rdblue commented on code in PR #14117: URL: https://github.com/apache/iceberg/pull/14117#discussion_r2445424680
########## format/udf-spec.md: ########## @@ -0,0 +1,285 @@ +--- +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 and executes a function body. +Depending on the function type, the result can be: + +- **Scalar functions (UDFs)** – return a scalar value, which may be a primitive type (e.g., `int`, `string`) or a non-primitive type (e.g., `struct`, `list`). +- **Table functions (UDTFs)** – return a table, i.e., a table with zero or more rows and columns with a uniform schema. + +Many 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, and atomically swaps in the new file as the current metadata. +* 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 | Type | Description | +|-------------|-------------------|------------------------|-----------------------------------------------------------------------------------------------------------------| +| *required* | `function-uuid` | `string` | A UUID that identifies the function, generated once at creation. | +| *required* | `format-version` | `int` | Metadata format version (must be `1`). | +| *required* | `definitions` | `list<overload>` | List of function [overload](#overload) entities. | +| *required* | `definition-log` | `list<definition-log>` | History of [definition snapshots](#definition-log). | +| *required* | `max-overload-id` | `long` | Highest `overload-id` currently assigned for this UDF. Used to allocate new overload identifiers monotonically. | +| *optional* | `location` | `string` | Storage location of metadata files. | +| *optional* | `properties` | `map` | A string to string map of properties. | +| *optional* | `secure` | `boolean` | Whether it is a secure function. Default: `false`. | +| *optional* | `doc` | `string` | Documentation string. | + +Notes: +1. When `secure` is `true`, + - Engines **SHOULD NOT** expose the function definition through any inspection (e.g., `SHOW FUNCTIONS`). + - Engines **SHOULD** ensure that execution does not leak sensitive information through any channels, such as error messages, logs, or query plans. + +### Overload + +Function overloads allow multiple implementations of the same function name with different signatures. Each overload has +the following fields: + +| Requirement | Field name | Type | Description | +|-------------|----------------------------|--------------------------------------------------------|-----------------------------------------------------------------------------------------------| +| *required* | `overload-id` | `long` | Monotonically increasing identifier of this function overload. | +| *required* | `parameters` | `list<parameter>` | Ordered list of [function parameters](#parameter). Invocation order **must** match this list. | +| *required* | `return-type` | `string` (Iceberg data type) | Return type. Example: `"string"`, `"struct<...>"`. | +| *required* | `versions` | `list<overload-version>` | [Versioned implementations](#overload-version) of this overload. | +| *required* | `current-overload-version` | `long` | Monotonically increasing identifier of the current overload version. | +| *optional* | `function-type` | `string` (either `"udf"` or `"udtf"`, default `"udf"`) | If `"udtf"`, `return-type` must be a `struct` describing the output schema. | +| *optional* | `doc` | `string` | Documentation string. | + +### Parameter +| Requirement | Field | Type | Description | +|-------------|--------|------------------------------|--------------------------| +| *required* | `name` | `string` | Parameter name. | +| *required* | `type` | `string` (Iceberg data type) | Parameter data type. | +| *optional* | `doc` | `string` | Parameter documentation. | + +Notes: +1. The `name` and `type` of a `parameter` 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 | Type | Description | +|-------------|-----------------------|--------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------| +| *required* | `overload-version-id` | `long` | Monotonically increasing identifier of the overload version. | Review Comment: How about `version-id` since we don't need more than one version concept in this spec? -- This is an automated message from the Apache Git Service. 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