Rule Definition
If the cardinality MANY TO ONE is specified, any database that supports this addition assumes that the results set defined by the left outer join matches this cardinality and SQL Optimizer attempts to suppress any surplus joins. For exemple, When defining outer joins as LEFT OUTER MANY TO ONE joins, if no field is requested from the joined table, the database can avoid executing the join at runtime.

Remediation
Add "MANY TO ONE" cardinality on "left outer join".

Violation Code Sample

define view demo_cds_join
  as select from spfli
    left outer join   scarr on
      spfli.carrid = scarr.carrid
  {
    scarr.carrname  as carrier,
    spfli.connid    as flight,
    spfli.cityfrom  as departure,
    spfli.cityto    as arrival
  }

Fixed Code Sample

define view demo_cds_join
  as select from spfli
    left outer many to one join   scarr on
      spfli.carrid = scarr.carrid
  {
    scarr.carrname  as carrier,
    spfli.connid    as flight,
    spfli.cityfrom  as departure,
    spfli.cityto    as arrival
  }

https://blogs.sap.com/2018/12/20/safeguard-performance-of-abap-cds-views-part-3-rules-for-good-performance-of-cds-views/