PushJoinsIntoUnions optimization with NULL constants in UNION ALL operand causes unexpected exception

Detail name	Value
Changelog Number	30753
Type	Bug
Status	Resolved
Affected Versions	Exasol 8.34.0
Fix Versions	Exasol 2026.1.0, Exasol 2025.1.11
Resolution Date	2026-05-15

Description

Query eligible for PushJoinsIntoUnions optimization with NULL constants in a wrapping select in UNION ALL operands throws unexpected exception like “Feature not supported: Incomparable Types"

Example

-- Preparation
create table A (A1 int);
create table A_ (A1 bigint);
create table B (B1 int, B2 int);

-- Expected: query runs without errors
-- Observed: "Feature not supported: Incomparable Types: BOOLEAN and DECIMAL(18,0)!" exception
select * from (
   select A1 Y from A
     union all
   select N from (select null N order by false))
   inner join B 
   on B1=y and B2 = y+2 and B2=3;

Workaround

1. Add explicit cast to a desired type in the UNION ALL operand OR on the lookup referencing UNION ALL. For the example from above:
2. Disable PushJoinsIntoUnions optimization by rewriting the query (e.g. wrapping UNION ALL with a simple select with ORDER BY FALSE). For the example from above:

select * from (
   select A1 Y from A
     union all
   select cast(N as int) from (select null N order by false))
   inner join B 
   on B1=y and B2 = y+2 and B2=3;

select * from (
   select * from (
     select A1 Y from A
       union all
     select N from (select null N order by false))
   order by false)
   inner join B 
   on B1=y and B2 = y+2 and B2=3;

Fix

Query runs as expected.