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https://issues.apache.org/jira/browse/CALCITE-6640?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17900902#comment-17900902
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Stamatis Zampetakis commented on CALCITE-6640:
----------------------------------------------
I logged CALCITE-6704 to pursue further the idea of restricting the size of the
result set using a new "limit" parameter.
> RelMdUniqueKeys generates non-minimal keys when columns are repeated in
> projections
> ------------------------------------------------------------------------------------
>
> Key: CALCITE-6640
> URL: https://issues.apache.org/jira/browse/CALCITE-6640
> Project: Calcite
> Issue Type: Bug
> Components: core
> Reporter: Stamatis Zampetakis
> Assignee: Stamatis Zampetakis
> Priority: Major
> Labels: pull-request-available
> Fix For: 1.39.0
>
>
> Consider the following table where empno is a unique key column.
> {code:sql}
> CREATE TABLE emp (
> empno INT,
> ename VARCHAR,
> job VARCHAR
> PRIMARY KEY (empno));
> {code}
> The results of RelMetadataQuery#getUniqueKeys for the following queries are
> as follows:
> {code:sql}
> SELECT empno FROM emp;
> {0}
> SELECT ename, empno FROM emp;
> {1}
> SELECT empno, ename, empno FROM emp;
> {0}, {2}, {0, 2}
> SELECT empno, ename, empno, empno FROM emp;
> {0}, {2}, {3}, {0, 2}, {0 3}, {2, 3}, {0, 2, 3}
> {code}
> When key columns are repeated in the project the result grows exponentially.
> This makes the unique key computation very expensive when there are many keys
> or when keys are repeated multiple times. The problem can lead to OOM errors
> and queries/rules hanging forever while trying to extract the keys.
> Observe, that the results above are not minimal so currently we are creating
> and returning a lot of redundant information.
> {noformat}
> {0}, {2}, {3}, {0, 2}, {0 3}, {2, 3}, {0, 2, 3}
> {noformat}
> If we know that \{0\}, \{2\}, and \{3\} are unique keys individually then any
> superset of those is also a unique key so it is sufficient to return just
> those.
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