[IMPALA-7942] Add query hints for cardinalities and selectivities

• Type: New Feature
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: Impala 3.2.0
• Component/s: Frontend
• Epic Color: ghx-label-2
• Assignee: Sheng Wang
• Reporter: Lars Volker
• Created: 07/Dec/18 01:27
• Updated: 27/Apr/23 18:27

Description

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.

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;

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:

select count(*) from T /*+ num_rows(100) */
  where id < 100 /*+ selectivity(0.1) */;

This is a first step towards giving users more control over the planner / optimizer.

Issue Links

• links to: Code Review