Re: libpq simple SELECT - Mailing list pgsql-general

Laurenz,

On Wed, Dec 17, 2025 at 12:22 PM Laurenz Albe <laurenz.albe@cybertec.at> wrote:
>
> On Wed, 2025-12-17 at 09:42 -0800, Igor Korot wrote:
> > On Tue, Dec 16, 2025 at 11:34 PM Laurenz Albe <laurenz.albe@cybertec.at> wrote:
> > > On Tue, 2025-12-16 at 21:49 -0800, Igor Korot wrote:
> > > > I just tried the following code:
> > > >
> > > > [code]
> > > >      std::wstring query1 = L"SELECT t.table_catalog AS catalog,
> > > > t.table_schema AS schema, t.table_name AS table, u.usename AS owner,
> > > > c.oid AS table_id FROM information_schema.tables t,
> > > > pg_catalog.pg_class c, pg_catalog.pg_user u WHERE t.table_name =
> > > > c.relname AND c.relowner = usesysid AND (t.table_type = 'BASE TABLE'
> > > > OR t.table_type = 'VIEW' OR t.table_type = 'LOCAL TEMPORARY') ORDER BY
> > > > table_name;";
> > > >          res = PQexec( m_db, m_pimpl->m_myconv.to_bytes( query1.c_str()
> > > > ).c_str() );
> > > >           for( int i = 0; i < PQntuples( res ); i++ )
> > > >           {
> > > > some code handling the results
> > > >           }
> > > > [/code]
> > > >
> > > > The loop is executed exactly 1 time.
> > > >
> > > > My question is - according to the comment above I don't have to use cursor,
> > > > but it looks like it's unavoidable?
> > >
> > > I see no cursor in your example...
> >
> > That is my question.
> >
> > The code shows cursor but the comment indicate that the cursor is not required.
>
> I am still confused.
>
> You quoted a comment from a sample program in the documentation, and that
> example is actually using a cursor:
>
>     res = PQexec(conn, "DECLARE myportal CURSOR FOR select * from pg_database");
>
> But your code does not use a cursor.  So why should the comment apply to it?

This is a comment:

/*
     * Our test case here involves using a cursor, for which we must be inside
     * a transaction block.  We could do the whole thing with a single
     * PQexec() of "select * from pg_database", but that's too trivial to make
     * a good example.
     */

When I read it my understanding is:

It is possible to write the code without a cursor..
However it will be trivial and so the code will use CURSOR.
But the usage of CURSOR is actually not required - it is used
in the docs only to make a good example.

However, when I tried to write the code without the CURSOR
it seems that CURSOR usage is a must and not optional, which
contradicts the comment above.

I hope now my question is clear. I want to know if using CURSR
is a must.

Thank you.


>
> Yours,
> Laurenz Albe