Read-consistency and multi-version of Oracle

Oracle always makes decisions based on a certain time point of the query (the query is consistent from this time point ). That is to say, Oracle will ensure that the opened result set must be the current result set at one of the following two points in time:

The time when the cursor is opened. This is the default behavior of the read committed isolation mode, which is the default transaction mode (Chapter 1 describes the differences between the read committed, read only, and SERIALIZABLE transaction levels ).

Query the start time of the transaction. This is the default behavior at the READ-ONLY and SERIALIZABLE isolation levels.

Flash back technology flashback --

Alter table t_nik enable row movement;

The role is to allow Oracle to modify the rowid assigned to the row. In Oracle, when a row is inserted, A rowid is assigned to it,

And this row will always have this rowid

Variable SCN number

Exec: scn: = dbms_flashback.get_system_change_number

Select * from t_nik as of scn: scn;

Data can be queried from the rollback segment at a specific time point.