huaxingao commented on code in PR #17822:
URL: https://github.com/apache/iceberg/pull/17822#discussion_r4139099356
##########
format/spec.md:
##########
@@ -654,6 +655,127 @@ 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 property that a table's rows are expected to
satisfy. A constraint's definition is stored in table metadata. Whether a
constraint holds is recorded for each snapshot, see [Constraint
Validation](#constraint-validation).
+
+Iceberg does not evaluate constraints. Enforcement and validation are
performed by engines that write to a table. Iceberg stores constraint
definitions and records the status that a writer reports for a commit without
verifying it.
+
+Three constraint types are defined:
+
+* `check` -- every row must satisfy a predicate
+* `unique` -- the non-null values of a set of fields must be distinct across
all rows; more than one row may have a null value
+* `primary-key` -- the values of a set of fields must be distinct across all
rows and must not be null
+
+Constraints are stored separately from schemas because they span multiple
fields and evolve independently. Every constraint references the fields that it
applies to by field ID, so a constraint continues to apply to the same columns
after a column is renamed or reordered.
+
+#### 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
verify that the rows they add 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 list 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 that are either top-level fields or nested in required
structs, and must not reference fields within a `list` or a `map`. These are
the same restrictions that apply to [identifier fields](#identifier-field-ids).
+
+When a constraint is `enforced`, writers must verify that the rows they add
satisfy the constraint and must fail the write if they do not. A writer that
cannot verify an enforced constraint must reject writes to the table rather
than add rows that have not been verified. When a constraint is not enforced,
writers are not required to verify the rows they add.
Review Comment:
Good catch, that was not the intent. A writer that cannot evaluate a check
should still be able to run a delete or a compaction, since those add no rows
and cannot create a violation.
I will change to
A writer that cannot verify an enforced constraint must reject writes
that add rows to the table.
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