select var1, var2, var3 from TBL_DUMMY
where (var1= <some value>
  and  var2= <some value>)
   or  var3= <some value>

idx5 (var1, var2)
idx6 (var1, var2, var3)

Scenario 1:
TBL_DUMMY, idx1(var1), 20million records, 30 variables (var1 - var30)
select * from TBL_DUMMY where var1= <some value> and var2= <some value>

Scenario 2:
TBL_DUMMY, idx1(var1, var2), 20million records, 30 variables (var1 - var30)
select * from TBL_DUMMY where var1= <some value> and var2= <some value>

Scenario 3:
TBL_DUMMY, idx1(var2, var1), 20million records, 30 variables (var1 - var30)
select * from TBL_DUMMY where var1= <some value> and var2= <some value>