SQL> explain plan for select 1 from dual where nullif(dummy,'x') is null;

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
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Plan hash value: 272002086

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |     2 |     2   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| DUAL |     1 |     2 |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
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PLAN_TABLE_OUTPUT
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   1 - filter(CASE "DUMMY" WHEN 'x' THEN NULL ELSE "DUMMY" END  IS NULL)

13 rows selected.

SQL>