If I run a query as an Oracle **18c** fiddle, it works:

    select * from dual

 | DUMMY |
 | :---- |
 | X     |

https://dbfiddle.uk/?rdbms=oracle_18&fiddle=2b82d5420b7c18626b6116960bc0fafe

------------

But if I run the same query as an Oracle **21c** fiddle, it fails:

    select * from dual
    
![image.png](/image?hash=9bb895896facbd87ec63b6e5be6b9636660788fef58624d3f5c3f4d542f24510)

    Error:
    
    dbfiddle.uk says
    
    Run failed
    
(I can't provide a db<>fiddle link of the result, since a query needs to ***run successfully*** in order to generate a permalink.)
    
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**Question:**

Why do Oracle 21c fiddles fail?