Underlying mechanism mvcc

Isolation level implementation principle (MVCC)

MVCC( Multi version concurrency control Multi-Version Concurrent Control) yes MySql One way to improve performance , coordination Undo log And version chain , Let different things read - Write , Write - Loss operations can be executed concurrently , So as to improve the system performance .

MVCC Make the database read will not lock the data , ordinary SELECT The request will not be locked , It improves the concurrent processing ability of data . With the help of MVCC, The database can be implemented ( Read submitted )READ COMMITTED,( Repeatable )REPETABLE READ Wait for the isolation level .

InnoDB Of MVCC This is achieved by saving two hidden columns after each row of records , A thing that preserves rows Id(DB_TRX_ID), A rollback pointer that holds the row (DB_ROLL_PT).

db_trx_id: Every time a cluster index record is changed , Will put the corresponding transaction id Assign a value to trx_id Hide columns .

db_roll_pt: Every time a cluster index record is changed , Will write the old version to undo log In the log , Then the hidden column is equivalent to a pointer , It can be used to find the information before the modification of the record .

ReadView

Read View When the transaction is started , A collection of all current transactions , This data structure stores the current Read View The largest of ID And the smallest ID

For the use of READ UNCOMMITTED( Read uncommitted ) For isolation level transactions , Just read the latest version directly , For the use of SERIALIZABLE( Serialization ) For isolation level transactions , Using lock to access records .

For the use of READ COMMITTED( Read submitted ) and REPETABLE READ( Repeatable ), You need to use version chain , Core issues : You need to determine which version in a version chain is visible to the current transaction .

ReadView: It mainly includes those active read-write transactions in the current system , Put their affairs id Put it in a table .

m_ids: It means generating ReadView Active read / write transactions in the current system id list .

min_trx_id: Generate ReadView It is the smallest thing in the active read-write transactions in the current system id, That is to say m_ids Minimum of .

max_trx_id: Generate ReadView Should be assigned to the next transaction in the system id, Not an active list m_ids Maximum of .

creator_trx_id: Generate this ReadView Affairs of things id;

Current version transactions id, At present ReadView Of create_trx_id(cid).

if id=cid, Indicates that the current transaction is modified by itself , Can be accessed , Directly return to this version .

if id<cid, Indicates the modification of the transaction committed before the current transaction , Can be accessed .

if id>ReadView Of mix_trx_id, Indicates that the version generated by transaction modification is enabled after the transaction , Can't be accessed , Continue to access backward along the version chain .

If this version of the transaction id At present ReadView Of min_trx_id and max_trx_id We need to judge , Modified this version of the transaction id Is it active , That is to say m_ids In the list , It means that the transaction of this version has not been committed at this time , Then this version cannot be accessed . If not active , The description is modified by the submitted transaction , Can be accessed .

If the version chain of this record is accessed , Are inaccessible , There is no such record in the query result .

READ COMMITTED-- Generate a... Before each read ReadView. The current reading

REPEATBLE READ-- Generate a... When reading data for the first time ReadView Read the snapshot .