Dyntex.net Support Centre: MySQL Reference Manual p.519

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Table of Contents
1. General Information
2. MySQL Installation
3. Tutorial Introduction
4. Database Administration
5. MySQL Optimisation
6. MySQL Language Reference
7. MySQL Table Types
8. MySQL APIs
9. Extending MySQL

Chapter 7: MySQL Table Types 519 copying all the relevant les, which we already listed in the previous section on backing up a database. If the oating-point formats on the machines are dierent but you have not used FLOAT or DOUBLE data types in your tables then the procedure is the same: just copy the relevant les. If the formats are dierent and your tables contain oating-point data, you have to use `mysqldump' and `mysqlimport' to move those tables. A performance tip is to switch o the auto commit when you import data into your database, assuming your tablespace has enough space for the big rollback segment the big import transaction will generate. Do the commit only after importing a whole table or a segment of a table. 7.5.8 InnoDB Transaction Model In the InnoDB transaction model the goal has been to combine the best properties of a multi-versioning database to traditional two-phase locking. InnoDB does locking on row level and runs queries by default as non-locking consistent reads, in the style of Oracle. The lock table in InnoDB is stored so space-eciently that lock escalation is not needed: typically several users are allowed to lock every row in the database, or any random subset of the rows, without InnoDB running out of memory. In InnoDB all user activity happens inside transactions. If the auto-commit mode is used in MySQL, then each SQL statement will form a single transaction. If the auto commit mode is switched o, then we can think that a user always has a transaction open. If he issues the SQL COMMIT or ROLLBACK statement, that ends the current transaction, and a new starts. Both statements will release all InnoDB locks that were set during the current transaction. A COMMIT means that the changes made in the current transaction are made permanent and become visible to other users. A ROLLBACK on the other hand cancels all modications made by the current transaction. 7.5.8.1 Consistent Read A consistent read means that InnoDB uses its multi-versioning to present to a query a snapshot of the database at a point in time. The query will see the changes made by exactly those transactions that committed before that point of time, and no changes made by later or uncommitted transactions. The exception to this rule is that the query will see the changes made by the transaction itself which issues the query. When a transaction issues its rst consistent read, InnoDB assigns the snapshot, or the point of time, which all consistent reads in the same transaction will use. In the snapshot are all transactions that committed before assigning the snapshot. Thus the consistent reads within the same transaction will also be consistent with respect to each other. You can get a fresher snapshot for your queries by committing the current transaction and after that issuing new queries. Consistent read is the default mode in which InnoDB processes SELECT statements. A consistent read does not set any locks on the tables it accesses, and therefore other users are free to modify those tables at the same time a consistent read is being performed on the table.

MySQL Reference Manual

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