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Note that the third column is a bitvector of columns being selected.
For the first row, none of the columns are being selected.
For the second row, only the first column is being selected, which explains the value 1.
For the third row, both the columns are being selected (and the second column happens to be null), which explains the value 0.
For the third row, only the first column is being selected, which explains the value 1.

Grouping function

The grouping function indicates whether an expression in a GROUP BY clause is aggregated or not for a given row. The value 0 represents a column that is part of the grouping set, while the value 1 represents a column that is not part of the grouping set. 

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