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Add indices and stats file format specification
Add a specification for a container file format to store indices and stats for Iceberg tables.
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landing-page/content/common/index-and-statistics-format.md
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| <!-- | ||
| - Licensed to the Apache Software Foundation (ASF) under one or more | ||
| - contributor license agreements. See the NOTICE file distributed with | ||
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| - distributed under the License is distributed on an "AS IS" BASIS, | ||
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| # Index and statistics file format | ||
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| This is a specification for the Plain Format for Iceberg Statistics, a file | ||
| format designed to store information such as statistics about data managed in an | ||
| Iceberg table that cannot be stored directly within the Iceberg manifest. A | ||
| statistics file contains arbitrary pieces of information (here called "blobs"), | ||
| along with metadata necessary to interpret them. The blobs supported by Iceberg | ||
| are documented at [Blob types](#blob-types). | ||
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| ## Format specification | ||
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| A file conforming to the format specification should have the structure as | ||
| described below. | ||
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| ### Versions | ||
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| Currently, there is a single version of the file format, described below. | ||
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| ### File structure | ||
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| The file has the following structure | ||
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| ``` | ||
| Magic Blob₁ Blob₂ ... Blobₙ Footer | ||
| ``` | ||
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| where | ||
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| - `Magic` is four bytes 0x50, 0x46, 0x42, 0x31 (short for: Plain Format for | ||
| Blobs, version 1), | ||
| - `Blobᵢ` is i-th blob contained in the file, to be interpreted by application | ||
| according to the footer, | ||
| - `Footer` is defined below. | ||
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| ### Footer structure | ||
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| Footer has the following structure | ||
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| ``` | ||
| Magic FooterPayload FooterPayloadSize Flags Magic | ||
| ``` | ||
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| where | ||
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| - `Magic`: four bytes, same as at the beginning of the file. | ||
| - `FooterPayload`: optionally compressed, UTF-8 encoded JSON payload describing the | ||
| blobs in the file, with the structure described below, | ||
| - `FooterPayloadSize`: a length in bytes of the `FooterPayload` (compressed), | ||
| stored as 4 byte integer, | ||
| - `Flags`: 4 bytes for boolean flags | ||
| - byte 0 (first) | ||
| - bit 0 (lowest bit): whether `FooterPayload` is compressed | ||
| - all other bits are reserved for future use and should be set to 0 on write | ||
| - all other bytes are reserved for future use and should be set to 0 on write | ||
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| A 4 byte integer is always signed, in a two's complement representation, stored | ||
| little-endian. | ||
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| ### Footer Payload | ||
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| Footer payload bytes is either uncompressed or LZ4-compressed (as a single | ||
| [LZ4 compression frame](https://github.com/lz4/lz4/blob/77d1b93f72628af7bbde0243b4bba9205c3138d9/doc/lz4_Frame_format.md) | ||
| with content size present), UTF-8 encoded JSON payload representing a single | ||
| `FileMetadata` object. | ||
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| #### FileMetadata | ||
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| `FileMetadata` has the following fields | ||
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| | Field Name | Field Type | Required | Description | | ||
| | ---------- | --------------------------------------- | -------- | ----------- | | ||
| | blobs | list of BlobMetadata objects | yes | | ||
| | properties | JSON object with string property values | no | storage for arbitrary meta-information, like writer identification/version. See [Common properties](#common-properties) for properties that are recommended to be set by a writer. | ||
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| #### BlobMetadata | ||
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| `BlobMetadata` has the following fields | ||
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| | Field Name | Field Type | Required | Description | | ||
| |-------------------|-------------------| -------- | ----------- | | ||
| | type | JSON string | yes | See [Blob types](#blob-types) | ||
| | fields | list of JSON long | yes | List of field IDs the blob was computed for; the order of items is used to compute sketches stored in the blob. | ||
| | offset | JSON long | yes | The offset in the file where the blob contents start | ||
| | length | JSON long | yes | The length of the blob stored in the file | ||
| | compression-codec | JSON string | no | See [Compression codecs](#compression-codecs). If omitted, the data is assumed to be uncompressed. | ||
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| ### Blob types | ||
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| The blobs can be of a type listed below | ||
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| #### `ndv-long-little-endian` blob type | ||
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| 8-bytes unsigned integer stored little-endian and representing number of distinct values | ||
| of a single field. | ||
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| #### `apache-datasketches-theta-v1` blob type | ||
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| A serialized form of a "compact" Theta sketch produced by the [Apache | ||
| DataSketches](https://datasketches.apache.org/) library. The sketch is obtained by | ||
| constructing Alpha family sketch with default seed, and feeding it with individual | ||
| distinct values converted to bytes using Iceberg's single-value serialization. | ||
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| ### Compression codecs | ||
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| The data can also be uncompressed. If it is compressed the code should be one of | ||
| codecs listed below. For maximal interoperability, other codecs are not supported. | ||
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| | Codec name | Description | | ||
| |------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | ||
| | lz4 | Single [LZ4 compression frame](https://github.com/lz4/lz4/blob/77d1b93f72628af7bbde0243b4bba9205c3138d9/doc/lz4_Frame_format.md), with content size present | | ||
| | zstd | Single [Zstandard compression frame](https://github.com/facebook/zstd/blob/8af64f41161f6c2e0ba842006fe238c664a6a437/doc/zstd_compression_format.md#zstandard-frames), with content size present | | ||
| __ | ||
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| ### Common properties | ||
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| When writing a file it is recommended to set the following fields in the | ||
| [FileMetadata](#filemetadata)'s `properties` field. | ||
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| - `created-by` - human-readable identification of the application writing the stats file, | ||
| along with it's version. Example "Trino version 381". | ||
| - `source-snapshot-id` - a table snapshot which was used to calculate blob contents | ||
| - `source-sequence-number` - sequence number of the table snapshot used to calculate blob contents |