willholley opened a new pull request, #4810:
URL: https://github.com/apache/couchdb/pull/4810

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   ## Overview
   
   Adds a `$beginsWith` operator to selectors, with json and text index 
support. This is a compliment / precursor to optimising `$regex` support as 
proposed in https://github.com/apache/couchdb/pull/4776.
   
   For `json` indexes, a $beginsWith operator translates into a key range 
query, as is common practice for _view queries. For example, to find all rows 
with a key beginning with "W", we can use a range `start_key="W", 
end_key="W\ufff0"`. Given Mango uses compound keys, this is slightly more 
complex in practice, but the idea is the same. As with other range operators 
(`$gt`, `$gte`, etc), `$beginsWith` can be used in combination with equality 
operators and result sorting but must result in a contiguous key range. That 
is, a range of `start_key=[10, "W"], end_key=[10, "W\ufff0", {}]` would be 
valid, but `start_key=["W", 10], end_key=["W\ufff0", 10, {}]` would not, 
because the second element of the key may result in a non-contiguous range.
   
   For text indexes, `$beginsWith` translates to a Lucene query on the 
specified field of `W*`.
   
   If a non-string operand is provided to `$beginsWith`, the request will fail 
with a 400 / `invalid_operator` error.
   
   ## Testing recommendations
   
   I've only tested this with json indexes so far because the dev container 
doesn't have `text` index support.
   
   A basic eunit test is added for the new `selector` support. You can run 
these using `make eunit apps=mango`.
   
   Integration tests are added to the Mango test suite, which can be run via 
`make mango-test`.
   
   ## Related Issues or Pull Requests
   
   n/a
   
   ## Checklist
   
   - [ ] Code is written and works correctly
   - [ ] Changes are covered by tests
   - [ ] Any new configurable parameters are documented in 
`rel/overlay/etc/default.ini`
   - [ ] Documentation changes were made in the `src/docs` folder
   - [ ] Documentation changes were backported (separated PR) to affected 
branches
   


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