> What's the expected result if the tuple from subselect is more than 1?
Error, per SQL99 section 7.14:
1) If the cardinality of a <row subquery> is greater than 1
then an exception condition is raised: cardinality violation.
I expect no update at all in case of void result set, is this the case ?
No, you get nulls; it's a subquery not a join. Per SQL99 7.1:
c) If the <row value constructor> is a <row subquery>, then:
i) Let R be the result of the <row subquery> and let D be
degree of R.
ii) If the cardinality of R is 0 (zero), then the result of
<row value constructor> is D null values.
iii) If the cardinality of R is 1 (one), then the result of
<row value constructor> is R.
regards, tom lane
To allow both of the above to hold, I think the subselect will have to be
treated like a EXPR_SUBLINK subquery. I was wondering if we have a similar
mechanism for plain selects/subselects to check and restrict their output to
a single row.