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https://issues.apache.org/jira/browse/CRUNCH-133?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Gabriel Reid updated CRUNCH-133:
--------------------------------
Issue Type: New Feature (was: Bug)
Change to New Feature (from Bug) so that it comes up in the right place in the
release notes later on.
> Add Aggregator support for combineValues ops on secondary keys via maps and
> collections
> ---------------------------------------------------------------------------------------
>
> Key: CRUNCH-133
> URL: https://issues.apache.org/jira/browse/CRUNCH-133
> Project: Crunch
> Issue Type: New Feature
> Reporter: Josh Wills
> Attachments: CRUNCH-133.patch
>
>
> Sawzall has a neat trick where you can do aggregations on secondary keys via
> maps, which is useful in cases where you might want to aggregate some data at
> (for example) both a country and at a city level within a single MapReduce
> job. We had a thread on crunch-user about this pattern:
> http://mail-archives.apache.org/mod_mbox/incubator-crunch-user/201212.mbox/%3CCAH29n6O-aHXTPHCRpSuAkAGUjvDR%3D56%3D-OLq9K9mZje%2BwVB4-Q%40mail.gmail.com%3E
> The pattern ends up looking something like this:
> // Define a table that has long values at both the K and the <K, String>
> levels.
> PTable<K, Pair<Long, Map<String, Long>>> in = ...;
> // Define and apply an Aggregator that can handle sums at both levels within
> a single MR job.
> Aggregator<Pair<Long, Map<String, Long>>> a = pairAggregator(SUM_LONGS(),
> map(Aggregators.SUM_LONGS()));
> PTable<K, Pair<Long, Map<String, Long>>> out =
> in.groupByKey().combineValues(a);
> ...which would run substantially faster than executing two dependent MR jobs,
> one that did the city aggregation and then a second follow-up job that did
> the country aggregation.
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