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https://issues.apache.org/jira/browse/IMPALA-7942?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17438506#comment-17438506
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Sheng Wang commented on IMPALA-7942:
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These kinds of hints is quite useful, since Impala's estimation for compute
predicate selectivity for is simple without histograms. We can set
cardinalities and selectivities manually to get better planner in some cases. I
will assign this Jira to me and try to do. [~stigahuang]
> 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
> 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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