oracle的声音：

This can happen because the Oracle database presents the user with a consistent view of the data as it exists at the time the query starts running. If there are uncommitted changes to the table being queried, the Oracle database reads the undo data to get the committed version of data. This is read consistency. If the query runs so long that in the meantime those modifications are indeed committed, and subsequently their undo data is released and overwritten, then the long-running query no longer can see a consistent view of the data as of when it first began to run. For this reason, undo retention should be configured to accommodate the longest-running query.