ESQL: No rows with JOIN/ENRICH + EVAL + KEEP

[ivancea]

Weird interaction resulting in no rows:

Correct:

FROM languages
| DROP language_code
| EVAL language_code = 3
| LOOKUP JOIN languages_lookup ON language_code

# Results:
 language_code | language_name 
---------------+---------------
3              |Spanish
3              |Spanish
3              |Spanish
3              |Spanish

Wrong results:

FROM languages
| DROP language_code
| EVAL language_code = 3
| LOOKUP JOIN languages_lookup ON language_code
| KEEP language_code

# Results:
 language_code 
---------------

You can remove the first DROP if the original index doesn't have the JOIN key column

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[ivancea]

After some investigation, it's happening because of fieldNames() returning fields from the lookup to be searched through field-caps, which return no concrete indices for them.

The same is also happening with ENRICH, also in 8.17.

Also, chaining any of those commands may render a similar result: They will add "their own fields" to the field_caps