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Status

Current state: Under Discussion

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

JIRA: Unable to render Jira issues macro, execution error.

Released: <Flink Version>

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

Basic usage of pre-aggregated merge

To use pre-aggregated merge in Flink Table Store, two kind of configurations should be added to WITH clause when creating table.

  1. assign 'aggregation' to 'merge-engine' 
  2. designate aggregate function for each column of table.

For example,

-- DDL
CREATE TABLE T (
    pk STRING PRIMARY KEY NOT ENFOCED,
    sum_field1 BIGINT,
    max_field1 BIGINT
) WITH (
     'merge-engine' = 'aggregation',
     'sum_field1.aggregate-function' = 'sum',    -- sum up all sum_field1 with same pk
     'max_field.aggregate-function' = 'max'      -- get max value of all max_field1 with same pk
);
-- DML
INSERT INTO T VALUES ('pk1', 1, 1);
INSERT INTO T VALUES ('pk1', 1, 1);
-- verify
SELECT * FROM T;
=> output 'pk1', 2, 2

Advanced usage of pre-aggregated merge

Apart from creating table with pre-aggregated merge engine, using materialized view to get pre-aggregated merge result from a source table is another choice.


CREATE MATERIALIZED VIEW T
with (
    'merge-engine' = 'aggregation'
) AS SELECT
    pk,
    SUM(field1) AS sum_field1,
    MAX(field2) AS max_field1
FROM source_t
GROUP BY pk ;

This will start a stream job to synchronize data, consume source data, and write incrementally to T. This data synchronization job has no state.

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

Proposed Changes

Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.

Compatibility, Deprecation, and Migration Plan

This is new feature, no compatibility, deprecation, and migration plan.

Test Plan

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

Rejected Alternatives

None.

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