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https://issues.apache.org/jira/browse/HIVE-11022?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15400781#comment-15400781
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Dudu Markovitz commented on HIVE-11022:
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In order to support complex ordering expressions I would suggest to add the
sorting option as an enhanced syntax for the current functions and not as
additional functions, in a similar way to MySQL's GROUP_CONCAT or Oracle's
LISTAGG.
It would also be nice to add the SEPARATOR /DELIMITER option.
http://dev.mysql.com/doc/refman/5.7/en/group-by-functions.html#function_group-concat
http://docs.oracle.com/cd/E11882_01/server.112/e41084/functions089.htm#SQLRF30030
> Support collecting lists in user defined order
> ----------------------------------------------
>
> Key: HIVE-11022
> URL: https://issues.apache.org/jira/browse/HIVE-11022
> Project: Hive
> Issue Type: New Feature
> Components: UDF
> Reporter: Michael Haeusler
>
> Hive currently supports aggregation of lists "in order of input rows" with
> the UDF collect_list. Unfortunately, the order is not well defined when
> map-side aggregations are used.
> Hive could support collecting lists in user-defined order by providing a UDF
> COLLECT_LIST_SORTED(valueColumn, sortColumn[, limit]), that would return a
> list of values sorted in a user defined order. An optional limit parameter
> can restrict this to the n first values within that order.
> Especially in the limit case, this can be efficiently pre-aggregated and
> reduces the amount of data transferred to reducers.
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