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https://issues.apache.org/jira/browse/FLINK-685?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14631824#comment-14631824
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Fabian Hueske commented on FLINK-685:
-------------------------------------
Hi [~pietropinoli],
that's a good question ;-)
If we decide to implement the feature, there are basically two ways to
implement it:
1) light-weight on top of a CoGroup (and Map). This is basically using Flink's
existing API but won't be super efficient.
2) with a custom hash-based execution strategy. This means extending the
optimizer and adding runtime code that operates on binary data, a lot of
implementation effort. However, it should have much better performance than
option 1).
I haven't seen somebody asking for this feature, so I would opt for option 1)
if we want to add this feature.
We can add a more efficient execution strategy later if there is need for
better performance.
> Add support for semi-joins
> --------------------------
>
> Key: FLINK-685
> URL: https://issues.apache.org/jira/browse/FLINK-685
> Project: Flink
> Issue Type: New Feature
> Reporter: GitHub Import
> Priority: Minor
> Labels: github-import
> Fix For: pre-apache
>
>
> A semi-join is basically a join filter. One input is "filtering" and the
> other one is "filtered".
> A tuple of the "filtered" input is emitted exactly once if the "filtering"
> input has one (ore more) tuples with matching join keys. That means that the
> output of a semi-join has the same type as the "filtered" input and the
> "filtering" input is completely discarded.
> In order to support a semi-join, we need to add an additional physical
> execution strategy, that ensures, that a tuple of the "filtered" input is
> emitted only once if the "filtering" input has more than one tuple with
> matching keys. Furthermore, a couple of optimizations compared to standard
> joins can be done such as storing only keys and not the full tuple of the
> "filtering" input in a hash table.
> ---------------- Imported from GitHub ----------------
> Url: https://github.com/stratosphere/stratosphere/issues/685
> Created by: [fhueske|https://github.com/fhueske]
> Labels: enhancement, java api, runtime,
> Milestone: Release 0.6 (unplanned)
> Created at: Mon Apr 14 12:05:29 CEST 2014
> State: open
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