Re: [HACKERS] PostgreSQL 10 parenthesized single-column updates canproduce errors

According to the spec, the elements of a parenthesized
SET list should be assigned from the fields of a composite RHS.  If
there's just one element of the SET list, the RHS should be a single-field
composite value, and this syntax should result in extracting its field.
This patch doesn't do that; it preserves our previous broken behavior,
and thus just puts off the day of reckoning that inevitably will come.