Incearca sa pui nolock la tabele.

Incearca sa scapi de conditia where .... pk_key =(SELECT MAX(pk_key) as pk_key FROM tabel t2 WHERE t1.cod = t2.cod AND t2.data <= data) printr-un join intre t1 si t2

ceva de genul:

SELECT t1.cod, t1.sold, SUBSTRING(t1.cod,3,4) as grupa FROM tabel t1 (nolock)

join (SELECT MAX(pk_key) as pk_key FROM tabel t2(nolock) WHERE t1.cod = t2.cod AND t2.data <= data) xt on t1.pk_key = xt.pk_key

Nu am testat join-ul asta, dar stiu sigur ca ajuta la performnta.