如何检查spark数据框是否为空? [英] How to check if spark dataframe is empty?

问题描述

现在，我必须使用 df.count >0 检查 DataFrame 是否为空.但这有点低效.有没有更好的方法来做到这一点?

Right now, I have to use df.count > 0 to check if the DataFrame is empty or not. But it is kind of inefficient. Is there any better way to do that?

谢谢.

PS:我想检查它是否为空，以便我只保存 DataFrame 如果它不为空

PS: I want to check if it's empty so that I only save the DataFrame if it's not empty

推荐答案

对于 Spark 2.1.0，我的建议是使用 head(n: Int) 或 take(n:Int) 和 isEmpty，以您最清楚的意图为准.

For Spark 2.1.0, my suggestion would be to use head(n: Int) or take(n: Int) with isEmpty, whichever one has the clearest intent to you.

df.head(1).isEmpty
df.take(1).isEmpty

与 Python 等效:

with Python equivalent:

len(df.head(1)) == 0  # or bool(df.head(1))
len(df.take(1)) == 0  # or bool(df.take(1))

如果 DataFrame 为空，使用 df.first() 和 df.head() 都会返回 java.util.NoSuchElementException.first() 直接调用head()，即调用head(1).head.

Using df.first() and df.head() will both return the java.util.NoSuchElementException if the DataFrame is empty. first() calls head() directly, which calls head(1).head.

def first(): T = head()
def head(): T = head(1).head

head(1) 返回一个数组，因此当 DataFrame 为空时，对该数组使用 head 会导致 java.util.NoSuchElementException.

head(1) returns an Array, so taking head on that Array causes the java.util.NoSuchElementException when the DataFrame is empty.

def head(n: Int): Array[T] = withAction("head", limit(n).queryExecution)(collectFromPlan)

所以不要调用head()，直接使用head(1)来获取数组，然后就可以使用isEmpty.

So instead of calling head(), use head(1) directly to get the array and then you can use isEmpty.

take(n) 也等价于 head(n)...

def take(n: Int): Array[T] = head(n)

而limit(1).collect()等价于head(1)(注意limit(n).queryExecution在head(n: Int) 方法)，所以以下都是等价的，至少据我所知，您不必捕获 java.util.NoSuchElementException DataFrame 为空时的异常.

And limit(1).collect() is equivalent to head(1) (notice limit(n).queryExecution in the head(n: Int) method), so the following are all equivalent, at least from what I can tell, and you won't have to catch a java.util.NoSuchElementException exception when the DataFrame is empty.

df.head(1).isEmpty
df.take(1).isEmpty
df.limit(1).collect().isEmpty

我知道这是一个较旧的问题，所以希望它可以帮助使用较新版本 Spark 的人.

I know this is an older question so hopefully it will help someone using a newer version of Spark.

这篇关于如何检查spark数据框是否为空?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！