resolves #838

Problem

The pruning logic introduced in #776 disrupted the grouping behavior in the at_least_one test.

Specifically, the initial pruned_rows CTE includes a limit 1 statement, which selects a single record where the target column is not null.

As a result, the test passes inappropriately when the group_by_columns parameter is configured, as long as the target column has at least one not-null value.

Essentially, the group_by_columns parameter currently has no effect on the test. It evaluates target column values as a single group, regardless of whether or not the parameter is present.

Solution

Restore the at_least_one test to its prior state.

Since the goal of #776 was to improve the performance of the test, I also considered a set-logic approach to evaluating a column at the group level:

• identify all group-by column values
• identify group-by column values with at least one not-null target column value
• return group-by column values without at least one not-null target column value as errors

In BigQuery, however, this version of the test took twice as long as the simple reverted version.

Checklist

• This code is associated with an issue which has been triaged and accepted for development.
• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the README.md (if applicable)