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Add statistics information in table snapshot #77

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0c603c4
Add Puffin, indices and stats file format specification
findepi Apr 1, 2022
934ec5e
Add statistics information in table snapshot
findepi May 5, 2022
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144 changes: 144 additions & 0 deletions landing-page/content/common/puffin.md
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---
url: puffin
toc: false
---
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# Puffin file format

This is a specification for the Puffin, a file format designed to store
information such as indexes and statistics about data managed in an
Iceberg table that cannot be stored directly within the Iceberg manifest. A
Puffin 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).

## Format specification

A file conforming to the Puffin file format specification should have the structure
as described below.

### Versions

Currently, there is a single version of the Puffin file format, described below.

### File structure

The Puffin file has the following structure

```
Magic Blob₁ Blob₂ ... Blobₙ Footer
```

where

- `Magic` is four bytes 0x50, 0x46, 0x41, 0x31 (short for: Puffin _Fratercula
arctica_, version 1),
- `Blobᵢ` is i-th blob contained in the file, to be interpreted by application
according to the footer,
- `Footer` is defined below.

### Footer structure

Footer has the following structure

```
Magic FooterPayload FooterPayloadSize Flags Magic
```

where

- `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

A 4 byte integer is always signed, in a two's complement representation, stored
little-endian.

### Footer Payload

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.

#### FileMetadata

`FileMetadata` has the following fields


| 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.

#### BlobMetadata

`BlobMetadata` has the following fields

| 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.

### Blob types

The blobs can be of a type listed below

#### `ndv-long-little-endian` blob type

8-bytes unsigned integer stored little-endian and representing number of distinct values
of a single field.

#### `apache-datasketches-theta-v1` blob type

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.

### Compression codecs

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.

| 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 |
__

### Common properties

When writing a Puffin file it is recommended to set the following fields in the
[FileMetadata](#filemetadata)'s `properties` field.

- `created-by` - human-readable identification of the application writing the file,
along with its 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
12 changes: 12 additions & 0 deletions landing-page/content/common/spec.md
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Expand Up @@ -475,6 +475,7 @@ A snapshot consists of the following fields:
| _optional_ | | **`manifests`** | A list of manifest file locations. Must be omitted if `manifest-list` is present |
| _optional_ | _required_ | **`summary`** | A string map that summarizes the snapshot changes, including `operation` (see below) |
| _optional_ | _optional_ | **`schema-id`** | ID of the table's current schema when the snapshot was created |
| | _optional_ | **`statistics`** | A list of statistics files' metadata (see below) |

The snapshot summary's `operation` field is used by some operations, like snapshot expiration, to skip processing certain snapshots. Possible `operation` values are:

Expand All @@ -493,6 +494,17 @@ Manifests for a snapshot are tracked by a manifest list.

Valid snapshots are stored as a list in table metadata. For serialization, see Appendix C.

Statistics files' metadata within `statistics` field is a struct with the following fields:

| Field name | Type | Description |
|---------------------------------|------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **`location`** | `string` | Location of the statistics file. See [Puffin file format](../puffin). |
| **`file-size-in-bytes`** | `long` | Size of the statistics file. |
| **`file-footer-size-in-bytes`** | `long` | Size of the statistics file's footer. See [Puffin file format](../puffin) for footer definition. |
| **`source-sequence-number`** | `long` | Table sequence number at which the stats were calculated |
| **`statistics-fields-sets`** | `map<string, list<list<integer>>>` | A map indicating which statistics are contained in the statistics file and on which columns they were calculated. The map keys are statistics sketch names and map values represent sets of columns, given by column ID. |

Snapshot's statistics field should be retained by writers, unless writer updates the statistics, or knows they became obsolete.

#### Manifest Lists

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