Draft: Secondary index specification

format/index.md

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+---
+title: "Index Spec"
+---
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+
+# Iceberg Index Specification
+
+## Background and Motivation
+
+Indexes enable query engines to locate relevant rows without scanning entire datasets.
+They can accelerate point lookups, range predicates, and other retrieval patterns
+while preserving Iceberg's table format, snapshot isolation, and interoperability.
+
+Indexes are optional. Engines may choose to create, maintain, consume, or ignore them.
+
+## Goals
+
+- Define a portable metadata format for indexes
+- Provide a common storage architecture for index data
+- Allow indexes to evolve independently from table metadata as catalog-managed objects
+- Enable index sharing across engines
+- Provide a framework for defining new index types and transform functions
+
+## Overview
+
+Indexes are stored as a collection of files with some Iceberg table like semantics. At a high level they consist of a tracking file (similar to a root manifest file) which contains listings for a defined set of leaf files (similar to data files.) Leaf files store an ordered set of rows containing at least a key and the path of a Iceberg Table data file and the position within that file where the row where that key is stored. The organization of leaf files is defined by an Indexing Transform which varies based on the type of index. This structure is recorded in an Index metadata.json file which contains a set of snapshots, each of which points to a single tracking file mapping to the complete state of an Iceberg table at a given Iceberg table snapshot.
+
+Like Iceberg tables, views, and functions:
+
+- Metadata and data files are immutable
+- Updates create new metadata files
+- Catalogs perform atomic metadata swaps
+
+Index data is stored separately from metadata.
+
+Each index snapshot references a tracking file which describes the leaf files belonging to the snapshot.
+
+```text
+Index Metadata
+    |
+    +-- Index Snapshot
+            |
+            +-- Tracking File
+                    |
+                    +-- Leaf Data Files
+```
+
+Transform functions derive a transform value from the key columns and determine how index entries are organized within
+the leaf files.
+- The transform value space is divided into non-overlapping ranges.
+- Each leaf file stores entries for a single range.
+- The tracking file stores range bounds for each leaf file.
+
+This structure enables efficient planning while keeping the data layout flexible for different index implementations.
+
+## Definitions
+
+### Index Type
+
+The index type defines the logical category of an index and the class of queries it is designed to accelerate.
+
+The following index type is defined in this specification:
+
+| Type   |
+|--------|
+| SCALAR |
+
+The following index types are reserved for future specifications:
+
+| Type   |
+|--------|
+| VECTOR |
+| TERM   |
+
+The index type communicates the capabilities of an index to query engines and helps determine whether an index is
+applicable to a particular query.
+
+### Index Transform Function
+
+The index transform function defines how the index organization key is derived from source table columns when rows are
+stored in the index.
+
+The transform function determines the physical organization of the indexed data and therefore influences which query
+patterns can efficiently leverage the index.
+
+The following transform functions are defined in this specification:
+
+| Transform |
+|-----------|
+| IDENTITY  |
+| HASH      |
+| HILBERT   |
+
+The following transform function is reserved for future specifications:
+
+| Transform |
+|-----------|
+| IVF       |
+
+Index Instances may share the same index type while using different transform functions.
+
+### Index Instance
+
+An index instance is a concrete realization of an index type and function applied to a specific table.
+
+Users create index instances by specifying:
+
+- The source table
+- The index type
+- The transform function
+- The indexed columns
+- The included columns
+- Index properties
+
+Multiple instances of the same index type may exist for a table.
+
+### Index Snapshot
+
+An index snapshot is an immutable version of the index data generated from a specific table snapshot.
+
+Each index snapshot references a complete set of index files and contains all data from the referenced table snapshot.
+
+## Index Metadata
+
+The index metadata file stores the index definition and snapshot history.
+
+### Index Metadata File
+
+| Requirement | Field               | Type                     | Description                                     |
+|-------------|---------------------|--------------------------|-------------------------------------------------|
+| required    | format-version      | int                      | Index specification version                     |
+| required    | uuid                | string                   | Stable UUID assigned at creation                |
+| required    | table-uuid          | string                   | UUID of the indexed table                       |
+| required    | location            | string                   | Index root location                             |
+| required    | type                | string                   | Logical index type                              |
+| required    | transform-function  | string                   | Physical organization transform                 |
+| required    | key-column-ids      | list<int>                | Indexed columns                                 |
+| optional    | included-column-ids | list<int>                | Included columns                                |
+| optional    | properties          | map<string,string>       | Index properties applicable for every snapshot  |
+| required    | snapshots           | list<index-snapshot>     | Index snapshots                                 |
+
+## Index Snapshot
+
+Each index snapshot corresponds to one version of the index data.
+
+| Requirement | Field                    | Type               | Description                                                         |
+|-------------|--------------------------|--------------------|---------------------------------------------------------------------|
+| required    | snapshot-id              | long               | Index snapshot identifier                                           |
+| required    | source-table-snapshot-id | long               | Source table snapshot                                               |
+| required    | timestamp-ms             | long               | Snapshot creation timestamp                                         |
+| required    | tracking-file            | string             | Tracking file location                                              |
+| optional    | properties               | map<string,string> | Snapshot properties specific to this snapshot                       |
+| optional    | key-metadata             | binary             | Implementation-specific key metadata, for tracking file encryption. |
+
+## Tracking File
+
+Each index snapshot references exactly one tracking file.
+
+It contains summary metadata about all leaf files belonging to the index snapshot and enables efficient planning
+without scanning every leaf file.
+
+The tracking file may be stored using any supported metadata file format.
+
+### Tracking File Entry
+
+Each tracking file contains a collection of tracking file entries. A tracking file entry describes a single leaf file
+tracked by an index snapshot. The fields are the subset of the V4 manifest entry fields that are relevant to planning
+queries against the index.
+Entries contain aggregated statistics for all referenced leaf files, enabling engines to perform pruning and planning
+without opening every leaf file.
+
+| Field ID | Name               | Type    | Requirement  | Description                                                                                                  |
+|----------|--------------------|---------|--------------|--------------------------------------------------------------------------------------------------------------|
+| 100      | location           | string  | required     | Location of the referenced file.                                                                             |
+| 101      | file_format        | string  | required     | File format name, such as parquet, avro, or orc.                                                             |
+| 103      | record_count       | long    | required     | Number of records contained in the referenced leaf file.                                                     |
+| 104      | file_size_in_bytes | long    | required     | Total file size in bytes.                                                                                    |
+| 146      | content_stats      | struct  | optional     | Statistics used for planning and pruning, including transform-key statistics and optional column statistics. |
+| 131      | key_metadata       | binary  | optional     | Implementation-specific key metadata, used for leaf file encryption.                                         |
+
+### Content Statistics
+
+The content statistics structure contains transform-key statistics and optional column statistics for the referenced
+file. The transform-key statistics are always present, while column statistics are optional and may be omitted for
+performance reasons.
+
+## Leaf Files
+
+Leaf files contain the actual index entries and represent the lowest level of the index hierarchy.
+
+Leaf files must be standard Iceberg data files and may be stored using any Iceberg-supported file format:
+- Parquet
+- Avro
+- ORC - May be removed if ORC support is deprecated in Iceberg.
+
+The schema of a leaf file is determined by the index definition and contains:
+- All key columns defined by the index
+- All included columns defined by the index
+- The transform value produced by the transform function
+- The source file path
+- The source row position
+
+### Transform functions
+
+The transform function produces a transform value for each indexed row. To enable efficient planning, the transform
+value space is divided into non-overlapping ranges. Each leaf file contains entries for a single range, while the
+tracking file stores the corresponding bounds for every leaf file. If, and only if, a single transform value produces
+more rows than fit in one leaf file, multiple leaf files may be created for that value, and engines must read all of
+them.
+
+When a query predicate can be mapped to transform value ranges, engines can use these bounds to prune leaf files that
+cannot contain matching entries, avoiding unnecessary reads.
+
+A well-designed transform function also preserves locality between the source columns and the resulting transform value,
+allowing additional pruning using column statistics stored in the tracking file. For example, a Hilbert transform can
+cluster similar multi-column keys together, reducing the number of leaf files that must be read for range scans and
+partial-key lookups.
+
+The following transform functions are currently supported:
+
+| Transform | Bound Interpretation |
+|-----------|----------------------|
+| IDENTITY  | Original value range |
+| HASH      | Hash bucket range    |
+| HILBERT   | Hilbert key range    |
+
+The following transform type is reserved for future specifications:
+
+| Transform | Bound Interpretation       |
+|-----------|----------------------------|
+| IVF       | Centroid identifier range  |
+
+
+### Leaf Schema
+
+Columns originating from the source table must preserve their original Iceberg field identifiers.
+Reusing the original field IDs ensures that schema evolution, column renames, and type compatibility semantics remain
+consistent between the table and the index.
+
+The index-specific columns are:
+
+| Field Id  | Column          | Type   | Description                                                            |
+|-----------|-----------------|--------|------------------------------------------------------------------------|
+| TBD       | transform_value | long   | The result of applying the index transform function to the key columns |
+| TBD       | file_path       | string | The path of the source data file the entry references                  |
+| TBD       | position        | long   | The row position of the entry within the source data file              |
+
+## Example: Key Lookup Index
+
+Index Type:
+
+```text
+SCALAR
+```
+
+Transform Function:
+
+```text
+HASH(primary_key)
+```
+
+Leaf Schema:
+
+| Column           |
+|------------------|
+| primary_key      |
+| transform_value  |
+| file_path        |
+| position         |
+
+The leaf files are organized by hash key, while the tracking file stores summary information and pruning statistics.
+
+## Snapshot Evolution
+
+Index snapshots are immutable.
+
+Updating an index creates:
+
+1. New leaf files
+2. A new tracking file pointing to new leaf files, and potentially some old leaf files that are still valid for the new snapshot.
+3. A new index metadata file
+
+The catalog commits the update by atomically replacing the metadata location.
+
+## Maintenance
+
+An index maintains a mapping between source table snapshots and index snapshots.
+
+Engines may use this mapping to determine whether a compatible index snapshot exists for a given table snapshot.
+
+## Future Extensions
+
+Future specifications may define:
+
+- VECTOR indexes
+- TERM indexes