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https://issues.apache.org/jira/browse/CALCITE-6640?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17893120#comment-17893120
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Julian Hyde commented on CALCITE-6640:
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Alternative suggestion. Add parameter “Boolean ignoreRepeats”. If we know that
a0 is repeated as a1 and a2 and b0 is repeated as b1 then we know that the key
set willl be the core set (on a0 and b0) multiplied 8 times by the repetitions,
so why even return the repetitions? The consumer can do that multiplication if
they wish.
> RelMdUniqueKeys grows exponentially when key 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
>
> 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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