Update explain plan to show when topk operator is used

[alamb]

Is your feature request related to a problem or challenge?

After #7721 a SortExec with a limit will use a special TopK operator

However, I don't think it is clear from the EXPLAIN plan that this will be used (you have to know that sort with a limit is specially optimized to not actually sort).

Using DataFusion CLI and the dataset described here: #7721 (review), the explain plan looks like

❯ explain select trace_id from 'traces.parquet' order by time desc limit 10;
+---------------+-------------------------------------------------------------------------------------------------------------------+
| plan_type     | plan                                                                                                              |
+---------------+-------------------------------------------------------------------------------------------------------------------+
| logical_plan  | Projection: traces.parquet.trace_id                                                                               |
|               |   Limit: skip=0, fetch=10                                                                                         |
|               |     Sort: traces.parquet.time DESC NULLS FIRST, fetch=10                                                          |
|               |       Projection: traces.parquet.trace_id, traces.parquet.time                                                    |
|               |         TableScan: traces.parquet projection=[time, trace_id]                                                     |
| physical_plan | ProjectionExec: expr=[trace_id@0 as trace_id]                                                                     |
|               |   GlobalLimitExec: skip=0, fetch=10                                                                               |
|               |     SortExec: fetch=10, expr=[time@1 DESC]         <----- ****  this uses the TopK operator |                                                               
|               |       ProjectionExec: expr=[trace_id@1 as trace_id, time@0 as time]                                               |
|               |         ParquetExec: file_groups={1 group: [[traces.parquet]]}, projection=[time, trace_id] |
|               |                                                                                                                   |
+---------------+-------------------------------------------------------------------------------------------------------------------+
2 rows in set. Query took 0.004 seconds.

Describe the solution you'd like

I would like to make it clearer in the explain plan that this new operator is used. Perhaps something like

SortExec: TopK(fetch=10), expr=[time@1 DESC]         <----- **** This is updated |                                                               

See comment in https://github.com/apache/arrow-datafusion/pull/7721/files#r1343969678 for exactly where this code that controls the output is

Describe alternatives you've considered

We can leave the explain plans alone, but I think that is confusing

Additional context

No response

[alamb]

I think this is an excellent task for new users as the code is quite straightforward and most of this work will be to learn how to run the various tests and update them for the new explain plans.

[fansehep]

Let me try it. 😄

[alamb]

Note there is already a partial version in #7751

With a good comment here: #7751 (comment)

Maybe you can base your efforts on that PR

[MayurShirodkarOfficial]

hello !! can i try working on this issue?

[alamb]

hello !! can i try working on this issue?

Absolutely -- I think we are just waiting on someone to make a PR that passes CI :)