Re: BUG #16149: Prepared COPY queries always report 0 parameters when described

On Wed, Dec 4, 2019 at 5:22 PM PG Bug reporting form <[email protected]>
wrote:

> The following bug has been logged on the website:
>
> Bug reference:      16149
> Logged by:          Steven Fackler
> Email address:      [email protected]
> PostgreSQL version: 12.1
> Operating system:   Debian Buster
> Description:
>
> When a Postgres backend describes a prepared `COPY ... TO STDOUT query`, it
> always reports 0 query parameters regardless of how many are actually
> present


Working as designed though I agree with the comment below that somehow it
has escaped documentation that this command is a utility command and
"query" cannot contain parameters.

        PGresult *result = PQprepare(conn, "a", "COPY (SELECT $1::TEXT) TO
> STDOUT",
> 0, NULL);
>
 [...]

>         result = PQexecPrepared(conn, "a", 0, NULL, NULL, NULL, 0);
> When run, it prints the following:
>
> ```
> nparams: 0
> error: there is no parameter $1
> ```
>

Yep - though this is the odd error message - but it almost reads like
because you are using execPrepared the code is expecting a planned and
parameterized statement and is complaining, oddly, that it wasn't provided
one.  Its OK to give it a statement that can accept parameters but has zero
but NOT OK to give it one that doesn't accept parameters at all.

>
> If you change the query to just the inner `SELECT $1::TEXT`, the number of
> parameters is correctly reported, but interestingly the error message
> changes:
>
> ```
> nparams: 1
> error: bind message supplies 0 parameters, but prepared statement "a"
> requires 1
> ```
>

Not that interesting, the PQexecPrepared call supplies zero parameter
values but the SELECT query is indeed expecting one.


> From some quick Googling, I did see this StackOverflow post[1] stating that
> COPY queries don't support parameters,


Yes

> but if that's the case it seems like
> an error should be reported at the preparation stage.


Seems reasonable...


> I also don't see
> anything about that in the documentation for COPY[2], though I may have
> missed it!


I do not either.

I see the same behavior on Postgres 11.1 as well, if that's
> relevant.
>

It supports the conclusion that the behavior is likely intentional.

David J.