Crash or wrong results with table operator with LIMIT where first operand has a LIMIT clause

Detail name	Value
Changelog Number	27499
Type	Bug
Status	Resolved
Affected Versions	Exasol 7.1.0, Exasol 8.0.0, Exasol 8.29.0, Exasol 2025.1.0, Exasol 2025.2.0
Fix Versions	Exasol 2026.1.0, Exasol 2025.2.1
Resolution Date	2026-03-20

Description

If a table operator such as UNION or EXCEPT has a LIMIT clause and its first operand is not a query expression (i.e. is not "SELECT ..."), then the database can crash or give wrong results.

Example

-- Setup
create table A (A1 int);
insert into A values (5), (6);

-- Expected: 3 rows
-- Observed: internal server error
values between 1 and 5 union select * from A limit 3;

-- Expected: 3 rows
-- Observed: internal server error
(explain query graph select 1) union all select null, null, null, null, null, null, null, null, null, null limit 1;

-- Expected: 3 rows
-- Observed: 5 rows
values between 1 and 5 except select * from B limit 3;

Workaround

Wrapping the first operand of the UNION or EXCEPT in a SELECT fixes the problem.

select * from (values between 1 and 5) union all select * from B limit 3;
select * from (explain query graph select 1) union all select null, null, null, null, null, null, null, null, null, null limit 1;
select * from (values between 1 and 5) except select * from B limit 3;

Fix

Query doesn’t crash in these scenarios and gives the correct answer.