[jira] [Created] (CALCITE-3131) In LatticeSuggester, record whether columns are used as "dimensions" or "measures"

["Julian Hyde (JIRA)" <ji...@apache.org>]

Julian Hyde created CALCITE-3131:
------------------------------------

             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.)



--
This message was sent by Atlassian JIRA
(v7.6.3#76005)