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From Jerry Lam <chiling...@gmail.com>
Subject Re: Spark SQL DataFrame 1.5.0 is extremely slow for take(1) or head() or first()
Date Mon, 21 Sep 2015 17:43:22 GMT
Hi Yin,

You are right! I just tried the scala version with the above lines, it
works as expected.
I'm not sure if it happens also in 1.4 for pyspark but I thought the
pyspark code just calls the scala code via py4j. I didn't expect that this
bug is pyspark specific. That surprises me actually a bit. I created a
ticket for this (SPARK-10731
<https://issues.apache.org/jira/browse/SPARK-10731>).

Best Regards,

Jerry


On Mon, Sep 21, 2015 at 1:01 PM, Yin Huai <yhuai@databricks.com> wrote:

> btw, does 1.4 has the same problem?
>
> On Mon, Sep 21, 2015 at 10:01 AM, Yin Huai <yhuai@databricks.com> wrote:
>
>> Hi Jerry,
>>
>> Looks like it is a Python-specific issue. Can you create a JIRA?
>>
>> Thanks,
>>
>> Yin
>>
>> On Mon, Sep 21, 2015 at 8:56 AM, Jerry Lam <chilinglam@gmail.com> wrote:
>>
>>> Hi Spark Developers,
>>>
>>> I just ran some very simple operations on a dataset. I was surprise by
>>> the execution plan of take(1), head() or first().
>>>
>>> For your reference, this is what I did in pyspark 1.5:
>>> df=sqlContext.read.parquet("someparquetfiles")
>>> df.head()
>>>
>>> The above lines take over 15 minutes. I was frustrated because I can do
>>> better without using spark :) Since I like spark, so I tried to figure out
>>> why. It seems the dataframe requires 3 stages to give me the first row. It
>>> reads all data (which is about 1 billion rows) and run Limit twice.
>>>
>>> Instead of head(), show(1) runs much faster. Not to mention that if I do:
>>>
>>> df.rdd.take(1) //runs much faster.
>>>
>>> Is this expected? Why head/first/take is so slow for dataframe? Is it a
>>> bug in the optimizer? or I did something wrong?
>>>
>>> Best Regards,
>>>
>>> Jerry
>>>
>>
>>
>

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