Re: V8.3.0: PQprepare()/PQexecPrepared() and DECLARE CURSOR - Mailing list pgsql-hackers

Sebastien FLAESCH wrote:
> Hi All,
>
> I am new to this mailing list and want to participate to the 8.3.0 
> beta program.
> (Sorry to be late BTW)
>
> My name is Sebastien FLAESCH and I am in charge of the database 
> interfaces at Four J's Development Tools.
>
> Our product is a Informix 4gl compatible compiler / runtime system.
>
> I wrote all the database interfaces to:
>
>   - Oracle (OCI),
>   - DB2 UDB (CLI),
>   - SQL Server (ODBC and Native Client),
>   - PostgreSQL (libpq),
>   - MySQL (libmysqlclient),
>   - Sybase ASA (dblib*),
>   - ANTs (ODBC).
>
> Understand it's about a real database driver for our virtual machine 
> (kind of php db or jdbc driver).
>
> We have a large customer base using Informix and some of them have 
> migrated / want to migrate to PostgreSQL.
>
> We support a libpq-based driver for PostgreSQL since version 7, we 
> support currently 8.1, 8.2 and now I am working in the 8.3 driver.
>
> I do use prepared statements with the PQprepare() / PQexecPrepared() 
> API since first version 8 - thanks for that by the way.
>
> Now I want to take benefit of server cursors, using the 
> DECLARE/FETCH/CLOSE instructions.
>
> 8.3 also introduced positioned update/deletes (WHERE CURRENT OF), so 
> we do not more need to emulate this with oids.
>
> ...
>
> The problem: It appears that the server gets confused when doing 
> PQprepare("DECLARE...) followed by several PQexecPrepared().
>
> Basically I do libpq API calls like this:
>
> For SQL that does not return a result set:
>
>   PQprepare(... "cu1", "INSERT INTO ..." );
>   PQexecPrepared( ... "cu1" ... );
>   PQexecPrepared( ... "cu1" ... );
>   PQexecPrepared( ... "cu1" ... );
>   PQexec( "DEALLOCATE cu1" );
>
> For SQL producing a result set:
>
>   PQprepare(... "cu1", "DECLARE cu1 CURSOR WITH HOLD ..." );
>   PQexecPrepared( ... "cu1" ... );  -- opens the cursor...
>   PQexec( "FETCH NEXT FROM cu1 ..." ... );
>   PQexec( "FETCH NEXT FROM cu1 ..." ... );
>   PQexec( "FETCH NEXT FROM cu1 ..." ... );
>   PQexec( "CLOSE cu1" ); -- frees cursor resources (need to re-execute 
> DECLARE)
>   PQexecPrepared( ... "cu1" ... );  -- opens the cursor...
>   PQexec( "FETCH NEXT FROM cu1 ..." ... );
>   PQexec( "FETCH NEXT FROM cu1 ..." ... );
>   PQexec( "CLOSE cu1" ); -- frees cursor resources (need to re-execute 
> DECLARE)
>   PQexecPrepared( ... "cu1" ... );  -- Here I get error: 
> [42P03][cursor "cu1" already exists]
>
> I wonder why the second PQexecPrepare() executes and the third fails...
>
> To make this work, I need to de-allocate the statement and re-prepare 
> with PQprepare() ...
>
> I will try to provide you with a little sample to reproduce, but 
> wanted to post this early to let you known.
>
>
>

This example would be clearer if you used different names for the cursor 
and the prepared statement.

cheers

andrew