based on its `type`. Each type carries only the metadata that it
requires: a `check` constraint has an `expression` and must not declare
`field-ids`, and a `unique` or `primary-key` constraint has `field-ids` and
must not declare an `expression`. This keeps a single source of truth for the
fields that a constraint references.
+
+The `field-ids` of a `unique` or `primary-key` constraint must reference
primitive fields and must not reference fields within a `list` or a `map`.
Fields of type `float`, `double`, `geometry`, or `geography` must not be used
because Iceberg does not define equality for them. Fields of type `unknown`
must not be used because their values are always null.
+
+The fields of a `primary-key` constraint must be `required` and must not be
nested in an optional struct, so that the schema rules out null keys even when
the constraint is not enforced. These are the same restrictions that apply to
[identifier fields](#identifier-field-ids). The fields of a `unique` constraint
may be optional or nested in optional structs, because a row in which any field
is null is not compared.
+
+When a constraint is `enforced`, writers must only add rows they can prove
follow the constraint. A constraint that is not enforced can be written to by
any writer, regardless of their ability to prove that the constraint is
followed by new rows.
+
+For constraints, a row whose values are changed by a commit counts as a
removed row and an added row, whether the engine models the change as deleting
and inserting the row or as a modification that preserves the row ID (see [Row
Lineage](#row-lineage)).
+
+When a commit is retried against a new parent snapshot, the rows that a writer
adds must still satisfy the table's enforced constraints. A `check` constraint
is evaluated for each row on its own, so a retry cannot change its result. A
`unique` or `primary-key` constraint compares rows to each other, so a writer
must verify that added rows have distinct keys among themselves and do not
conflict with rows remaining from the new parent after applying the commit's
removals; verifying them against the original parent is not sufficient. A
writer must also verify the rows that it adds against any constraint that a
concurrent commit added or changed to enforced, and must recompute
`constraint-statuses` from the new parent.
+
+Whether to trust a constraint that is not enforced is left to engines and is
not tracked in table metadata.
+
+A table may have at most one `primary-key` constraint. A `primary-key`
constraint replaces [identifier field IDs](#identifier-field-ids), which
express the same concept: a set of fields that identifies a row. Iceberg never
guaranteed uniqueness for identifier fields and did not track whether it held.
A `primary-key` constraint makes both expressible, through `enforced` and
`constraint-statuses`. Identifier field IDs are not used in v4.
+
+When a table is upgraded to v4, the `identifier-field-ids` of the table's
current schema are rewritten as a single `primary-key` constraint that is not
enforced. The constraint is assigned a `constraint-id` from
`last-constraint-id` in the same way as any other constraint, its `name` is
`pk`, and its `timestamp-ms` is the time of the upgrade. If any of those fields
cannot be used in a `primary-key` constraint, such as a `geometry` or
`geography` field, no constraint is created. A table whose current schema has
no identifier fields has no constraints until they are added. Writers must not
set `identifier-field-ids` in a schema that is added to a v4 table, and readers
must ignore `identifier-field-ids` in a v4 table.
+
+Constraint IDs are assigned from the table's `last-constraint-id`, which is
treated as 0 when it is not present. Writers must assign a new constraint an ID
that is higher than the table's current `last-constraint-id` and must update
`last-constraint-id` to the highest assigned ID. Constraint IDs must not be
reused after the constraint that used an ID is removed, because retained
snapshots may still reference the removed ID. Readers must not assume that
every `constraint-id` referenced by a snapshot is present in `constraints`.
+
+#### Check Constraint Expressions
+
+The `expression` of a `check` constraint is serialized as described in the
[Iceberg expressions spec](expressions-spec.md) and must use ID references so
that it remains bound to the same fields when columns are renamed or reordered.
+
+A check expression is evaluated for each row over the values of that row. An
expression may reference more than one field of the row, such as `start_date <=
end_date`. Expressions that depend on more than one row, such as aggregates
over rows and window functions, and expressions that depend on another table,
such as subqueries, must not be used. An expression must not reference a field
within a `list` or a `map`, because such a field has a value for each element
rather than one value for the row. An expression may pass a whole `list` or
`map` field to a function, for example to check that a list is not empty.
+
+A check expression must produce the same result every time it is evaluated for
the same row. A function that depends on anything other than its arguments,
such as the current time or a random value, must not be called, because the
status recorded for a snapshot describes the table's data and an expression
whose result can change on its own would make a recorded status wrong without
any write. A [user-defined function](udf-spec.md) must not be called unless its
current definition version declares `deterministic` as true.
+
+The status of a `check` constraint that calls a user-defined function
describes the function definitions that were current when the snapshot was
written. Changing a function definition can make the status of a constraint
that calls it wrong, and engines are responsible for validating the constraint
again.
Review Comment:
Agreed, there is no reliable way for a reader to tell which UDF version was
used. I changed it so that a check constraint that calls a UDF is always
unvalidated. If it is enforced, writers still check the rows they add using the
current UDF. This avoids pinning UDF versions or storing them in
constraint-statuses. Does this work for you?
##########
format/spec.md:
##########
@@ -965,11 +1087,25 @@ A snapshot consists of the following fields:
| | | _required_ | **`first-row-id`** |
The first `_row_id` assigned to the first row in the first data file in the
first manifest, see [Row Lineage](#row-lineage) |
| | | _required_ | **`added-rows`** |
The upper bound of the number of rows with assigned row IDs, see [Row
Lineage](#row-lineage) |
| | | _optional_ | **`key-id`** | ID
of the encryption key that encrypts the manifest list key metadata |
+=== "v4"
+ | v4 | Field | Description |
+ |------------|------------------------------|-------------|
+ | _required_ | **`snapshot-id`** | A unique long ID |
+ | _optional_ | **`parent-snapshot-id`** | The snapshot ID of the
snapshot's parent. Omitted for any snapshot with no parent |
+ | _required_ | **`sequence-number`** | A monotonically increasing
long that tracks the order of changes to a table |
+ | _required_ | **`timestamp-ms`** | A timestamp when the
snapshot was created, used for garbage collection and table inspection |
+ | _required_ | **`manifest-list`** | The location of a manifest
list for this snapshot that tracks manifest files with additional metadata |
+ | _required_ | **`summary`** | A string map that summarizes
the snapshot changes, including `operation` as a _required_ field (see below) |
+ | _optional_ | **`schema-id`** | ID of the table's current
schema when the snapshot was created |
+ | _required_ | **`first-row-id`** | The first `_row_id` assigned
to the first row in the first data file in the first manifest, see [Row
Lineage](#row-lineage) |
+ | _required_ | **`added-rows`** | The upper bound of the
number of rows with assigned row IDs, see [Row Lineage](#row-lineage) |
+ | _optional_ | **`key-id`** | ID of the encryption key
that encrypts the manifest list key metadata |
+ | _optional_ | **`constraint-statuses`** | A struct (optional) that
records the status of the table's constraints for the snapshot, see [Constraint
Validation](#constraint-validation) |
The snapshot summary's `operation` field is used by some operations, like
snapshot expiration, to skip processing certain snapshots. Possible `operation`
values are:
* `append` -- Only data files were added and no files were removed.
-* `replace` -- Data and delete files were added and removed without changing
table data; i.e., compaction, changing the data file format, or relocating data
files.
+* `replace` -- Data and delete files were added and removed without changing
table data; i.e., compaction, changing the data file format, or relocating data
files. A snapshot that only records [constraint
statuses](#constraint-validation) and does not add or remove files also uses
`replace`.
Review Comment:
Done. Thanks!
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