This is a live demo for the database locking rules.

Following are the high-level main rules:
No locks put when the user is just reading data.
Oracle locks data at the row-level when the user updates the data, no lock escalation to the table, or block-level (only locking the modified rows).
The user updating data doesn't block a user from reading the same data. (Readers are not blocked by Writers).
A user modifying data will only be blocked if another user modifying the same data at the same time.


See also:
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