What are the four isolation levels of transactions?

The four isolation levels of transactions are: 1. Read uncommitted content (Read Uncommitted); 2. Read submitted content (Read Committed); 3. Repeatable Read (Repeatable Read); 4. Can Serializable.

Four isolation levels of transactions:

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1. Read Uncommitted -- Read uncommitted content

• A transaction can view uncommitted content

• Dirty reading problems often occur (dirty reading: reading uncommitted (executed) content of other transactions)

Open two transactions A and B (A , B transaction cross) start transaction

A transaction only queries the content in the data table, B transaction performs addition, deletion and modification operations but does not commit(submit)

A transaction can still query the data in the table Change (query uncommitted content--dirty reading)

2. Read Committed -- read the submitted content

• One transaction only Ability to view submitted content

• often causes non-repeatable read problems (non-repeatable read: executing the same select statement in the same transaction to obtain different results)

Open two transactions A and B for the same data table (A and B transactions intersect) start transaction

A transaction only queries the content in the data table, and B transaction performs addition, deletion and modification operations but does not commit ( Submit)

A transaction query cannot find the content of the data changes in the table

B transaction submission

A finds the data changes (A queries twice, resulting in different Result - non-repeatable read)

3. Repeatable Read -- rereadable

• When multiple instances of the same transaction read data concurrently Get the same result

• MySQL's default transaction isolation level

• often causes phantom reading problems (phantom reading: different results when reading multiple times) (Phantom row))

Open two transactions A and B for the same data table (A and B transactions intersect) start transaction

A transaction only queries the data table Content, transaction B performs addition, deletion and modification operations but does not commit

A transaction query cannot find the content of the data changes in the table

B transaction submission

A transaction query The content of the data changes in the table cannot be found

A submits

A can query the changes in the data in the table

4. Serializable -- Serializable

• The highest isolation level

• Add a shared lock to the transaction, and there can only be one transaction operation at the same time to solve the phantom read problem

• Will lead to a large number of timeouts and lock competition issues

Open transaction A

This operation cannot be added or deleted when transaction B is opened

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