ORA-01466: unable to read data - table definition has changed

What does it mean?

A query reading data from a past point in time (Flashback Query, AS OF or consistent read) failed because the table definition (DDL) changed between the queried past time and now. Oracle cannot safely read the data at that point in time with the current definition.

Why does it happen?

A DDL (adding/dropping a column, type change) was applied to the table after the timestamp targeted by the Flashback/AS OF query. Also, clock differences between client and server or queries of the very recent past (a few seconds) can raise this error due to SCN/time resolution.

How to fix it

Run the flashback query against a time/SCN before the table's DDL change. Where possible, use an exact SCN instead of a timestamp (AS OF SCN). For very recent past queries, go back a few seconds. If permanent historical access is needed, consider a versioning or archive table design instead of DDL.