feat: query to fetch unique columns

[aaqilniz]

Checklist

• [x] Sign off your commits with DCO (Developer Certificate of Origin)
• [x] npm test passes on your machine
• [ ] New tests added or existing tests modified to cover all changes
• [x] Code conforms with the style guide
• [x] Commit messages are following our guidelines

[coveralls]

Pull Request Test Coverage Report for Build 8679499746

Details

• 15 of 19 (78.95%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.05%) to 84.715%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/datasource.js	15	19	78.95%
<!--	Total:	15	19	78.95%	-->

Files with Coverage Reduction	New Missed Lines	%
lib/datasource.js	4	72.21%
<!--	Total:	4		-->

Totals
Change from base Build 8670044630:	-0.05%
Covered Lines:	7281
Relevant Lines:	8287

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💛 - Coveralls