UNIQUE from Group By Key Reduction in IF/ENDIF causes internal server error

Detail name	Value
Changelog Number	30917
Type	Bug
Status	Resolved
Affected Versions	Exasol 2026.1.0, Exasol 2025.1.11
Fix Versions	Exasol 2026.1.1, Exasol 2025.1.13
Resolution Date	2026-07-15

Description

A valid query with GROUP BY and an IF/ENDIF operator, which the Group By Key Reduction optimization transforms, can cause an internal server error.

-- Setup
CREATE OR REPLACE TABLE t1 ( c1 INT );
CREATE OR REPLACE TABLE t2 ( c1 INT, c2 VARCHAR(10));
ENFORCE GLOBAL INDEX ON t2( c1 );
ENFORCE GLOBAL INDEX ON t2( c2 );

-- Expected: Success
-- Observed: Error "Successfully reconnected after internal server error, transaction was rolled back"
SELECT IF t1.c1 THEN 1 ENDIF AS c
FROM t1 JOIN t2 ON t1.c1 = t2.c1
GROUP BY local.c, t2.c2;

Workaround

1. Use CASE instead of IF/ENDIF.
2. Reduce or disable Group By Key Reduction using the DB parameter to -groupByKeyReductionLevel=2, -groupByKeyReductionLevel=1 or -groupByKeyReductionLevel=0 (see Add database parameters).

-- Expected/Observed: Success
SELECT case when t1.c1 THEN 1 END AS c
FROM t1 JOIN t2 ON t1.c1 = t2.c1
GROUP BY local.c, t2.c2;

Fix

The query succeeds as expected.