Affects Version/s: None
Fix Version/s: 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
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.)