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This page is meant as a template for writing a KIP. To create a KIP choose Tools->Copy on this page and modify with your content and replace the heading with the next KIP number and a description of your issue. Replace anything in italics with your own description.

Status

Current state: [One of "Under Discussion", "Accepted", "Rejected"]

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

Motivation

A self-join is a join whose left and right-side arguments are the same entity (a stream or a table reading from the same topic). Although self-joins are currently supported in Streams, their implementation is inefficient as they are implemented like regular joins where a state store is created for both left and right join arguments. Since both these arguments represent the same entity, we don't need to create two state stores (as they will contain the exact same data) but only one. This optimization is only suitable for inner joins whose join condition is on the primary key. We do not consider foreign-key joins as we would need to create a state store for both arguments in order to be able to do efficient lookups. Hence, we will handle foreign-key self joins as regular inner foreign-key joins. Moreover, we do not consider outer joins since we are focusing on primary key joins and there will always be at least one join result, the current record joining with itself. 

One should consider this work as a logical plan optimization rule. If the graph contains a join whose arguments are the same entity, then we will rewrite it by applying the aforementioned self-join optimization that will create only one state store. 

Public Interfaces

No public interfaces will be impacted but the config TOPOLOGY_OPTIMIZATION_CONFIG will be used to enable or disable this optimization along the other ones that it already controls. This introduces backwards compatibility issues. However, this is the case with any optimization happening under the umbrella of this config and the decision has been that users who have this config turned on should be prepared to build new topologies when upgrading. Another way to go about this is to introduce a config per optimization. 

Proposed Changes

The changes required to implement this proposal are:

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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

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

Test Plan

Describe in few sentences how the KIP 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

Add to the DSL the operator selfJoin. We did not go with this approach as we prefer to push the complexity of the optimization to streams instead of to the user.

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