pig的limit无效（返回所有记录）sample有效

pig中，limit可以取样少部分数据，但有很多问题，比如数据不能少于10条，否则返回全部。

今天又遇到另一个问题：

group后的数据，limit无效：也就是group后的数据，不能用limit，估计是含group结构不行（没验证）

相比而言sample比较好，我试了一下是可以的，对group后的数据也能有作用

测试代码如下：

origin_cleaned_data = LOAD '$cleanedLog' as 省略；

STORE  origin_cleaned_data INTO '/user/wizad/tmp/origin_cleaned_data' USING PigStorage(',');

DESCRIBE origin_cleaned_data;





test_data = foreach origin_cleaned_data generate $0,$1,$2,$3, $4;

ts_limit = limit origin_cleaned_data 100;--可以看到返回100条数据

STORE ts_limit INTO '/user/wizad/tmp/my' USING PigStorage(',');





g_log = group test_data by ($2,$4);

DESCRIBE g_log;





alldata = limit g_log 10;

dump alldata;--返回了全部数据，limit 无效

返回的group结构如下

origin_cleaned_data: 

{

wizad_ad_id: chararray,

guid: chararray,

android_id: chararray,

imei: chararray,

app_category_id: chararray

}





g_log: {

group: (android_id: chararray,app_category_id: chararray),

test_data: {

wizad_ad_id: chararray,

guid: chararray,

android_id: chararray,

imei: chararray,

app_category_id: chararray

}

}

对sample的测试：

test_data = foreach origin_cleaned_data generate wizad_ad_id,guid,android_id,imei,app_category_id;

g_log = group test_data by ($2,$4);

DESCRIBE g_log;



myts = sample g_log 0.0001;

STORE myts INTO '/user/wizad/tmp/my' USING PigStorage(',');



dump myts;

describe myts;

结果如下：

g_log: {

group: (android_id: chararray,app_category_id: chararray),

test_data: {wizad_ad_id: chararray,guid: chararray,android_id: chararray,imei: chararray,app_category_id: chararray}

}



myts: {

group: (android_id: chararray,app_category_id: chararray),

test_data: {wizad_ad_id: chararray,guid: chararray,android_id: chararray,imei: chararray,app_category_id: chararray}

}



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((a50a17bde79ac018,),{(74,863010025134441,a50a17bde79ac018,863010025134441,)})

((a51779f736cd3f54,),{(74,862949029595753,a51779f736cd3f54,862949029595753,)})

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