[
https://issues.apache.org/jira/browse/HADOOP-485?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12490071
]
Runping Qi commented on HADOOP-485:
-----------------------------------
A real example is "session analysis" on the log data.
Let's say we have a data set containing a collection of records, each has a
timestamp field and other fields.
We want to group the records by certain fields (the primary key fields), but we
want the records within each group to be sorted by timestamps so that the
user's reduce function can analyse the records in one pass. Otherwise, the
user's code has to cache all the records within each group, sort them by the
timestamps first
(this is effectively the common approach people take, but it will run out
memory if the number of records per
group is very large).
> allow a different comparator for grouping keys in calls to reduce
> -----------------------------------------------------------------
>
> Key: HADOOP-485
> URL: https://issues.apache.org/jira/browse/HADOOP-485
> Project: Hadoop
> Issue Type: New Feature
> Components: mapred
> Affects Versions: 0.5.0
> Reporter: Owen O'Malley
> Assigned To: Tahir Hashmi
> Attachments: Hadoop-485-pre.patch, TestUserValueGrouping.java.patch
>
>
> Some algorithms require that the values to the reduce be sorted in a
> particular order, but extending the key with the additional fields causes
> them to be handled by different calls to reduce. (The user then collects the
> values until they detect a "real" key change and then processes them.)
> It would be much easier if the framework let you define a second comparator
> that did the grouping of values for reduces. So your reduce inputs look like:
> A1, V1
> A2, V2
> A3, V3
> B1, V4
> B2, V5
> instead of getting calls to reduce that look like:
> reduce(A1, {V1}); reduce(A2, {V2}); reduce(A3, {V3}); reduce(B1, {V4});
> reduce(B2, {V5});
> you could define the grouping comparator to just compare the letters and end
> up with:
> reduce(A1, {V1,V2,V3}); reduce(B1, {V4,V5});
> which is the desired outcome. Note that this assumes that the "extra" part of
> the key is just for sorting because the reduce will only see the first
> representative of each equivalence class.
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
