Rule Definition
Usually the result of joining the two tables will be a set of data consisting of Order Headers and their Items. The select will gather all the requested data from both tables. That means, even if a business user is interested only in header information the view still fetch data from both tables. This will impact application performance mainly when perform join on large set of tables

Remediation
Use "association" instead of "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
  association to scarr as _scarr on
    spfli.carrid = _scarr.carrid
  {
    _scarr.carrname        as carrier,
    spfli.connid           as flight,
    spfli.cityfrom         as departure,
    spfli.cityto           as arrival
  }

https://www.alexfediuc.com/cds-views-associations-vs-joins/ https://blogs.sap.com/2016/08/12/from-open-sql-joins-to-cds-associations/ https://blogs.sap.com/2018/12/20/safeguard-performance-of-abap-cds-views-part-3-rules-for-good-performance-of-cds-views/