在Oracle DB使用PL/SQL時，其中一段查詢DBA_TABLES的SQL，出現了一個很少見的錯誤：ORA-25191: cannot reference overflow table of an index-organized table
SQL：select * from dba_tables where …，這段SQL沒有什麼特殊的語法，但是就出現了ORA-25191，其發生錯誤的表是這個：SYS_IOT_OVER_403318。
在網路查找問題文章時，發現有人說這是一個Bug：
See [MOSC](http://support.oracle.com/), there is a bug in Oracle 11.2 that makes the automated task that gathers statistics raise the ORA-25191 error on some databases upgraded from 10.2 that contain IOT overflow tables.
If the bug does not apply to you, your query *where* clause should filter out all IOT_OVERFLOW Tables. You need a WHERE clause to filter out index-organized table mapping segments & overflow segments.
而這個SYS_IOT_OVER_403318的IOT_TYPE的確是IOT_OVERFLOW，所以在SQL加上where iot_type IS NULL or iot_type != 'IOT_OVERFLOW'，果然就解決了。
資料來源：[ORA-25191: Cannot reference overflow table of an index-organized table tips (dba-oracle.com)](https://www.dba-oracle.com/t_ora_25191_cannot_reference_overflow_of_an_iot.htm)
