Re: MERGE and parsing with prepared statements

On Fri, Jul 15, 2022 at 12:17:51PM -0700, David G. Johnston wrote:
> On Fri, Jul 15, 2022 at 11:40 AM Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
> > On 2022-Jul-15, Justin Pryzby wrote:
> >
> > > It seems a bit odd that it's impossible to use merge with prepared statements
> > > without specifically casting the source types (which I did now to continue my
> > > experiment).
> >
> > I have no comments on this.  Maybe it can be improved, but I don't know
> > how.
>
> Not tested, but the example prepare command fails to make use of the
> optional data types specification.  Using that should remove the need to
> cast the parameter placeholder within the query itself.

What optional data type specification ?

> That said, in theory the INSERT specification of the MERGE could be used to
> either resolve unknowns or even forcibly convert the data types of the
> relation produced by the USING clause to match the actual types required
> for the INSERT (since that will happen at insert time anyway).

Yeah.  I hadn't looked before, but just noticed this:

https://www.postgresql.org/docs/devel/sql-merge.html
| If the expression for any column is not of the correct data type, automatic type conversion will be attempted.

That appears to be copied from the INSERT page.
What does that mean, if not that data types will be resolved as needed ?

Note that if I add casts to the "ON" condition, MERGE complains about the
INSERT VALUES.

PREPARE p AS
MERGE INTO CustomerAccount CA
USING (SELECT $1 AS CustomerId, $2 AS TransactionValue) AS T
ON CA.CustomerId = T.CustomerId::int
WHEN NOT MATCHED THEN
  INSERT (CustomerId, Balance)
  VALUES (T.CustomerId, T.TransactionValue)
WHEN MATCHED THEN
  UPDATE SET Balance = Balance + TransactionValue;

ERROR:  column "customerid" is of type integer but expression is of type text
LINE 7:   VALUES (T.CustomerId, T.TransactionValue)

        postgres: pryzbyj postgres [local] PREPARE(transformAssignedExpr+0x3b6) [0x5605f9699e8e]
        postgres: pryzbyj postgres [local] PREPARE(transformInsertRow+0x2ce) [0x5605f9653a47]
        postgres: pryzbyj postgres [local] PREPARE(transformMergeStmt+0x7ec) [0x5605f968fe3b]
        postgres: pryzbyj postgres [local] PREPARE(transformStmt+0x70) [0x5605f9656071]
        postgres: pryzbyj postgres [local] PREPARE(+0x1fa350) [0x5605f9657350]
        postgres: pryzbyj postgres [local] PREPARE(transformTopLevelStmt+0x11) [0x5605f9657385]
        postgres: pryzbyj postgres [local] PREPARE(parse_analyze_varparams+0x5b) [0x5605f96574f4]
        postgres: pryzbyj postgres [local] PREPARE(pg_analyze_and_rewrite_varparams+0x38) [0x5605f991edfe]
        postgres: pryzbyj postgres [local] PREPARE(PrepareQuery+0xcc) [0x5605f96f4155]
        postgres: pryzbyj postgres [local] PREPARE(standard_ProcessUtility+0x4ea) [0x5605f99251a0]
        postgres: pryzbyj postgres [local] PREPARE(ProcessUtility+0xdb) [0x5605f992587e]

-- 
Justin