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https://issues.apache.org/jira/browse/DATAFU-63?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16249793#comment-16249793
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Eyal Allweil commented on DATAFU-63:
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Hi [~cur4so],
I'll quickly answer your last comment - I'll get to the previous one as soon as
I can. We do indeed use still gradle 2.4 in the master branch. We're [about to
update to Gradle 3.5|https://issues.apache.org/jira/browse/DATAFU-125], but it
hasn't been merged yet.
However, when I did the gradle bootstrapping, it didn't modify my _gradlew_
file - what OS are you on? (BTW - we can't add it to the gitignore because it's
checked into the repository, and you can't ignore files that are checked in)
> SimpleRandomSample by a fixed number
> ------------------------------------
>
> Key: DATAFU-63
> URL: https://issues.apache.org/jira/browse/DATAFU-63
> Project: DataFu
> Issue Type: New Feature
> Reporter: jian wang
> Assignee: jian wang
>
> SimpleRandomSample currently supports random sampling by probability, it does
> not support random sample a fixed number of items. ReserviorSample may do the
> work but since it relies on an in-memory priority queue, memory issue may
> happen if we are going to sample a huge number of items, eg: sample 100M from
> 100G data.
> Suggested approach is to create a new class "SimpleRandomSampleByCount" that
> uses Manuver's rejection threshold to reject items whose weight exceeds the
> threshold as we go from mapper to combiner to reducer. The majority part of
> the algorithm will be very similar to SimpleRandomSample, except that we do
> not use Berstein's theory to accept items and replace probability p = k / n,
> k is the number of items to sample, n is the total number of items local in
> mapper, combiner and reducer.
> Quote this requirement from others:
> "Hi folks,
> Question: does anybody know if there is a quicker way to randomly sample a
> specified number of rows from grouped data? I’m currently doing this, since
> it appears that the SAMPLE operator doesn’t work inside FOREACH statements:
> photosGrouped = GROUP photos BY farm;
> agg = FOREACH photosGrouped {
> rnds = FOREACH photos GENERATE *, RANDOM() as rnd;
> ordered_rnds = ORDER rnds BY rnd;
> limitSet = LIMIT ordered_rnds 5000;
> GENERATE group AS farm,
> FLATTEN(limitSet.(photo_id, server, secret)) AS (photo_id, server,
> secret);
> };
> This approach seems clumsy, and appears to run quite slowly (I’m assuming the
> ORDER/LIMIT isn’t great for performance). Is there a less awkward way to do
> this?
> Thanks,
> "
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