yyanyy commented on code in PR #11041: URL: https://github.com/apache/iceberg/pull/11041#discussion_r2412249832
########## format/view-spec.md: ########## @@ -42,12 +42,28 @@ An atomic swap of one view metadata file for another provides the basis for maki Writers create view metadata files optimistically, assuming that the current metadata location will not be changed before the writer's commit. Once a writer has created an update, it commits by swapping the view's metadata file pointer from the base location to the new location. +### Materialized Views + +Materialized views are a type of view with precomputed results from the view query stored as a table. +When queried, engines may return the precomputed data for the materialized views, shifting the cost of query execution to the precomputation step. + +Iceberg materialized views are implemented as a combination of an Iceberg view and an underlying Iceberg table, known as the storage table, which stores the precomputed data. +The metadata for a materialized view extends the Iceberg view metadata, adding a pointer to the precomputed data and refresh information to determine if the data is still fresh. +The refresh information is composed of data about the so-called "source tables", which are the tables referenced in the query definition of the materialized view. +The storage table can be in the states of "fresh", "stale" or "invalid", which are determined from the following situations: +* **fresh** -- The `snapshot_id`s of the last refresh operation match the current `snapshot_id`s of the source tables. +* **stale** -- The `snapshot_id`s do not match, indicating that a refresh operation needs to be performed to capture the latest source table changes. Review Comment: I personally don't have first hand experience working with MV by here's my 2 cents: I'm not completely sure if we need to couple this with the lineage discussion, since I feel that they may serve different purposes. I think from the doc Steven shared, the main purpose and advantage of option 2 is to help engine to determine if the MV is stale or not. Obviously the exact criteria for determining this is engine specific, but just from a high level guess, I think when the use case of someone creating MV1 from MV2 and MV3 emerges, most likely this user/engine would expect MV1 to be refreshed based on MV2 and MV3's data most of the time, instead of recursively obtain the most deeply nested source table and start from there; otherwise it feels that there's not much point creating MV2 and MV3 as materialized view and use them as the children for MV1, normal views would be enough. Because of this, I think the fact that it's a nested MV makes the "materialized" part of the child views more interesting for engine processing. -- 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: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
