Re: MERGE and parsing with prepared statements

Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> writes:
> On 2022-Jul-15, Justin Pryzby wrote:
>> I see now that the same thing can happen with "ON CONFLICT" if used with a
>> subselect.
>>
>> PREPARE p AS INSERT INTO t SELECT a FROM (SELECT $1 AS a)a
>> ON CONFLICT (i) DO UPDATE SET i=excluded.i;
>> ERROR: column "i" is of type integer but expression is of type text

> Right, I didn't think that MERGE was doing anything peculiar in this
> respect.

Yeah. The current theory about this is that if we haven't assigned a
type to an unknown-type parameter (or literal) that is an output
column of a sub-SELECT, we will as a rule force it to text.
That MERGE USING clause is a sub-SELECT, so that rule applies.

There is a hoary old exception to that rule, which is that if you
write INSERT INTO tab SELECT ..., $1, ...
we will figure out the type of the column of "tab" that $1 is going
into, and force $1 to that type instead of text. It looks like this
also works in INSERT ... VALUES. You could make a case that MERGE
should be equally smart, but it's not clear to me that the info is
available sufficiently close by to make it reasonable to do that.
It looks like the MERGE syntax has a couple of levels of indirection,
which'd probably be enough to put the kibosh on that idea -- in
particular, AFAICS there might not be a unique target column
corresponding to a given data_source column.

regards, tom lane