danielcweeks commented on code in PR #17822: URL: https://github.com/apache/iceberg/pull/17822#discussion_r4109174684
########## 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: ```suggestion 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. When a constraint is not enforced, writers are not required to verify the rows they add. ``` -- 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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