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https://issues.apache.org/jira/browse/CALCITE-1644?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16008349#comment-16008349
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Julian Hyde commented on CALCITE-1644:
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A few minor things:
* You accidentally changed indentation in a couple of places. Please revert.
* The error message "Pattern var 'STRT' has been defined" doesn't work for me.
Why not re-use "Pattern variable ''{0}'' has already been defined"?
* Can you update reference.md. The SUBSET clause doesn't look right.
* Deep-copy the map you assign to Match.subsets. Currently the TreeSets are
still mutable. Maybe you can use Match.copy or something similar.
> subset clause syntax support for MATCH_RECOGNIZE
> ------------------------------------------------
>
> Key: CALCITE-1644
> URL: https://issues.apache.org/jira/browse/CALCITE-1644
> Project: Calcite
> Issue Type: Sub-task
> Components: core
> Affects Versions: 1.11.0
> Reporter: Zhiqiang He
> Assignee: Zhiqiang He
> Labels: features
>
> h1. SUBSET: Defining Union Row Pattern Variables
> At times, it is helpful to create a grouping of multiple pattern variables
> that can be referred to with a variable name of its own. These groupings are
> called union row pattern variables, and you create them with the SUBSET
> clause. The union row pattern variable created by SUBSET can be used in the
> MEASURES and DEFINE clauses. The SUBSET clause is optional. It is used to
> declare union row pattern variables. For example, here is a query using
> SUBSET to calculate an average based on all rows that are mapped to the union
> of STRT and DOWN variables, where STRT is the starting point for a pattern,
> and DOWN is the downward (left) leg of a V shape.
> Example 20-5 illustrates creating a union row pattern variable.
> **Example 20-5 Defining Union Row Pattern Variables**
> {code:sql}
> SELECT *
> FROM Ticker MATCH_RECOGNIZE(
> PARTITION BY symbol
> ORDER BY tstamp
> MEASURES FIRST(STRT.tstamp) AS strt_time,
> LAST(DOWN.tstamp) AS bottom,
> AVG(STDN.Price) AS stdn_avgprice
> ONE ROW PER MATCH
> AFTER MATCH SKIP TO LAST UP
> PATTERN (STRT DOWN+ UP+)
> SUBSET STDN= (STRT, DOWN)
> DEFINE
> UP AS UP.Price > PREV(UP.Price),
> DOWN AS DOWN.Price < PREV (DOWN.Price)
> );
>
> SYMBOL STRT_TIME BOTTOM STDN_AVGPRICE
> ------ --------- --------- -------------
> ACME 05-APR-11 06-APR-11 18.5
> ACME 10-APR-11 12-APR-11 19.6666667
> ACME 14-APR-11 16-APR-11 17
> {code}
> This example declares a single union row pattern variable, STDN, and defines
> it as the union of the rows mapped to STRT and the rows mapped to DOWN. There
> can be multiple union row pattern variables in a query. For example:
> {code:sql}
> PATTERN (W+ X+ Y+ Z+)
> SUBSET XY = (X, Y),
> WZ = (W, Z)
> {code}
> The right-hand side of a SUBSET item is a comma-separated list of distinct
> primary row pattern variables within parentheses. This defines the union row
> pattern variable (on the left-hand side) as the union of the primary row
> pattern variables (on the right-hand side).
> Note that the list of pattern variables on the right-hand side may not
> include any union row pattern variables (there are no unions of unions).
> For every match, there is one implicit union row pattern variable called the
> universal row pattern variable. The universal row pattern variable is the
> union of all primary row pattern variables. For instance, if your pattern has
> primary pattern variable A, B, and C, then the universal row pattern variable
> is equivalent to a SUBSET clause with the argument (A, B, C). Thus, every row
> of a match is mapped to the universal row pattern variable. Any unqualified
> column reference within the MEASURES or DEFINE clauses is implicitly
> qualified by the universal row pattern variable. Note that there is no
> keyword to explicitly specify the universal row pattern variable.
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