BUG #16149: Prepared COPY queries always report 0 parameters when described
The following bug has been logged on the website:
Bug reference: 16149
Logged by: Steven Fackler
Email address: sfackler@gmail.com
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. Here's a simple example program that demonstrates the issue:
```
#include <libpq-fe.h>
#include <assert.h>
#include <stdio.h>
int main() {
PGconn *conn = PQconnectdb("host=localhost port=5433 user=postgres");
assert(PQstatus(conn) == CONNECTION_OK);
PGresult *result = PQprepare(conn, "a", "COPY (SELECT $1::TEXT) TO STDOUT",
0, NULL);
assert(PQresultStatus(result) == PGRES_COMMAND_OK);
result = PQdescribePrepared(conn, "a");
assert(PQresultStatus(result) == PGRES_COMMAND_OK);
printf("nparams: %d\n", PQnparams(result));
result = PQexecPrepared(conn, "a", 0, NULL, NULL, NULL, 0);
assert(PQresultStatus(result) == PGRES_FATAL_ERROR);
printf("error: %s\n", PQresultErrorField(result,
PG_DIAG_MESSAGE_PRIMARY));
```
When run, it prints the following:
```
nparams: 0
error: there is no parameter $1
```
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
```
From some quick Googling, I did see this StackOverflow post[1]https://stackoverflow.com/a/22963085 stating that
COPY queries don't support parameters, but if that's the case it seems like
an error should be reported at the preparation stage. I also don't see
anything about that in the documentation for COPY[2]https://www.postgresql.org/docs/12/sql-copy.html, though I may have
missed it! I see the same behavior on Postgres 11.1 as well, if that's
relevant.
[1]: https://stackoverflow.com/a/22963085
[2]: https://www.postgresql.org/docs/12/sql-copy.html
On Wed, Dec 4, 2019 at 5:22 PM PG Bug reporting form <noreply@postgresql.org>
wrote:
The following bug has been logged on the website:
Bug reference: 16149
Logged by: Steven Fackler
Email address: sfackler@gmail.com
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.