flyrain commented on code in PR #14117: URL: https://github.com/apache/iceberg/pull/14117#discussion_r2719355279
########## format/udf-spec.md: ########## @@ -0,0 +1,322 @@ +--- +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 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 function (UDF)** – returns a single value, which may be a primitive type (e.g., `int`, `string`) or a non-primitive type (e.g., `struct`, `list`). +- **Table function (UDTF)** – returns a table with zero or more rows of 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 definition, 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<definition>` | List of function [definition](#definition) entities. | +| *required* | `definition-log` | `list<definition-log>` | History of [definition snapshots](#definition-log). | +| *optional* | `location` | `string` | The function's base location; used to create metadata file locations. | +| *optional* | `properties` | `map<string,string>` | 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. Engines must prevent leakage of sensitive information when a function is marked as `secure` by setting it to `true`. +2. Entries in `properties` are treated as hints, not strict rules. + +### Definition + +Each `definition` represents one function signature (e.g., `add_one(int)` vs `add_one(float)`). + +| Requirement | Field name | Type | Description | +|-------------|----------------------|-------------------------------------------------|---------------------------------------------------------------------------------------------------------------| +| *required* | `definition-id` | `string` | An identifier derived from canonical parameter-type tuple (lowercase, no spaces; e.g., `"(int,int,string)"`). | +| *required* | `parameters` | `list<parameter>` | Ordered list of [function parameters](#parameter). Invocation order **must** match this list. | +| *required* | `return-type` | `string` | Declared return type (see [Parameter Type](#parameter-type)). | +| *optional* | `nullable-return` | `boolean` | A hint to indicate whether the return value is nullable or not. Default: `true`. | +| *required* | `versions` | `list<definition-version>` | [Versioned implementations](#definition-version) of this definition. | +| *required* | `current-version-id` | `int` | Identifier of the current version for this definition. | +| *optional* | `function-type` | `string` (`"udf"` or `"udtf"`, default `"udf"`) | If `"udtf"`, `return-type` must be an Iceberg type `struct` describing the output schema. | +| *optional* | `doc` | `string` | Documentation string. | + +### Parameter +| Requirement | Field | Type | Description | +|-------------|--------|----------|--------------------------------------------------------------| +| *required* | `type` | `string` | Parameter data type (see [Parameter Type](#parameter-type)). | +| *required* | `name` | `string` | Parameter name. | +| *optional* | `doc` | `string` | Parameter documentation. | + +Notes: +1. Function definitions are identified by the tuple of `type`s and there can be only one definition for a given tuple. + The type tuple is immutable across versions. +2. Variadic (vararg) parameters are not supported. Each definition must declare a fixed number of parameters. +3. Each parameter input MUST be assignable to its declared Iceberg type. For complex types, the value’s + structure must match (correct field names, element/key/value types, and nesting). If a parameter—or any nested + field/element—is marked required, engines MUST reject null at that position (including inside structs, lists, and maps). +4. The `return-type` is immutable across versions. To change it, users must create a new definition and remove the old one. +5. The function MUST return a value assignable to the declared `return-type`, meaning the returned value’s type and + structure must match the declared Iceberg type (including field names, element types, and nesting for complex types), + and any field or element marked as required MUST NOT be null. Engines MUST reject results that violate these rules. + +#### Parameter-Type +Primitive types are encoded using the [Iceberg Type JSON Representation](https://iceberg.apache.org/spec/#appendix-c-json-serialization), +for example `"int"`, `"string"`. + +Three composite types are supported. +* `list<T>`: Ordered, homogeneous collection of elements of type `T` (any Iceberg type); example `list<int>`. +* `map<K,V>`: Key–value collection; `K` must be a primitive, comparable type (`string` recommended); example `map<string,int>`. +* `struct<...>`: Closed record with explicitly named fields; field order is significant and undeclared fields are not allowed; example `struct<id:int,name:string>`. + +### Definition-Version + +Each definition can evolve over time by introducing new versions. +A `definition version` represents a specific implementation of that definition at a given point in time. + +| Requirement | Field name | Type | Description | +|-------------|-------------------|---------------------------------------------------------------------------------------------------------|----------------------------------------------------------------| +| *required* | `version-id` | `int` | Monotonically increasing identifier of the definition version. | +| *required* | `representations` | `list<representation>` | [Dialect-specific implementations](#representation). | +| *optional* | `deterministic` | `boolean` (default `false`) | Whether the function is deterministic. | +| *optional* | `on-null-input` | `string` (`"returns_null_on_null_input"` or `"called_on_null_input"`, default `"called_on_null_input"`) | Defines how the UDF behaves when any input parameter is NULL. | +| *required* | `timestamp-ms` | `long` (epoch millis) | Creation timestamp of this version. | + +Note: + +`on-null-input` provides an optimization hint for query engines, its value can be either `"returns_null_on_null_input"` or `"called_on_null_input"`: +1. If set to `returns_null_on_null_input`, the function always returns `NULL` if any input argument is `NULL`. This allows engines to apply predicate pushdown or skip function evaluation for rows with `NULL` inputs. For a function `f(x, y) = x + y`, +the engine can safely rewrite `WHERE f(a,b) > 0` as `WHERE a IS NOT NULL AND b IS NOT NULL AND f(a,b) > 0`. +2. If set to `called_on_null_input`, the function may handle `NULL`s internally (e.g., `COALESCE`, `NVL`, `IFNULL`), so the engine must execute the function even if some inputs are `NULL`. + +### Representation +A representation encodes how the definition version is expressed in a specific SQL dialect. + +| Requirement | Field name | Type | Description | +|-------------|--------------|-------------------|---------------------------------------------------------------------------------------------| +| *required* | `type` | `string` | Must be `"sql"` | +| *required* | `dialect` | `string` | SQL dialect identifier (e.g., `"spark"`, `"trino"`). | +| *required* | `body` | `string` | SQL expression text. | + +Note: The `body` must be valid SQL in the specified dialect; validation is the responsibility of the consuming engine. + +### Definition-Log +| Requirement | Field name | Type | Description | +|-------------|-----------------------|----------------------------------------------------|------------------------------------------------------------------| +| *required* | `timestamp-ms` | `long` (epoch millis) | When the definition snapshot was created or updated. | +| *required* | `definition-versions` | `list<{ definition-id: string, version-id: int }>` | Mapping of each definition to its selected version at this time. | + +## Function Call Convention and Resolution in Engines +Resolution rule is decided by engines, but engines SHOULD: Review Comment: Fixed per suggestion -- This is an automated message from the Apache Git Service. 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