Fix precedence of EXCEPT

Detail name	Value
Changelog Number	28180
Type	Bug
Status	Resolved
Affected Versions	Exasol 6.2.0, Exasol 7.0.0, Exasol 7.1.0, Exasol 8.0.0, Exasol 8.1.0
Fix Versions	Exasol 2026.1.0, Exasol 2025.2.1
Resolution Date	2026-03-20

Description

The table operators UNION and EXCEPT have equal operator precedence while INTERSECT has higher operator precedence. Table operators with equal operator precedence are left-associative, that is they are evaluated from left to right.

In Exasol, EXCEPT had an incorrect operator precedence higher than the other operators. This can lead to queries that give wrong results.

Example

-- Setup
create table A (C1 int);
insert into A values (1);
create table B (C1 int);
insert into B values (2);
create table C (C1 int);
insert into C values (4);

-- Q1: EXCEPT and INTERSECT
-- Expected: 1 row, values (1)
-- Observed: 0 rows
select * from A except select * from B intersect select * from C;

-- The database executes Q1 like this.
-- Expected/Observed: 0 rows
(select * from A except select * from B) intersect select * from C;

-- Q1 should be executed like this.
-- Workaround for Q1
-- Expected/Observed: 1 row, values (1)
select * from A except (select * from B intersect select * from C);

-- Q2: EXCEPT and UNION
-- Expected: 1 row, values (2)
-- Observed: 2 rows, values (1), (2)
select * from A union select * from B except select * from A;

-- The database executes Q2 like this.
-- Expected/Observed: 2 rows, values (1), (2)
select * from A union (select * from B except select * from A);

-- Q2 should be executed like this.
-- Workaround for Q2
-- Expected/Observed: 1 row, values (2)
(select * from A union select * from B) except select * from A;

Workaround

Use brackets to choose the correct order of evaluation.

Fix

The query returns the correct results.

Changed behavior

The operator precedence of the table operator EXCEPT was incorrectly higher than INTERSECT. We changed the operator precedence to be the same as UNION.