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https://issues.apache.org/jira/browse/PIG-1846?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14319322#comment-14319322
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Rohini Palaniswamy commented on PIG-1846:
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I have wondered when I have encountered cases like this why we don't have a
syntax which enables a user to perform distinct on select columns in the alias
like below.
distinct_user_data = distinct user_data by (user, gender);
This would avoid the need to do a group by and distinct inside nested foreach
and the distinct job will use its own combiner to reduce the number of records.
Changes to distinct implementation should not be also that much as you will
only have to rearrange key and values additionally at the map and reduce end.
> optimize queries like - count distinct users for each gender
> ------------------------------------------------------------
>
> Key: PIG-1846
> URL: https://issues.apache.org/jira/browse/PIG-1846
> Project: Pig
> Issue Type: Improvement
> Affects Versions: 0.9.0
> Reporter: Thejas M Nair
>
> The pig group operation does not usually have to deal with skew on the
> group-by keys if the foreach statement that works on the results of group has
> only algebraic functions on the bags. But for some queries like the
> following, skew can be a problem -
> {code}
> user_data = load 'file' as (user, gender, age);
> user_group_gender = group user_data by gender parallel 100;
> dist_users_per_gender = foreach user_group_gender
> {
> dist_user = distinct user_data.user;
> generate group as gender, COUNT(dist_user) as
> user_count;
> }
> {code}
> Since there are only 2 distinct values of the group-by key, only 2 reducers
> will actually get used in current implementation. ie, you can't get better
> performance by adding more reducers.
> Similar problem is there when the data is skewed on the group key. With
> current implementation, another problem is that pig and MR has to deal with
> records with extremely large bags that have the large number of distinct user
> names, which results in high memory utilization and having to spill the bags
> to disk.
> The query plan should be modified to handle the skew in such cases and make
> use of more reducers.
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