pg_last_error() returns the last
error message for given
connection.
Error messages may be overwritten by internal
PostgreSQL(libpq) function calls. It may not return
appropriate error message, if multiple errors are occured
inside a PostgreSQL module function.
Use pg_result_error(),
pg_result_status() and
pg_connection_status() for better error handling.
注: This function used to be called pg_errormessage().
See also pg_result_error().