Rule Definition
If the cardinality is specified, any databases that support this addition assume that the results set defined by association matches this cardinality and SQL Optimizer attempts to suppress any surplus joins.

Remediation
Set cardinality to associations.

Violation Code Sample

define view demo_cds_wrong_cardinality
  as select from
    scarr
    association to spfli as _spfli on
      _spfli.carrid = scarr.carrid
    {
      scarr.carrid   as carrid,
      scarr.carrname as carrname,
      _spfli.connid  as connid
    }

Fixed Code Sample

define view demo_cds_explicit_cardinality
  as select from
    scarr
    association [1..*] to spfli as _spfli on
      _spfli.carrid = scarr.carrid
    {
      scarr.carrid   as carrid,
      scarr.carrname as carrname,
      _spfli.connid  as connid
    }

https://blogs.sap.com/2018/12/20/safeguard-performance-of-abap-cds-views-part-3-rules-for-good-performance-of-cds-views/ https://blogs.sap.com/2018/08/20/cardinality-of-association-in-cds-view/