Views should not use constant values or expressions in aggregates

Problem/Motivation

Follow up from #2159347: Aggregation not working simple test case →

Views is generating under some circumstances queries that have constant expressions inside the aggregation clause.

This, besides making no sense as constant values have no effect on aggregates, makes these queries incompatible with SQL Server and Oracle (probably Postgre too, but have not tested).

This is a query with a literal in aggregate:

SELECT uid FROM users GROUP BY 'literal', uid

This is without it:

SELECT uid FROM uers GROUP BY uid

Steps to reproduce

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Proposed resolution

Skip constant values and expressions when generating the aggregation part of the query.

Remaining tasks

Review
RTBC
Commit

User interface changes

None.

API changes

None.