findepi commented on code in PR #69: URL: https://github.com/apache/iceberg-docs/pull/69#discussion_r859498987
########## landing-page/content/common/index-and-statistics-format.md: ########## @@ -0,0 +1,126 @@ +--- +url: index-and-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. + --> + +# Index and statistics file format + +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). + +## 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 0x50, 0x46, 0x49, 0x53 (short for: Plain Format for + Indices and 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 + +``` +Magic FooterPayload FooterPayloadSize Reserved Flags FileFormatVersion 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` is a length in bytes of the `FooterPayload` (compressed), + stored as 4 byte integer, little-endian, +- `Reserved` is 4 bytes reserved for future use, currently should be written as + 0x00, 0x00, 0x00, 0x00, +- `Flags` a 4 byte integer, stored little-endian, for boolean flags + - 0 (lowest bit): whether `FooterPayload` is compressed + - all other bits are reserved for future use and should be set to 0 on write +- `FileFormatVersion` is a number, stored as 4 byte integer, little-endian, + +### Footer Payload + +Footer payload bytes is either uncompressed or LZ4-compressed (as a single LZ4 +compression frame 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 + +#### BlobMetadata + +`BlobMetadata` has the following fields + +| Field Name | Field Type | Required | Description | +| ---------- | ---------------------- | -------- | ----------- | +| type | JSON string | yes | See [Blob types](#blob-types) +| columns | list of JSON long | yes | list of column IDs the blob was computed for +| offset | JSON long | yes | The offset in the file where the blob contents start. Reader should assume the value can be more than 2^32. +| 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 integer stored little-endian and representing number of distinct values. Review Comment: Making it signed requires me to say "two's complement", which is unnecessary complexity for what is a non-negative number. Note that I took a different approach for integer numbers in the footer, because footer _may_ gain some integer fields that can be negative, and having to deal with unsigned and signed numbers there would be impractical. Here, the definition is well-isolated, independent, so I it's actually simpler to define it as unsigned. -- This is an automated message from the Apache Git Service. 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