if not connected
SPI_connect opens a connection to the Postgres backend. You should call this function if you will need to execute queries. Some utility SPI functions may be called from un-connected procedures.
You may get SPI_ERROR_CONNECT error if SPI_connect is called from an already connected procedure - e.g. if you directly call one procedure from another connected one. Actually, while the child procedure will be able to use SPI, your parent procedure will not be able to continue to use SPI after the child returns (if SPI_finish is called by the child). It's bad practice.
SPI_connect performs the following:
Initializes the SPI internal structures for query execution and memory management.