New optimizer: Wrong cardinality estimation for left joins
Details
| Detail name | Value |
|---|---|
| Changelog Number | 13232 |
| Type | Bug |
| Status | Resolved |
| Affected Versions | Exasol 7.0.0, Exasol 7.1.0 |
| Fix Versions | Exasol 8.0.0, Exasol 7.1.5 |
| Resolution Date | 2022-01-26 |
Description
The new non-default join order optimizer does not estimate cardinalities correctly if the following conditions are met in the joins
- at least one join is a left join
- at least one table is empty
For left joins with more than two tables (including at least one empty table) this can lead to bad join queues and hence long run times.
Workaround
Use the default join order optimizer.
Fix
Cardinality is estimated correctly potentially leading to better join queues and lower run times.
