rdblue commented on code in PR #3188: URL: https://github.com/apache/iceberg/pull/3188#discussion_r841295454
########## format/view-spec.md: ########## @@ -0,0 +1,264 @@ +<!-- + - 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. + --> + +# Iceberg View Spec + +## Background and Motivation + +Most compute engines (e.g. Trino and Apache Spark) support views. A view is a logical table that can be referenced by future queries. Views do not contain any data. Instead, the query stored by the view is executed every time the view is referenced by another query. + +Each compute engine stores the metadata of the view in its proprietary format in the metastore of choice. Thus, views created from one engine can not be read or altered easily from another engine even when engines share the metastore as well as the storage system. This document standardizes the view metadata for ease of sharing the views across engines. + +## Goals + +* A common metadata format for view metadata, similar to how Iceberg supports a common table format for tables. +* The view metadata format specification + * Includes storage format as well as APIs to write/read the metadata. + * Supports versioning of views to track how a view evolved over time. + +## Overview + +View metadata storage mirrors how Iceberg table metadata is stored and retrieved. View metadata is maintained in metadata files. All changes to view state create a new view metadata file and completely replace the old metadata using an atomic swap. Like Iceberg tables, this atomic swap is delegated to the metastore that tracks tables and/or views by name. The view metadata file tracks the view schema, custom properties, current and past versions, as well as other metadata. +Each metadata file is self-sufficient. It contains the history of the last few operations performed on the view and can be used to roll back the view to a previous version. Review Comment: I think it is a little misleading to say the view metadata file "can be used to roll back the view to a previous version". Like tables, the view metadata files should only go forward. You can roll back by updating a pointer to an older version tracked in metadata but we do not want people to think that they can roll back by pointing to an older metadata file. -- 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
