Optimize TEMP_DB_RAM usage by applying GROUP BY on UNION ALL branches
Details
| Detail name | Value |
|---|---|
| Changelog Number | 27055 |
| Type | Improvement |
| Status | Resolved |
| Fix Versions | Exasol 2026.1.0 |
| Resolution Date | 2026-05-15 |
Background
Consider an aggregated query over a UNION ALL subselect, for example:
SELECT X, SUM(Y) FROM (SELECT * FROM A UNION ALL SELECT * FROM B WHERE X < 100 ) GROUP BY X SELECT SUM(X) FROM (SELECT * FROM A UNION ALL SELECT * FROM B WHERE X < 100);
In such cases, the UNION ALL subselect was materialized in its entirety, and then the aggregation takes place. This may lead to very large TEMP_DB_RAM usage due to this materialization.
Improvement
The aggregation is performed on each UNION ALL branch and then the results are combined, with potentially large gains in TEMP_DB_RAM usage and query duration.
