这两段代码之间是否有任何差异，性能方面：
df = //read the file here
df = df.select(//some columns here)
df = df.groupBy().agg() // some operation
df = df.filter() //some filtering
df.show()
和
df = //read the file here
df1 = df.select(//some columns here)
df2 = df1.groupBy().agg() // some operation
df3 = df2.filter() //some filtering
df3.show()