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https://issues.apache.org/jira/browse/CALCITE-1643?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Julian Hyde resolved CALCITE-1643.
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Resolution: Fixed
Fix Version/s: 1.13.0
Fixed in http://git-wip-us.apache.org/repos/asf/calcite/commit/c850e227. Thanks
for the PR, [~ransom]!
> AFTER MATCH sub-clause of MATCH_RECOGNIZE clause
> ------------------------------------------------
>
> Key: CALCITE-1643
> URL: https://issues.apache.org/jira/browse/CALCITE-1643
> Project: Calcite
> Issue Type: Sub-task
> Components: core
> Affects Versions: 1.11.0
> Reporter: Zhiqiang He
> Assignee: Zhiqiang He
> Labels: features
> Fix For: 1.13.0
>
>
> h1. AFTER MATCH SKIP: Defining Where to Restart the Matching Process After a
> Match Is Found
> The AFTER MATCH SKIP clause determines the point to resume row pattern
> matching after a non-empty match was found. The default for the clause is
> AFTER MATCH SKIP PAST LAST ROW. The options are as follows:
> * AFTER MATCH SKIP TO NEXT ROW
> Resume pattern matching at the row after the first row of the current match.
> * AFTER MATCH SKIP PAST LAST ROW
> Resume pattern matching at the next row after the last row of the current
> match.
> * AFTER MATCH SKIP TO FIRST pattern_variable
> Resume pattern matching at the first row that is mapped to the pattern
> variable.
> * AFTER MATCH SKIP TO LAST pattern_variable
> Resume pattern matching at the last row that is mapped to the pattern
> variable.
> * AFTER MATCH SKIP TO pattern_variable
> The same as AFTER MATCH SKIP TO LAST pattern_variable.
> When using AFTER MATCH SKIP TO FIRST or AFTER MATCH SKIP TO [LAST], it is
> possible that no row is mapped to the pattern_variable. For example:
> {code:sql}
> AFTER MATCH SKIP TO A
> PATTERN (X A* X),
> {code}
> The pattern variable A in the example might have no rows mapped to A. If
> there is no row mapped to A, then there is no row to skip to, so a runtime
> exception is generated. Another problem condition is that AFTER MATCH SKIP
> may try to resume pattern matching at the same row that the last match
> started. For example:
> {code:sql}
> AFTER MATCH SKIP TO X
> PATTERN (X Y+ Z),
> {code}
> In this example, AFTER MATCH SKIP TO X tries to resume pattern matching at
> the same row where the previous match was found. This would result in an
> infinite loop, so a runtime exception is generated for this scenario.
> Note that the AFTER MATCH SKIP syntax only determines the point to resume
> scanning for a match after a non-empty match. When an empty match is found,
> one row is skipped (as if SKIP TO NEXT ROW had been specified). Thus an empty
> match never causes one of these exceptions. A query that gets one of these
> exceptions should be rewritten, as, for example, in the following:
> {code:sql}
> AFTER MATCH SKIP TO A
> PATTERN (X (A | B) Y)
> {code}
> This will cause a run-time error when a row is mapped to B, because no row
> was mapped to A. If the intent is to skip to either A or B, the following
> will work:
> {code:sql}
> AFTER MATCH SKIP TO C
> PATTERN (X (A | B) Y)
> SUBSET C = (A, B)
> {code}
> In the revised example, no runtime error is possible, whether A or B is
> matched.
> As another example:
> {code:sql}
> AFTER MATCH SKIP TO FIRST A
> PATTERN (A* X)
> {code}
> This example gets an exception after the first match, either for skipping to
> the first row of the match (if A* matches) or for skipping to a nonexistent
> row (if A* does not match). In this example, SKIP TO NEXT ROW is a better
> choice.
> When using ALL ROWS PER MATCH together with skip options other than AFTER
> MATCH SKIP PAST LAST ROW, it is possible for consecutive matches to overlap,
> in which case a row R of the row pattern input table might occur in more than
> one match. In that case, the row pattern output table will have one row for
> each match in which the row participates. If a row of the row pattern input
> table participates in multiple matches, the MATCH_NUMBER function can be used
> to distinguish among the matches. When a row participates in more than one
> match, its classifier can be different in each match.
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