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suggestions to code review #4

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  • Have you signed the contributor license agreement?
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@pgomulka pgomulka mentioned this pull request Jun 16, 2023
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rjernst pushed a commit that referenced this pull request Aug 17, 2023
@luigidellaquila
Fix Project after TopN (ESQL-541)
f5d2ca2 
Fixes elastic/elasticsearch-internal#497
Fixes ESQL-560

A query like `from test | sort data | limit 2 | project count` fails
because `LocalToGlobalLimitAndTopNExec` planning rule adds a collecting
`TopNExec` after last GATHER exchange, to perform last reduce, see

```
TopNExec[[Order[data{f}elastic#6,ASC,LAST]],2[INTEGER]]
\_ExchangeExec[GATHER,SINGLE_DISTRIBUTION]
  \_ProjectExec[[count{f}#4]]      // <- `data` is projected away but still used by the TopN node above
    \_FieldExtractExec[count{f}#4]
      \_TopNExec[[Order[data{f}elastic#6,ASC,LAST]],2[INTEGER]]
        \_FieldExtractExec[data{f}elastic#6]
          \_ExchangeExec[REPARTITION,FIXED_ARBITRARY_DISTRIBUTION]
            \_EsQueryExec[test], query[][_doc_id{f}elastic#9, _segment_id{f}elastic#10, _shard_id{f}elastic#11]
```

Unfortunately, at that stage the inputs needed by the TopNExec could
have been projected away by a ProjectExec, so they could be no longer
available.

This PR adapts the plan as follows:
- add all the projections used by the `TopNExec` to the existing
`ProjectExec`, so that they are available when needed
- add another ProjectExec on top of the plan, to project away the
originally removed projections and preserve the query semantics


This approach is a bit dangerous, because it bypasses the mechanism of
input/output resolution and validation that happens on the logical plan.
The alternative would be to do this manipulation on the logical plan,
but it's probably hard to do, because there is no concept of Exchange at
that level.
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