rdblue commented on code in PR #14117: URL: https://github.com/apache/iceberg/pull/14117#discussion_r2466553550
########## 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, executes a function body. +Depending on the function type, the result can be: + +- **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` | `array<overload>` | List of function [overload](#overload) entities. | +| *required* | `definition-log` | `array<definition-log>` | History of [definition snapshots](#definition-log). | +| *required* | `max-overload-id` | `long` | Highest `overload-id` currently assigned in this metadata file. Used to allocate new overload identifiers monotonically. | +| *optional* | `location` | `string` | Storage location of metadata files. | +| *optional* | `properties` | `map` | Arbitrary key–value pairs. | +| *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` | `array<parameter>` | Ordered list of [function parameters](#parameter). Invocation order **must** match this list. | +| *required* | `return-type` | `Type` (Iceberg data type; `struct` for UDTF) | Return type. Example: `"string"`, `"struct<...>"`. | +| *required* | `versions` | `array<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` | `enum { "udf", "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` | `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. Review Comment: After talking with Dan about the issue we discussed in the sync, I think that it makes sense to have a list of parameter names in the SQL representation. That way each representation is self-contained and consistent. And there's no need to have restrictions on whether names can change. The names in the definition and docs are shown as the definition, but the names used in SQL are specific to that SQL. It's the same idea as having a param name in a Java interface that can differ in the definition: ```java interface Definition { int do_something(String foo); } class Impl implements Definition { int do_something(String bar) { return bar.length(); } } ``` -- 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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