fix(batch): schedule single task for singleton table scan

[BugenZhao]

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What's changed and what's your intention?

For singleton table, we make the distribution of the table scan Single instead of UpstreamHashShard(vec![]).

Per discussion with @liurenjie1024, as the root stage is always executed on the frontend, we insert an additional Single if necessary to ensure the table scan runs on the compute node just like what we do for DMLs.

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)

Refer to a related PR or issue link (optional)

• Close batch: singleton seq scan on local mode #4164

[codecov]

Codecov Report

Merging #5907 (dc10e0b) into main (938f427) will decrease coverage by 0.02%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##             main    #5907      +/-   ##
==========================================
- Coverage   74.89%   74.86%   -0.03%     
==========================================
  Files         924      924              
  Lines      147014   146913     -101     
==========================================
- Hits       110104   109992     -112     
- Misses      36910    36921      +11     

Flag	Coverage Δ
rust	74.86% <81.81%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/frontend/src/handler/query.rs	20.49% <0.00%> (ø)
...ntend/src/optimizer/plan_node/stream_table_scan.rs	96.93% <ø> (ø)
src/frontend/src/scheduler/error.rs	8.33% <ø> (ø)
src/frontend/src/scheduler/local.rs	0.00% <0.00%> (ø)
src/frontend/src/scheduler/distributed/stage.rs	21.55% <33.33%> (ø)
src/frontend/src/scheduler/plan_fragmenter.rs	76.57% <74.24%> (-8.04%)	⬇️
src/frontend/src/optimizer/mod.rs	96.70% <100.00%> (+0.61%)	⬆️
...frontend/src/optimizer/plan_node/batch_seq_scan.rs	93.86% <100.00%> (-0.36%)	⬇️
src/frontend/src/optimizer/plan_visitor.rs	90.69% <100.00%> (+3.60%)	⬆️
src/frontend/src/scheduler/distributed/query.rs	74.81% <100.00%> (+0.91%)	⬆️
... and 4 more

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[xxchan]

LGTM

[liurenjie1024]

LGTM

[mergify]

Hey @BugenZhao, this pull request failed to merge and has been dequeued from the merge train. If you believe your PR failed in the merge train because of a flaky test, requeue it by clicking "Update branch" or pushing an empty commit with git commit --allow-empty -m "rerun" && git push.

[BowenXiao1999]

Take this for example, root fragment task, no matter before-and-after pr, is always executed on FE, while one is exchange singleton and another is hash agg.

What's the benefit of changing? 🤔 Unify with DML?

[liurenjie1024]

It's for this bug: #4164