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https://issues.apache.org/jira/browse/CALCITE-3131?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Julian Hyde reassigned CALCITE-3131:
------------------------------------
Assignee: Julian Hyde
> 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
> Assignee: Julian Hyde
> Priority: Major
> 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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