Lars Volker created IMPALA-7942:
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Summary: 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
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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