nastra commented on code in PR #17822: URL: https://github.com/apache/iceberg/pull/17822#discussion_r4359515091
########## format/spec.md: ########## @@ -693,6 +698,96 @@ 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 live 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 Status](#constraint-status). + +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 or modify rows so that they 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, using ID references, 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. Must contain at least one field ID and no duplicates. Required for a `unique` or `primary-key` constraint and must not be set for a `check` constraint | + +The `field-ids` of a `unique` or `primary-key` constraint must only reference primitive fields and must not reference fields within a `list` or a `map`. Fields of type `float` or `double` must not be used. Fields of type `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. Fields of type `file` and their sub-fields must not be used. + +A table may have at most one `primary-key` constraint. The fields of a `primary-key` constraint must be `required` and must not be nested in an optional struct, so that the schema rejects null keys even when the constraint is not enforced. 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 ensure that written data conform to the constraint, and a write that would violate the constraint must fail. When a constraint is not enforced, writers are not required to ensure that written data conform to the constraint. + +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, 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 and the upgrade continues without it. Writers must not set `identifier-field-ids` in a v4 table schema, 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. 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 a [predicate](expressions-spec.md#predicates), not a value expression, and is serialized as described in the [Iceberg expressions spec](expressions-spec.md). It must use ID references so that it remains bound to the same fields when columns are renamed. + +A check expression is evaluated for each row and may only reference the fields of that row by ID. Review Comment: nit: maybe highlight "check expression" as `check` expression here and below -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. 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