rdblue commented on code in PR #69: URL: https://github.com/apache/iceberg-docs/pull/69#discussion_r841290553
########## landing-page/content/common/statistics-format-spec.md: ########## @@ -0,0 +1,111 @@ +--- +url: statistics-format +toc: false +--- +<!-- + - 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. + --> + +# Plain Format for Iceberg Statistics + +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. + +## Format specification + +A file conforming to the format specification should have the structure as +described below. + +### File structure + +The file has the following structure + +``` +Magic Blob₁ Blob₂ ... Blobₙ Footer +``` + +where + +- `Magic` is four bytes 0x80, 0x70, 0x73, 0x83 (short for: Plain Format for + Iceberg Statistics), +- `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 + +``` +FooterPayload FooterPayloadSize Reserved FileFormatVersion Magic +``` + +where + +- `FooterPayload` LZ4-compressed, UTF-8 encoded JSON payload describing the + blobs in the file, with the structure described below, +- `FooterPayloadSize` is a length in bytes of the `FooterPayload`, stored as 4 + byte integer, little-endian, +- `Reserved` is 4 bytes reserved for future use, currently should be written as + 0x00, 0x00, 0x00, 0x00, +- `FileFormatVersion` is a number, stored as 4 byte integer, little-endian. + +### Footer Payload + +Footer payload bytes is LZ4-compressed, 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 Blob objects | yes | +| properties | map {string: string} | no | storage for arbitrary meta-information, like writer identification/version + +#### Blob + +`Blob` has the following fields + +| Field Name | Field Type | Required | Description | +| ---------- | ---------------------- | -------- | ----------- | +| type | string | yes | See [Blob types](#blob-types) +| columns | list of integer numbers| yes | list of column IDs the blob was computed for +| offset | integer number (long) | yes | The offset in the file where the blob contents start. Reader should assume the value can be more than 2^32. +| length | integer number | yes | The length of the blob stored in the file +| compression_codec | string | no | See [Compression codecs](#compression-codecs). If omitted, the data is assumed to be uncompressed. + +### Blob types + +The file format makes no assumptions about the information stored in it. The +applications using the format need to agree on type of information stored, the +format of the information and type designators. The list below is not part of +the file format specification itself, but is provided for readers' convenience. + +| Blob type | Description | +| ------------------------------ | ----------- | +| ndv-long-little-endian | 8-bytes integer stored little-endian and representing number of distinct values +| apache-datasketches-theta-v1 | A compact [Apache DataSketches](https://datasketches.apache.org/)'s Theta sketch representation. Review Comment: I suspect that we need to be a bit more specific here. Is there any more detail we can give besides a sketch? This could be interpreted as what you get when you Java serialize a sketch object. I think we want "the serialized form produced by the DataSketches library". -- 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. To unsubscribe, e-mail: dev-unsubscr...@iceberg.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org
