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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