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https://issues.apache.org/jira/browse/FLINK-6075?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16089436#comment-16089436
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ASF GitHub Bot commented on FLINK-6075:
---------------------------------------

Github user rtudoran commented on the issue:

    https://github.com/apache/flink/pull/4263
  
    @fhueske You were the one  that argue that retraction support is needed for 
offset and fetch. When we were discussing this i was not into having the 
retraction. I think finally it depends on what behavior we want to give for the 
function. If we have a SELECT x FROM stream ORDER BY *time FETCH 2 when we emit 
the new latest data - do we still want to invalidate what we have emitted 
previously (in this case we would emit the retraction), otherwise if we 
consider that that was the valid input at that point in time - we do not need 
the retraction. I am fine either way. Let me know. 


> Support Limit/Top(Sort) for Stream SQL
> --------------------------------------
>
>                 Key: FLINK-6075
>                 URL: https://issues.apache.org/jira/browse/FLINK-6075
>             Project: Flink
>          Issue Type: New Feature
>          Components: Table API & SQL
>            Reporter: radu
>              Labels: features
>         Attachments: sort.png
>
>
> These will be split in 3 separated JIRA issues. However, the design is the 
> same only the processing function differs in terms of the output. Hence, the 
> design is the same for all of them.
> Time target: Proc Time
> **SQL targeted query examples:**
> *Sort example*
> Q1)` SELECT a FROM stream1 GROUP BY HOP(proctime, INTERVAL '1' HOUR, INTERVAL 
> '3' HOUR) ORDER BY b` 
> Comment: window is defined using GROUP BY
> Comment: ASC or DESC keywords can be placed to mark the ordering type
> *Limit example*
> Q2) `SELECT a FROM stream1 WHERE rowtime BETWEEN current_timestamp - INTERVAL 
> '1' HOUR AND current_timestamp ORDER BY b LIMIT 10`
> Comment: window is defined using time ranges in the WHERE clause
> Comment: window is row triggered
> *Top example*
> Q3) `SELECT sum(a) OVER (ORDER BY proctime RANGE INTERVAL '1' HOUR PRECEDING 
> LIMIT 10) FROM stream1`  
> Comment: limit over the contents of the sliding window
> General Comments:
> -All these SQL clauses are supported only over windows (bounded collections 
> of data). 
> -Each of the 3 operators will be supported with each of the types of 
> expressing the windows. 
> **Description**
> The 3 operations (limit, top and sort) are similar in behavior as they all 
> require a sorted collection of the data on which the logic will be applied 
> (i.e., select a subset of the items or the entire sorted set). These 
> functions would make sense in the streaming context only in the context of a 
> window. Without defining a window the functions could never emit as the sort 
> operation would never trigger. If an SQL query will be provided without 
> limits an error will be thrown (`SELECT a FROM stream1 TOP 10` -> ERROR). 
> Although not targeted by this JIRA, in the case of working based on event 
> time order, the retraction mechanisms of windows and the lateness mechanisms 
> can be used to deal with out of order events and retraction/updates of 
> results.
> **Functionality example**
> We exemplify with the query below for all the 3 types of operators (sorting, 
> limit and top). Rowtime indicates when the HOP window will trigger – which 
> can be observed in the fact that outputs are generated only at those moments. 
> The HOP windows will trigger at every hour (fixed hour) and each event will 
> contribute/ be duplicated for 2 consecutive hour intervals. Proctime 
> indicates the processing time when a new event arrives in the system. Events 
> are of the type (a,b) with the ordering being applied on the b field.
> `SELECT a FROM stream1 HOP(proctime, INTERVAL '1' HOUR, INTERVAL '2' HOUR) 
> ORDER BY b (LIMIT 2/ TOP 2 / [ASC/DESC] `)
> ||Rowtime||   Proctime||      Stream1||       Limit 2||       Top 2|| Sort 
> [ASC]||
> |         |10:00:00  |(aaa, 11)       |               |             |         
>    |
> |         |10:05:00    |(aab, 7)  |           |             |            |
> |10-11          |11:00:00  |          |       aab,aaa |aab,aaa  |     aab,aaa 
>    |
> |         |11:03:00  |(aac,21)  |           |         |            |          
>         
> |11-12    |12:00:00  |          |     aab,aaa |aab,aaa  |     aab,aaa,aac|
> |         |12:10:00  |(abb,12)  |           |         |            |          
>         
> |         |12:15:00  |(abb,12)  |           |         |            |          
>         
> |12-13          |13:00:00  |          |       abb,abb | abb,abb |     
> abb,abb,aac|
> |...|
> **Implementation option**
> Considering that the SQL operators will be associated with window boundaries, 
> the functionality will be implemented within the logic of the window as 
> follows.
> * Window assigner – selected based on the type of window used in SQL 
> (TUMBLING, SLIDING…)
> * Evictor/ Trigger – time or count evictor based on the definition of the 
> window boundaries
> * Apply – window function that sorts data and selects the output to trigger 
> (based on LIMIT/TOP parameters). All data will be sorted at once and result 
> outputted when the window is triggered
> An alternative implementation can be to use a fold window function to sort 
> the elements as they arrive, one at a time followed by a flatMap to filter 
> the number of outputs. 
> !sort.png!
> **General logic of Join**
> ```
> inputDataStream.window(new [Slide/Tumble][Time/Count]Window())
> //.trigger(new [Time/Count]Trigger()) – use default
> //.evictor(new [Time/Count]Evictor()) – use default
>               .apply(SortAndFilter());
> ```



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