>I think you need to say where Numero IN (select Numero from #Temp)

It works in both ways. This is because #Temp only contains one field. But, for standardization, I have added Temp.Numero.

SELECT Numero INTO #Temp FROM Country WHERE Numero<10
SELECT * FROM Country WHERE Numero IN (SELECT #Temp.Numero FROM #Temp)

>Now, it may perform better for complex scenarios (e.g. selecting keys only into temp table first and then all info for these keys). In simple cases it may not give performance gain or may even be slower than original direct query without using a temp table.

Yes, I have applied a verification to detect which route to take. If the table has less than 1000 records, it goes as before. If it has more, it will use this approach.