Does someone know how to prepare a synamic SQL statement in plpgsql?
All the examples, PG documents describe only about preparing static SQL statement.
You might want to rephrase the question. From the pl/pgsql documentation:
"Note: The PL/pgSQL EXECUTE statement is not related to the EXECUTE SQL statement supported by the PostgreSQL server. The server's EXECUTE statement cannot be used directly within PL/pgSQL functions (and is not needed)."
So even if you could SQL-PREPARE you wouldn't be able to execute it.
The linked section describes how to construct and execute dynamic queries in pl/pgsql. The short answer is you create variable of type text and then EXECUTE it.