pg_lo_read() reads at most len bytes from a large object
and returns it as a string.
large_object specifies a valid large object resource
andlen specifies the
maximum allowable size of the large object segment. It
returns FALSE if there is an
error.
To use the large object (lo) interface, it is necessary
to enclose it within a transaction block.
注: This function used to be called pg_loread().
See also pg_lo_read_all().