Julian Hyde created CALCITE-3131:
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Summary: In LatticeSuggester, record whether columns are used as
"dimensions" or "measures"
Key: CALCITE-3131
URL: https://issues.apache.org/jira/browse/CALCITE-3131
Project: Calcite
Issue Type: Bug
Reporter: Julian Hyde
Fix For: 1.21.0
In {{LatticeSuggester}}, record whether columns are used as "dimensions" or
"measures" in the query set. And add a method
{{Lattice.isAlwaysMeasure(Column)}}, populated from these use counts.
To understand the difference between dimensions and measures, consider the SQL
queries
{code}
select deptno,
sum(salary + commission) as remuneration,
sum(salary) as salary
from Emp
group by first_name || ' ' || last_name, deptno;
select min(first_name || ' ' || last_name)
from Emp
where deptno = 10;
{code}
There are two expressions:
* {{salary + commission}} is only used as an argument two an aggregate
function, i.e. a measure.
* {{first_name || ' ' || last_name}} is used as a dimension in the first query
(in {{GROUP BY}}, but in {{WHERE}} or {{SELECT}} or {{ORDER BY}} would also
count), and as a measure in the second.
When applied to a collection of queries, {{LatticeSuggester}} should record
each individual use of a column or derived column (expression), and whether it
is a usage as a measure. Thus {{salary + commission}} has [true], and
{{first_name || ' ' || last_name}} has [false, true].
In future we might store further information about uses. For example, if a
column is used with aliases "expr$1", "profit", "profit" then we could go with
the majority decision and call it "profit". Currently we go with the alias the
first time it is used.
When the {{LatticeSuggester}} builds a {{Lattice}} it passes on the use counts,
and these power a new method {{Lattice.isAlwaysMeasure(Column)}}. A {{Lattice}}
that is made directly, not induced from a set of queries, will have an empty
use map. (It is a minor burden for these lattices, but it is outweighed by the
convenience when an existing lattice needs to be merged with a new set of
queries in an incremental {{LatticeSuggester}} run.)
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