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https://issues.apache.org/jira/browse/MADLIB-1002?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15311263#comment-15311263
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Nandish Jayaram commented on MADLIB-1002:
-----------------------------------------
The query posted above might not work correctly since the LAG({time_stamp},1)
does not seem to handle point (2) in the implementation notes.
The LAG() will always look at the previous row's timestamp instead of a row
that could potentially be multiple rows before the current row.
The problem of choosing a variable to specify how many rows back LAG() must
look at was the challenging part of using window function
with the min_time parameter for me.
> Sessionization - Phase 3 (minimum time)
> ---------------------------------------
>
> Key: MADLIB-1002
> URL: https://issues.apache.org/jira/browse/MADLIB-1002
> Project: Apache MADlib
> Issue Type: New Feature
> Components: Module: Utilities
> Reporter: Frank McQuillan
> Priority: Minor
> Labels: gsoc2016, starter
> Fix For: v1.9.2
>
>
> Story
> As a data scientist, I want to perform session reconstruction on my data set,
> so that I can prepare for input into other algorithms like path functions, or
> predictive analytics algorithms.
> This is a follow on to
> https://issues.apache.org/jira/browse/MADLIB-909
> https://issues.apache.org/jira/browse/MADLIB-1001
> to add minimum time.
> Details
> Add min time to the existing params:
> Proposed interface changes:
> {code}
> sessionize (
> source_table,
> output_table,
> partition_expr,
> order_expr,
> time_stamp,
> time_out,
> min_time, -- new
> output_cols,
> create_view
> )
> {code}
> where
> min_time (optional)
> Minimum delta time that must elapse for an event to be considered a valid
> event (default=0). If an event happens in less than min_time since the last
> valid event, it does not get included in the current session and is dropped.
> Same units as time_stamp.
> Implementation notes
> 1) Should be specified in the same units as the time_out parameter.
> 2) Always compare against the last valid session event, not against one(s)
> that just got dropped.
> For an example of how min_time could work, see Aster Analytics sessionization
> function [1].
> References
> [1] Aster Analytics users guide, see "sessionize" function
> http://www.info.teradata.com/edownload.cfm?itemid=143450001
> http://www.info.teradata.com/templates/eSrchResults.cfm?txtpid=&txtrelno=&prodline=all&frmdt=&txtsrchstring=aster%20analytics&srtord=Desc&todt=&rdSort=Date
> https://www.youtube.com/watch?v=C760M9ttK9Q
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