* exec without index, next try add index on ordered sequence, or reverse ( i don't understand data statistic )

  SELECT Etats
* INDEX ON ddebut TAG ddebut 
* INDEX ON dfin 	TAG dfin 
*1 INDEX ON ntype 	TAG ntype 
*2 INDEX ON nPoscat TAG nPoscat 


  SELECT DatesFiltred
*  INDEX ON ntype  TAG ntypeIndex 
*  INDEX ON nPoscat TAG oscatIndex

  SELECT Etats.nPoscat, ;
    Etats.ccat_physiq  ,;  && ATTENTION, NOT GROUPED
    Etats.ntype , ;
    COUNT( DatesFiltred.ddate ) AS "Ncat" , ;
    Etats.ltransfert  ;
    FROM FORCE DatesFiltred JOIN Etats  ;
    ON DatesFiltred.ddate BETWEEN m.D_debut AND m.d_fin; && this is VFP optimization for constant
	AND Etats.nPoscat = DatesFiltred.nPoscat AND Etats.ntype=DatesFiltred.ntype ;
	AND Etats.ddebut<=DatesFiltred.ddate AND Etats.dfin>=DatesFiltred.ddate  ;
    GROUP BY  ltransfert , Etats.ntype , Etats.nPoscat ; && IMPLICIT ORDER ON VFP
    INTO CURSOR Sum_Categ_per_periode

or try:

  SELECT Etats
* INDEX ON ddebut TAG ddebut 
* INDEX ON dfin 	TAG dfin 
* INDEX ON ntype 	TAG ntype 
* INDEX ON nPoscat TAG nPoscat 


  SELECT DatesFiltred
*1  INDEX ON ntype  TAG ntypeIndex 
*2  INDEX ON nPoscat TAG oscatIndex

  SELECT Etats.nPoscat, ;
    Etats.ccat_physiq  ,;  && ATTENTION, NOT GROUPED
    Etats.ntype , ;
    COUNT( DatesFiltred.ddate ) AS "Ncat" , ;
    Etats.ltransfert  ;
    FROM FORCE Etats JOIN  DatesFiltred ;
    ON DatesFiltred.ddate BETWEEN m.D_debut AND m.d_fin; && this is VFP optimization for constant
	AND DatesFiltred.nPoscat=Etats.nPoscat AND DatesFiltred.ntype = Etats.ntype;
	AND DatesFiltred.ddate BETWEEN Etats.ddebut AND Etats.dfin;
    GROUP BY  ltransfert , Etats.ntype , Etats.nPoscat ; && IMPLICIT ORDER ON VFP
    INTO CURSOR Sum_Categ_per_periode