radu created FLINK-6075:
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Summary: Support Limit/Top(Sort) for Stream SQL
Key: FLINK-6075
URL: https://issues.apache.org/jira/browse/FLINK-6075
Project: Flink
Issue Type: Bug
Components: Table API & SQL
Reporter: radu
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.
> see picture in the attached document
**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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