GitHub user tdas opened a pull request:
https://github.com/apache/spark/pull/20765
[SPARK-23406][SS] Enable stream-stream self-joins for branch-2.3
This is a backport of #20598.
## What changes were proposed in this pull request?
Solved two bugs to enable stream-stream self joins.
### Incorrect analysis due to missing MultiInstanceRelation trait
Streaming leaf nodes did not extend MultiInstanceRelation, which is
necessary for the catalyst analyzer to convert the self-join logical plan DAG
into a tree (by creating new instances of the leaf relations). This was causing
the error `Failure when resolving conflicting references in Join:` (see JIRA
for details).
### Incorrect attribute rewrite when splicing batch plans in
MicroBatchExecution
When splicing the source's batch plan into the streaming plan (by replacing
the StreamingExecutionPlan), we were rewriting the attribute reference in the
streaming plan with the new attribute references from the batch plan. This was
incorrectly handling the scenario when multiple StreamingExecutionRelation
point to the same source, and therefore eventually point to the same batch plan
returned by the source. Here is an example query, and its corresponding plan
transformations.
```
val df = input.toDF
val join =
df.select('value % 5 as "key", 'value).join(
df.select('value % 5 as "key", 'value), "key")
```
Streaming logical plan before splicing the batch plan
```
Project [key#6, value#1, value#12]
+- Join Inner, (key#6 = key#9)
:- Project [(value#1 % 5) AS key#6, value#1]
: +- StreamingExecutionRelation Memory[#1], value#1
+- Project [(value#12 % 5) AS key#9, value#12]
+- StreamingExecutionRelation Memory[#1], value#12 // two different
leaves pointing to same source
```
Batch logical plan after splicing the batch plan and before rewriting
```
Project [key#6, value#1, value#12]
+- Join Inner, (key#6 = key#9)
:- Project [(value#1 % 5) AS key#6, value#1]
: +- LocalRelation [value#66] // replaces
StreamingExecutionRelation Memory[#1], value#1
+- Project [(value#12 % 5) AS key#9, value#12]
+- LocalRelation [value#66] // replaces
StreamingExecutionRelation Memory[#1], value#12
```
Batch logical plan after rewriting the attributes. Specifically, for
spliced, the new output attributes (value#66) replace the earlier output
attributes (value#12, and value#1, one for each StreamingExecutionRelation).
```
Project [key#6, value#66, value#66] // both value#1 and value#12
replaces by value#66
+- Join Inner, (key#6 = key#9)
:- Project [(value#66 % 5) AS key#6, value#66]
: +- LocalRelation [value#66]
+- Project [(value#66 % 5) AS key#9, value#66]
+- LocalRelation [value#66]
```
This causes the optimizer to eliminate value#66 from one side of the join.
```
Project [key#6, value#66, value#66]
+- Join Inner, (key#6 = key#9)
:- Project [(value#66 % 5) AS key#6, value#66]
: +- LocalRelation [value#66]
+- Project [(value#66 % 5) AS key#9] // this does not generate value,
incorrect join results
+- LocalRelation [value#66]
```
**Solution**: Instead of rewriting attributes, use a Project to introduce
aliases between the output attribute references and the new reference generated
by the spliced plans. The analyzer and optimizer will take care of the rest.
```
Project [key#6, value#1, value#12]
+- Join Inner, (key#6 = key#9)
:- Project [(value#1 % 5) AS key#6, value#1]
: +- Project [value#66 AS value#1] // solution: project with aliases
: +- LocalRelation [value#66]
+- Project [(value#12 % 5) AS key#9, value#12]
+- Project [value#66 AS value#12]// solution: project with aliases
+- LocalRelation [value#66]
```
## How was this patch tested?
New unit test
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/tdas/spark SPARK-23406-2.3
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/spark/pull/20765.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #20765
commit c3ec9ef9355a3290d764dda0191165eaa4e49062
Author: Tathagata Das
Date: 2018-02-14T22:27:02Z
[SPARK-23406][SS] Enable stream-stream self-joins
## What changes were proposed in this pull request?
Solved two bugs to enable stream-stream self joins.
### Incorrect analysis due to missing MultiInstanceRelation trait
Streaming leaf nodes did not extend MultiInstanceRelation, which is
necessary for the catalyst analyzer to convert the self-join logical plan DAG
into a
