Correctly delegate BitColumnExpression::eval(*double out, bool& missing)

[TomHodson]

The Problem

When an odc sql statement includes both aggregate functions and bitfield columns, the bitfield columns are not printed (or aggregated) correctly.

>>> odc sql -i odc/docs/_static/data-1.odb -q "select avg(bitfield_column.flag_a),bitfield_column.flag_a,bitfield_column.flag_b"
avg(bitfield_column.flag_a)     bitfield_column.flag_a  bitfield_column.flag_b
                   0.000000                          0                       0
                   1.000000                        111                     111
                   0.000000                        222                     222
                   1.000000                        333                     333
                   0.000000                        444                     444
                   1.000000                        555                     555
                   0.000000                        666                     666
                   1.000000                        777                     777
                   0.000000                        888                     888
                   1.000000                        999                     999

The Fix

In the original code, on the code path where we mix aggregated and non-aggregated columns, SQLExpressionEvaluated callse.eval(&value_[0], missing_); whihc, when e is a bitfield column would actually call ColumnExpression::eval(*double out, bool& missing) which does not delegate to BitColumnExpression::eval(bool& missing)

This change overrides BitColumnExpression::eval(*double out, bool& missing) to use SQLExpression:eval(*double out, bool& missing) which correctly delegates to BitColumnExpression::eval(bool& missing).

After this fix:

avg(bitfield_column.flag_a)     bitfield_column.flag_a  bitfield_column.flag_b
                   0.000000                          0                       0
                   0.000000                          0                       1
                   0.000000                          0                       2
                   0.000000                          0                       3
                   1.000000                          1                       0
                   1.000000                          1                       1
                   1.000000                          1                       2
                   1.000000                          1                       3

[FussyDuck]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 63.76%. Comparing base (e29a620) to head (0e9a5ff).

Files with missing lines	Patch %	Lines
src/eckit/sql/expression/BitColumnExpression.h	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #144   +/-   ##
========================================
  Coverage    63.75%   63.76%           
========================================
  Files         1065     1066    +1     
  Lines        55342    55345    +3     
  Branches      4085     4085           
========================================
+ Hits         35286    35288    +2     
- Misses       20056    20057    +1     

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[simondsmart]

Thanks Tom. That looks good.
Please check what the downstream failures are. Otherwise this looks mergeable.