Status

Current state: Under Discussion

Discussion thread: here (<- link to https://mail-archives.apache.org/mod_mbox/flink-dev/)

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Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

We can introduce richer merge strategies, one of which is already introduced is PartialUpdateMergeFunction, which completes non-NULL fields when merging. We can introduce more powerful merge strategies, such as support for pre-aggregated merges. Currently the pre-aggregation is used by many big data systems, e.g. Apache Doris, Apache Kylin, Druid, to reduce storage cost and accelerate aggregation query. By introducing pre-aggregated merge to Flink table store, it can acquire the same benefit.  Aggregate functions which we plan to  implement includes sum, max/min, count, replace_if_not_null, replace, concatenate, or/and.

Public Interfaces

Briefly list any new interfaces that will be introduced as part of this proposal or any existing interfaces that will be removed or changed. The purpose of this section is to concisely call out the public contract that will come along with this feature.

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Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
  • They can decide whether to use pre-aggregated merge. If no, no impact on them. Otherwise, they have to  add some configurations in WITH clause when creating table. Configurations are described in 'Public Interfaces' section.
• If we are changing behavior how will we phase out the older behavior?
  • No need to phase out older behavior.
• If we need special migration tools, describe them here.
  • None.
• When will we remove the existing behavior?
  • No need.

Test Plan

Describe in few sentences how the FLIP will be tested. We are mostly interested in system tests (since unit-tests are specific to implementation details). How will we know that the implementation works as expected? How will we know nothing broke?

Rejected Alternatives

Each  pre-aggregated merge function will be covered with IT tests.

Rejected Alternatives

NoneIf there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.