MPP dispatch tasks error with TopN
Open
Nobody has claimed this yet.
affects-6.0
affects-6.1
component/compute
component/mpp
severity/minor
type/bug
- Dominant language
- C++
- Stars
- 1k
- Forks
- 423
- Avg merge
- 1d 15h
- Merged PRs (30d)
- 24
Description
Minimal reproduce:
create table t (a int); -- create any table
alter table t set tiflash replica 1;
set tidb_allow_mpp=2; -- enforce MPP
select cot(0) as field from t where a = 1 order by field limit 1;
ERROR 1105 (HY000): DB::TiFlashException: TopN executor without order by exprs
The perquisite of this issue:
- Projection has a constant expression that cannot be folded(may throw error during execution, e.g.
cot(0)) - Query will be run in MPP mode
- Query has a
TopNexecutor which will order by the expression mentioned in1
Contributor guide
No contributing guide indexed for this repository
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the SQL example with MPP enabled and inspect the TopN executor and MPP dispatch path implicated by the error. Trace why the constant projection expression leaves TopN without order-by expressions; done means the query completes without the reported TiFlashException.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- cpp, sql
- Domain
- databases
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100