Generalize aggregation on primitive (non-link) grouping columns

[asavinov]

Problem: the aggregation operation works only with link columns for grouping. It is a column operation which adds a new aggregate column to the group table. The group table must exist, and a link from a fact table (with data to be aggregated) must also exist. However, it cannot be applied for a tradition use case of groupby where we take a fact table by specifying one of its column as a grouping criterion. The problem is that the grouping table does not exist and hence we cannot define a new column for it.

In this task, we want to make the aggregate operation work in the case the table where a new aggregate column has to be added does not exist:

• Since the table for a new aggregate column does not exist, it has to be created, and hence it becomes a table-column operation which produces three new elements: one table, one link column and one aggregate column
• The grouping criterion can be an attribute of the fact table with the source data to be aggregated (not a link)
• We distinguish two cases:
  • Our currently implemented use case where the grouping criterion is an already existing link column
  • To be implemented use case where the grouping criterion is a list of attributes (or columns?) without an existing target table
• We actually need to combine two definition parameters:
  • define a projection (source attributes, link name, group table name)
  • define an aggregation (measure columns, link name, group table name, new aggregate column name)

It seems that it is difficult and not natural to combine two operations: projection and aggregation. Therefore, the groupby use case probably should be indeed implemented as two operations: project and aggregate. In this case, this task has not to be implemented.