Background: 1) Position Deletes
Writers determine what rows are deleted and mark them in a 1 for 1 representation. With delete vectors this means every data file has at most 1 delete vector that it is read in conjunction with to excise deleted rows. Reader overhead is more or less constant and is very predictable. The main cost of this mode is that deletes must be determined at write time which is expensive and can be more difficult for conflict resolution 2) Equality Deletes Writers write out reference to what values are deleted (in a partition or globally). There can be an unlimited number of equality deletes and they all must be checked for every data file that is read. The cost of determining deleted rows is essentially given to the reader. Conflicts almost never happen since data files are not actually changed and there is almost no cost to the writer to generate these. Almost all costs related to equality deletes are passed on to the reader. Proposal: Equality deletes are, in my opinion, unsustainable and we should work on deprecating and removing them from the specification. At this time, I know of only one engine (Apache Flink) which produces these deletes but almost all engines have implementations to read them. The cost of implementing equality deletes on the read path is difficult and unpredictable in terms of memory usage and compute complexity. We’ve had suggestions of implementing rocksdb inorder to handle ever growing sets of equality deletes which in my opinion shows that we are going down the wrong path. Outside of performance, Equality deletes are also difficult to use in conjunction with many other features. For example, any features requiring CDC or Row lineage are basically impossible when equality deletes are in use. When Equality deletes are present, the state of the table can only be determined with a full scan making it difficult to update differential structures. This means materialized views or indexes need to essentially be fully rebuilt whenever an equality delete is added to the table. Equality deletes essentially remove complexity from the write side but then add what I believe is an unacceptable level of complexity to the read side. Because of this I suggest we deprecate Equality Deletes in V3 and slate them for full removal from the Iceberg Spec in V4. I know this is a big change and compatibility breakage so I would like to introduce this idea to the community and solicit feedback from all stakeholders. I am very flexible on this issue and would like to hear the best issues both for and against removal of Equality Deletes. Thanks everyone for your time, Russ Spitzer
