[GitHub] [incubator-druid] clintropolis opened a new pull request #7973: expression language array constructor and sql multi-value string filtering support

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clintropolis opened a new pull request #7973: expression language array constructor and sql multi-value string filtering support
URL: https://github.com/apache/incubator-druid/pull/7973
 
 
   This PR adds a new function `array(expr1, ... ,exprN)` to allow constructing expression arrays, as well as a corresponding druid-sql `ARRAY` function which does the same. These functions were both added in the service of adding sql functions that allow filtering multi-value dimensions as expressively as native Druid queries, `MV_CONTAINS` and `MV_OVERLAP`, where the `MV_` stands for 'multi-value'. 
   
   ```SQL
   SELECT dim3 FROM druid.numfoo WHERE MV_CONTAINS(dim3, ARRAY['a','b']) LIMIT 5
   ```
   
   I've actually defined these as aliases for `ARRAY_CONTAINS` and `ARRAY_OVERLAP`, since both sets of these functions are driven internally by `array_contains` and `array_overlap` respectively, so they can share operator conversion implementations. The reasoning for the dupe naming is: most of the Druid expression array functions pose no problem with accepting either `VARCHAR` or `ARRAY` typed input, but the functions which _produce_ an array expression will need different implementations for differently typed outputs (`VARCHAR` for `MV_`, and `ARRAY` for `ARRAY_`), so this is done to encourage using the `MV_` prefix for multi-value string dimensions, and reserving `ARRAY_` for if/when we implement true array types natively in Druid. I have not documented any of the sql support for multi-value string dimensions yet, so there is still some wiggle room here as I continue to fill out the other functions.
   
   In the process of these changes, I've modified the output of `array_contains` and `array_overlap` expressions to return 1 or 0, instead of using `ExprEval.bestEffortOf` with a boolean input, which was ending up as `"true"` or `"false"`, which for `ExprEval` is never true. This allows these functions to actually work in filters.

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