2.9.3 Concurrent execution control of transactions

This subsection explains the processing method used when multiple transactions access the same row concurrently.

Organization of this subsection

(1) When transactions A and B reference the same row
(2) When transaction B references a row that has been updated by transaction A
(3) When transaction B attempts to update a row that has been updated by transaction A

(1) When transactions A and B reference the same row

Transactions A and B are not mutually affected.

(2) When transaction B references a row that has been updated by transaction A

The contents of the row to be referenced by transaction B are determined by the status of transaction A and the transaction isolation level of transaction B. The following table shows the row contents referenced by transaction B.

Table 2‒11: Contents of the row referenced by transaction B
No.	Status of transaction A	Transaction isolation level of transaction B
No.	Status of transaction A	READ COMMITTED	REPEATABLE READ
1	Being executed	Row's contents before being updated by transaction A	Row's contents before being updated by transaction A
2	Already committed	Row's contents after being updated by transaction A	Row's contents before being updated by transaction A

To Page Top

(3) When transaction B attempts to update a row that has been updated by transaction A

If transaction B attempts to update a row that has been updated by transaction A which is still executing, transaction B is rolled back.

To Page Top