JanKaul commented on code in PR #11041: URL: https://github.com/apache/iceberg/pull/11041#discussion_r2086100508
########## 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: Thanks @chenjian2664 and @yyanyy for your comments. These are great points. The current definition supports nested views in that it fully expands the query tree at refresh time and stores all directly and indirectly referenced tables in the refresh-state. This can include multiple levels of views. This currently would also apply to materialized views. However, I think you're making a good point. When referencing another materialized view it would be easier to just store the snapshot_id of its storage table instead of tracking all table references in the materialized view. You're correct in assuming that `fresh`/`stale`/`invalid` are introduced to have some nomenclature, these to not occur in the metadata. I will try to make this clearer. -- 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]
