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https://issues.apache.org/jira/browse/IMPALA-7942?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17565454#comment-17565454
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Quanlong Huang commented on IMPALA-7942:
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There is a patch submitted by [~skyyws] under review:
https://gerrit.cloudera.org/c/18023/
> Add query hints for cardinalities and selectivities
> ---------------------------------------------------
>
> Key: IMPALA-7942
> URL: https://issues.apache.org/jira/browse/IMPALA-7942
> Project: IMPALA
> Issue Type: New Feature
> Components: Frontend
> Affects Versions: Impala 3.2.0
> Reporter: Lars Volker
> Assignee: Sheng Wang
> Priority: Major
>
> The optimizer can pick suboptimal plans when tables don't have statistics. To
> allow users to help the optimizer, we should support query hints to specify
> cardinalities of scans, predicated (and possibly joins).
> This could look like the following example.
> {code:sql}
> select x from medium /*+ num_rows(1000000000) */
> join small /*+ num_rows(1000000) */
> join (select * from big /*+ num_rows(1000000000) */
> where c1 < 10 /*+ selectivity(0.00001) */) as big
> where medium.id = small.id and small.id = big.id;
> {code}
> Instead of cardinalities we could also support specifying the number of rows
> that pass a predicate (or join).
> We should not rely on the specified cardinalities to be accurate, e.g. the
> following should still execute a scan:
> {code:sql}
> select count(*) from T /*+ num_rows(100) */
> where id < 100 /*+ selectivity(0.1) */;
> {code}
> This is a first step towards giving users more control over the planner /
> optimizer.
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