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https://issues.apache.org/jira/browse/FLINK-7169?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16119855#comment-16119855
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ASF GitHub Bot commented on FLINK-7169:
---------------------------------------

Github user dawidwys commented on a diff in the pull request:

    https://github.com/apache/flink/pull/4331#discussion_r132173699
  
    --- Diff: 
flink-libraries/flink-cep/src/main/java/org/apache/flink/cep/nfa/compiler/NFACompiler.java
 ---
    @@ -150,6 +160,59 @@ long getWindowTime() {
                }
     
                /**
    +            * Check pattern after match skip strategy.
    +            */
    +           private void checkPatternSkipStrategy() {
    +                   AfterMatchSkipStrategy afterMatchSkipStrategy = 
currentPattern.getAfterMatchSkipStrategy();
    +                   if (afterMatchSkipStrategy.getStrategy() == 
AfterMatchSkipStrategy.SkipStrategy.SKIP_TO_FIRST ||
    +                           afterMatchSkipStrategy.getStrategy() == 
AfterMatchSkipStrategy.SkipStrategy.SKIP_TO_LAST) {
    +                           Pattern<T, ?> pattern = currentPattern;
    +                           while 
(!pattern.getName().equals(afterMatchSkipStrategy.getPatternName())) {
    +                                   pattern = pattern.getPrevious();
    +                           }
    +                           // pattern name match check.
    +                           if (pattern == null) {
    +                                   throw new 
MalformedPatternException("the pattern name specified in AfterMatchSkipStrategy 
" +
    +                                           "can not be found in the given 
Pattern");
    +                           } else {
    +                                   // can not be used with optional states.
    +                                   if 
(pattern.getQuantifier().hasProperty(Quantifier.QuantifierProperty.OPTIONAL)) {
    +                                           throw new 
MalformedPatternException("the AfterMatchSkipStrategy "
    +                                                   + 
afterMatchSkipStrategy.getStrategy() + " can not be used with optional 
pattern");
    +                                   }
    +                           }
    +
    +                           // start position check.
    +                           if (pattern.getPrevious() == null) {
    --- End diff --
    
    Had a second thought on all those cases, and I think you had a good point 
with one of the previous proposals, just to fall back to the 
`SKIP_TO_NEXT_EVENT`
    
    Maybe let's allow all those situations, but if within a match we cannot 
discard(e.g. skip is to the first pattern or the optional event is not present) 
then we discard nothing. The advantage in my opinion it is easier to understand 
than to understand and maintain all those cases. (As a side note the exceptions 
in SQL in all those cases in my opinion are just an implementation details as 
they keep a single partial match at a time). What do you think?  


> Support AFTER MATCH SKIP function in CEP library API
> ----------------------------------------------------
>
>                 Key: FLINK-7169
>                 URL: https://issues.apache.org/jira/browse/FLINK-7169
>             Project: Flink
>          Issue Type: Sub-task
>          Components: CEP
>            Reporter: Yueting Chen
>            Assignee: Yueting Chen
>             Fix For: 1.4.0
>
>
> In order to support Oracle's MATCH_RECOGNIZE on top of the CEP library, we 
> need to support AFTER MATCH SKIP function in CEP API.
> There're four options in AFTER MATCH SKIP, listed as follows:
> 1. AFTER MATCH SKIP TO NEXT ROW: resume pattern matching at the row after the 
> first row of the current match.
> 2. AFTER MATCH SKIP PAST LAST ROW: resume pattern matching at the next row 
> after the last row of the current match.
> 3. AFTER MATCH SKIP TO FIST *RPV*: resume pattern matching at the first row 
> that is mapped to the row pattern variable RPV.
> 4. AFTER MATCH SKIP TO LAST *RPV*: resume pattern matching at the last row 
> that is mapped to the row pattern variable RPV.
> I think we can introduce a new function to `CEP` class, which takes a new 
> parameter as AfterMatchSKipStrategy.
> The new API may looks like this
> {code}
> public static <T> PatternStream<T> pattern(DataStream<T> input, Pattern<T, ?> 
> pattern, AfterMatchSkipStrategy afterMatchSkipStrategy) 
> {code}
> We can also make `SKIP TO NEXT ROW` as the default option, because that's 
> what CEP library behaves currently.



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