Why is the ROW keyword needed when constructing a row with one element, but not two? In PostgreSQL 11, the following expression
UPDATE test_table SET (&quot;column_a&quot;,&quot;column_b&quot;) = ('value-a','value-b') WHERE &quot;column_a&quot; = 'value-c' 

is accepted and performs the update.
But for a single column / value, like
UPDATE test_table SET (&quot;column_a&quot;) = ('value-a') WHERE &quot;column_a&quot; = 'value-c'

it gives an error. ROW must be supplied, like
UPDATE test_table SET (&quot;column_a&quot;) = ROW ('value-a') WHERE &quot;column_a&quot; = 'value-c'

Is there an explanation why this behavior was chosen? Is there an ambiguity if ROW is missing?
As is often the case, the answer is to be found in documentation.
The UPDATE reference says, in particular:

According to the standard, the source value for a parenthesized sub-list of target column names can be any row-valued expression yielding the correct number of columns. PostgreSQL only allows the source value to be a row constructor or a sub-SELECT.

whereas &quot;row constructor&quot; is defined like so:

A row constructor is an expression that builds a row value (also called a composite value) using values for its member fields. A row constructor consists of the key word ROW, a left parenthesis, zero or more expressions (separated by commas) for the row field values, and finally a right parenthesis. [...] The key word ROW is optional when there is more than one expression in the list.

The last sentence kind of makes sense, because a single expression in parentheses is ambiguous by itself and can be confused with a simple parenthesised expression.
This syntax was introduced in version 10 -- HT to ypercube™.