flyrain commented on code in PR #14117: URL: https://github.com/apache/iceberg/pull/14117#discussion_r2373176598
########## format/udf-spec.md: ########## @@ -0,0 +1,221 @@ +--- +title: "SQL UDF Spec" +--- +<!-- + - Licensed to the Apache Software Foundation (ASF) under one or more + - contributor license agreements. See the NOTICE file distributed with + - this work for additional information regarding copyright ownership. + - The ASF licenses this file to You under the Apache License, Version 2.0 + - (the "License"); you may not use this file except in compliance with + - the License. You may obtain a copy of the License at + - + - http://www.apache.org/licenses/LICENSE-2.0 + - + - Unless required by applicable law or agreed to in writing, software + - distributed under the License is distributed on an "AS IS" BASIS, + - WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + - See the License for the specific language governing permissions and + - limitations under the License. + --> + +# Iceberg SQL UDF Spec + +## Background and Motivation + +A SQL user-defined function (UDF or UDTF) is a callable routine that accepts input parameters, executes a function body, +and returns either: + +* **Scalar functions (UDFs):** return a single value. +* **Table functions (UDTFs):** return a table with one or more rows and columns. + +Most compute engines (e.g., Spark, Trino) already support UDFs, but in different and incompatible ways. Without a common +standard, UDFs cannot be reliably shared across engines or reused in multi-engine environments. + +This specification introduces a standardized metadata format for UDFs in Iceberg. + +## Goals + +* Define a portable metadata format for both scalar and table SQL UDFs. The metadata is self-contained and can be moved across catalogs. +* Support function evolution through versioning and rollback. +* Provide consistent semantics for representing UDFs across engines. + +## Overview + +UDF metadata follows the same design principles as Iceberg table and view metadata: each function is represented by a +**self-contained metadata file**. Metadata captures definitions, parameters, return types, documentation, security, +properties, and engine-specific representations. + +* Any modification (new overload, updated representation, changed properties, etc.) creates a new metadata file. The old file is replaced atomically. +* Each metadata file includes recent definition versions, enabling rollbacks without external state. + +## Specification + +### UDF Metadata +The UDF metadata file has the following fields: + +| Requirement | Field name | Description | +|-------------|------------------------------|------------------------------------------------------------------| +| *required* | `function-uuid` | A UUID that identifies the function, generated once at creation. | +| *required* | `format-version` | Metadata format version (must be `1`). | +| *required* | `definition` | List of function overloads. | +| *required* | `definition-versions` | List of versioned function definitions. | +| *required* | `current-definition-version` | Identifier of the current definition version. | +| *optional* | `location` | The storage location of metadata files. | +| *optional* | `properties` | Arbitrary key-value pairs. | +| *optional* | `secure` | Whether it is a secure function. Default: `false`. | +| *optional* | `doc` | Documentation string. | + +### Overload + +Function overloads allow multiple implementations of the same function name with different signatures. Each overload has +the following fields: + +| Requirement | Field name | Description | +|-------------|-----------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| +| *required* | `overload-uuid` | A UUID that identifies this function overload. | +| *required* | `parameters` | Ordered list of function parameter definitions (name, type, optional doc). The order of parameters in this list **must exactly match** the order of arguments provided when invoking the function. | +| *required* | `return-type` | Return type (scalar or struct). Example: `"string"` or `"struct<...>"` | +| *required* | `versions` | List of overload versions. | +| *optional* | `udtf` | Whether this overload returns a table, default to `false`. If `true`, the `return-type` must be a `struct` describing the output schema. | +| *optional* | `doc` | Documentation string. | + +Notes: +1. The `name` and `type` of `parameters` are immutable. To change them, a new overload must be created. Only the optional documentation field (`doc`) can be updated in-place. +2. The `return type` is immutable. To change it, users must create a new overload and deprecate or remove the old one. + +### Overload-Version +Each overload can evolve over time by introducing new versions. An overload version represents a specific implementation +of the overload at a given point in time. + +| Requirement | Field name | Description | +|-------------|-----------------------|----------------------------------------------------------------------------------------------| +| *required* | `overload-version-id` | Monotonically increasing identifier (long) for this overload’s version history. Example: `1` | +| *required* | `representations` | Dialect-specific implementations of this overload. | +| *optional* | `deterministic` | Whether the function is deterministic. Defaults to `false`. | +| *required* | `timestamp-ms` | Creation timestamp of this version, in epoch milliseconds. | + +### Representation +A representation encodes how the overload version is expressed in a specific SQL dialect. + +| Requirement | Field name | Description | +|-------------|------------|--------------------------------------------------------| +| *required* | `dialect` | SQL dialect identifier. Example: `"spark"`, `"trino"`. | +| *required* | `body` | SQL expression text. | + +Note: The `body` must be valid SQL in the specified dialect; validation is the responsibility of the consuming engine. + +### Definition-Version + +| Requirement | Field name | Description | +|-------------|-------------------------|--------------------------------------------------------------------------------------------| +| *required* | `definition-version-id` | Unique identifier of the definition version, a monotonically increasing long. Example: `2` | +| *required* | `timestamp-ms` | Timestamp when the definition was created or updated. Example: `1734506000456` | +| *required* | `body` | List of mapping of overload uuids to their current version ids. | + +## Function Resolution in Engines +Resolution rule is decided by engines, but engines SHOULD: +1. Prefer exact signature matches over casting or subtyping. +2. Allow numeric widening (e.g., `INT` --> `BIGINT/FLOAT`) by default. +3. Require explicit casts for risky conversions (e.g., `STRING` --> `DATE`). + +## Appendix A: Example + +SQL statement: + +```sql +CREATE FUNCTION add_one(x INT COMMENT 'Input integer') +COMMENT 'Add one to the input value' +RETURNS INT +RETURN x + 1; + +CREATE FUNCTION add_one(x FLOAT COMMENT 'Input float') +COMMENT 'Add one to the input value' +RETURNS FLOAT +RETURN x + 1.0; +``` + +```json +{ + "function-uuid": "42fd3f91-bc10-41c1-8a52-92b57dd0a9b2", + "format-version": 1, + "definition": [ + { + "overload-uuid": "d2c7dfe0-54a3-4d5f-a34d-2e8cfbc34111", + "parameters": [ + { "name": "x", "type": "int", "doc": "Input integer" } + ], + "return-type": "int", + "doc": "Add one to the input integer", + "versions": [ + { + "overload-version-id": 1, + "deterministic": true, + "representations": [ + { + "type": "sql", + "dialect": "trino", + "body": "x + 2" + } + ], + "timestamp-ms": 1734507000123 + }, + { + "overload-version-id": 2, + "deterministic": true, + "representations": [ + { + "type": "sql", + "dialect": "trino", + "body": "x + 1" + } + ], + "timestamp-ms": 1735507000124 + } + ] + }, + { + "overload-uuid": "7c9f93b1-28b4-4ef5-90f5-70c73cda2222", + "parameters": [ + { "name": "x", "type": "float", "doc": "Input float" } + ], + "return-type": "float", + "doc": "Add one to the input float", + "versions": [ + { + "overload-version-id": 1, + "deterministic": true, + "representations": [ + { + "dialect": "trino", + "body": "x + 1.0" + } + ], + "timestamp-ms": 1734507001123 + } + ] + } + ], + "definition-versions": [ Review Comment: Added in the new commit -- This is an automated message from the Apache Git Service. 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