Rule Definition
The SELECT ... ENDSELECT works as a loop fetching single record for every loop pause. Basically, it works like a client cursor which will generate too much traffic on the network and communications between the application server and the database server. When used on extremely large tables, this statement becomes a performance killer.

Remediation
Use preferably the "SELECT... INTO TABLE...+ LOOP AT Table" tatement.

Violation Code Sample

* violation
SELECT name
    FROM dtab
    WHERE typeno = 2.
...
ENDSELECT.

* no violation
SELECT prodid INTO lv_prodid
    FROM prodlist
    UP TO 1 ROWS
    WHERE typeid = ps_alty-type_id.
ENDSELECT.

IF NOT lv_prodid IS INITIAL
...
ENDIF.

* no violation
SELECT prodid INTO lv_prodid
    FROM prodlist
    UP TO ONE ROWS
    WHERE typeid = ps_alty-type_id.
ENDSELECT.

IF NOT lv_prodid IS INITIAL
...
ENDIF.

Fixed Code Sample

* remediation
SELECT name INTO TABLE itab 
    FROM dtab
    WHERE typeno = 2.
IF sy-subrc = 0.
    LOOP AT itab.
...
    ENDLOOP.
ELSE.
    MESSAGE e000 WITH text-117.
ENDIF.