fix: short circuit SQL parsing

[timsaucer]

This PR avoids a problem encountered when creating a scalar index on a column that had a name that was not easily parseable by the SQL parser. If we are getting a raw expression that matches a column name in the schema, then return it directly.

More generally I wonder if we should be doing this sql parsing at all when we know we have columns. Should ProjectionPlan::from_expressions actually take in instead columns: &[(impl AsRef<str>, &Expr)] ?

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.83%. Comparing base (4780af9) to head (4c4a613).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4825      +/-   ##
==========================================
- Coverage   80.83%   80.83%   -0.01%     
==========================================
  Files         329      329              
  Lines      129559   129562       +3     
  Branches   129559   129562       +3     
==========================================
- Hits       104730   104728       -2     
- Misses      21148    21150       +2     
- Partials     3681     3684       +3     

Flag	Coverage Δ
unittests	80.83% <100.00%> (-0.01%)	⬇️

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[wjones127]

Seems reasonable. Thanks!

[wjones127]

More generally I wonder if we should be doing this sql parsing at all when we know we have columns. Should ProjectionPlan::from_expressions actually take in instead columns: &[(impl AsRef<str>, &Expr)] ?

Could you expand more on what you mean by that?

I think the intention of the expression was to allow indexing nested fields or fields within JSON blobs.

[wjones127]

This PR avoids a problem encountered when creating a scalar index on a column that had a name that was not easily parseable by the SQL parser.

Actually, before I merge, do you think this is worth a unit test?