(filed based on discussions with Igor)
Cost estimation code makes use of range analyzer to match (parts of) the WHERE
condition against column's Histogram.

Some other module (todo: which? MDEV#?) needs to know whether the estimate is
precise or not.

The main source of imprecision is that some of conditions are not [fully]
sargable. That is, range analyzer produces ranges that are wider than the
supplied conditions.

Examples of conditions with imprecise estimates:

  t.col < 10 AND t.col MOD 3 = 2 

  t.col LIKE 'foo%bar%'

Examples of conditions with precise estimates:

  t.col LIKE 'foo%'

  t.col < 3214

  t.col = 'bar'

  t.col IN (1,2,3) OR t.col BETWEEN 10 AND 20

(TODO: we don't need to handle the case of multi-column range analysis, do we?)