I issued a long running UPDATE query (an incorrect query) via the MySQL command line client and stopped it with Ctrl-C after a few seconds. The command had not finished running. Will my database be updated for some entries or does it happen in a transaction and everything is rolled back?
mysql> <LONG RUNNING INCORRECT UPDATE STATEMENT>
^CCtrl-C -- sending "KILL QUERY 12088743" to server ...
Ctrl-C -- query aborted.
ERROR 1317 (70100): Query execution was interrupted
mysql>
Update: All the tables involved in the query are InnoDB tables.
FOR INNODB : The Mysql manual says that InnoDB(a transactional storage engine) provides full ACID complaince. So therefore it will do complete all the operations at once or will not do the operations and rollback in case of an interruption. This is the default engine from MySQL 5.5 and up.
MySQL includes components such as the InnoDB storage engine that adhere closely to the ACID model, so that data is not corrupted and results are not distorted by exceptional conditions such as software crashes and hardware malfunctions.
FOR MYISAM :However for MyISAM storage engine which is non-transactional. Such storage engines follow a model where data is written one statement at a time . This is done using atomic operations. So if you interrupt the process then you will get it upto the point until you interrupted.
The nontransactional storage engines in MySQL Server (such as MyISAM) follow a different paradigm for data integrity called “atomic operations”. MyISAM tables effectively always operate in autocommit = 1 mode. Because changed data is written to disk one statement at a time, it is harder to guarantee the consistency of a sequence of related DML operations, which could be interrupted partway through. Thus, this mode is suitable for read-mostly workloads. In transactional terms, while each specific update is running, no other user can interfere with it, there can never be an automatic rollback, and there are no dirty reads.
However you can use LOCK TABLES as a workaround. This was the default storage engine before MySQL 5.5. *
So the answer depends on which storage engine you are using. Hope that helps :)
In InnoDb with autocommit enabled and a simple UPDATE it will initiate a rollback entirely. Rollbacks are important for ACID compliance, but can be a source of issues so careful use of forced rollback can mitigate those issues, which arise rarely compared to those of not having rollback.
There was a bug addressed in a previous version, where this did not occur as it should have: http://bugs.mysql.com/bug.php?id=45923.
From this question on the difference between autocommit on as a global var, vs. utilizing START TRANSACTION/COMMIT behavior you can learn more about some best practices.
As far as i know MySQL queries are "atomic" which means your database should looks as if your query was never run
Related
I was testing transaction support in innoDB tables, and just for the curriosity I tried to run the same transaction on MyIsam table, and surprisingly it worked. I am assuming that on myIsam table queries are executed one after another not in one atomic operation and I don't get any errors from START TRANSACTION and COMMIT and ROLLBACK operations. I am interested, is MyIsam engine just ignoring this operations or does it perform some actions?
MyISAM effectively works in auto-commit mode (as it's not a transactional engine), and it just ignores the commit/rollback.
Actually storage engine is a different layer in the MySQL architecture, separated from the SQL parser, the SQL layer communicates to the storage engine with lower-level API, and that's the reason there is a common SQL and engines, supporting different subset of featured. You can see very high-level overview of the architecture here
MyIsam tabels were not built for this. Not even in the 5+ versions. It only was meant to store data. It gives you no guarantee for transactions or data recovery. You should use InnoDB for this and, if needed, use MyIsam for replication purposes (it's faster to retrieve data with MyIsam tables since there's no cross-table checks).
You may read this post from MySQL
http://forums.mysql.com/read.php?21,68686,69229#msg-69229
It is my understanding that MySQL creates an execution plan from a SQL query, and then uses innodb (or any other storage engine) to execute the plan. If this is the case, then why does the innodb storage engine have its own parser, server main program, and user-session modules? It looks as if InnoDB could run on its own as a fully functional DBMS.
InnoDB began as an independent company in 1995. The founder wanted to create a standalone RDBMS server.
It wasn't until 2000 that InnoDB began working closely with MySQL, and by March 2001 they announced the InnoDB Table Handler, which allowed MySQL to delegate work to the storage engine.
But InnoDB wanted to support some features that MySQL did not support:
FOREIGN KEY constraints
Proprietary table options
Transactions
MySQL wanted to allow InnoDB and other storage engines to implement their own features too. So they allowed the storage engine layer to perform their own SQL parsing. There are a number of features (like CHECK constraints) that are validated for syntax by the MySQL storage-independent layer, without implementing the semantics. It's up to the storage engine to perform extra parsing and implement those features.
There have also been cases where the InnoDB storage engine wanted to implement features that had no SQL support at the higher level.
For example, the InnoDB monitor, to output periodic troubleshooting data to the server's error log, could be enabled not by sensible syntax like SET ENGINE INNODB MONITOR=ON or something like that, but by creating a table with a special name:
CREATE TABLE innodb_monitor (a INT) ENGINE=INNODB;
It doesn't matter which schema you create this table in, nor what columns you put in it. It doesn't need any rows of data. The name itself is special to InnoDB, and it's a signal to start logging monitor data to the log. Just so they didn't have to implement a new configuration option or SQL syntax!
In later versions of MySQL, you can enable the monitor in a less hacky way with SET GLOBAL innodb_status_output=ON.
I know there is one issue in MySQL with concurrent SELECT and INSERT. However, my question is if I open up two connections with MySQL and keep loading data using both of them, does MySQL takes data concurrently or waits for one to finish before loading another?
I’d like to know how MySQL behaves in both cases. Like when I am trying to load data in the same table or different tables concurrently when opening separate connections.
If you will create a new connection to the database and perform inserts from both the links, then from the database's perspective, it will still be sequential.
The documentation of Concurrent Inserts for MyISAM on the MySQL's documentation page says something like this:
If MyISAM storage is used and table has no holes, multiple INSERT statements are queued and performed in sequence, concurrently with the SELECT statements.
Mind that there is no control over the order in which two concurrent inserts will take place. The order in this concurrency is at the mercy of a lot of different factors. To ensure order, by default you will have to sacrifice concurrency.
MySQL does support parallel data inserts into the same table.
But approaches for concurrent read/write depends upon storage engine you use.
InnoDB
MySQL uses row-level locking for InnoDB tables to support simultaneous write access by multiple sessions, making them suitable for multi-user, highly concurrent, and OLTP applications.
MyISAM
MySQL uses table-level locking for MyISAM, MEMORY, and MERGE tables, allowing only one session to update those tables at a time, making them more suitable for read-only, read-mostly, or single-user applications
But, the above mentioned behavior of MyISAM tables can be altered by concurrent_insert system variable in order to achieve concurrent write. Kindly refer to this link for details.
Hence, as a matter of fact, MySQL does support concurrent insert for InnoDB and MyISAM storage engine.
You ask about Deadlock detection, ACID and particulary MVCC, locking and transactions:
Deadlock Detection and Rollback
InnoDB automatically detects transaction deadlocks and rolls back a
transaction or transactions to break the deadlock. InnoDB tries to
pick small transactions to roll back, where the size of a transaction
is determined by the number of rows inserted, updated, or deleted.
When InnoDB performs a complete rollback of a transaction, all locks
set by the transaction are released. However, if just a single SQL
statement is rolled back as a result of an error, some of the locks
set by the statement may be preserved. This happens because InnoDB
stores row locks in a format such that it cannot know afterward which
lock was set by which statement.
https://dev.mysql.com/doc/refman/5.6/en/innodb-deadlock-detection.html
Locking
The system of protecting a transaction from seeing or changing data
that is being queried or changed by other transactions. The locking
strategy must balance reliability and consistency of database
operations (the principles of the ACID philosophy) against the
performance needed for good concurrency. Fine-tuning the locking
strategy often involves choosing an isolation level and ensuring all
your database operations are safe and reliable for that isolation
level.
http://dev.mysql.com/doc/refman/5.5/en/glossary.html#glos_locking
ACID
An acronym standing for atomicity, consistency, isolation, and
durability. These properties are all desirable in a database system,
and are all closely tied to the notion of a transaction. The
transactional features of InnoDB adhere to the ACID principles.
Transactions are atomic units of work that can be committed or rolled
back. When a transaction makes multiple changes to the database,
either all the changes succeed when the transaction is committed, or
all the changes are undone when the transaction is rolled back. The
database remains in a consistent state at all times -- after each
commit or rollback, and while transactions are in progress. If related
data is being updated across multiple tables, queries see either all
old values or all new values, not a mix of old and new values.
Transactions are protected (isolated) from each other while they are
in progress; they cannot interfere with each other or see each other's
uncommitted data. This isolation is achieved through the locking
mechanism. Experienced users can adjust the isolation level, trading
off less protection in favor of increased performance and concurrency,
when they can be sure that the transactions really do not interfere
with each other.
http://dev.mysql.com/doc/refman/5.5/en/glossary.html#glos_acid
MVCC
InnoDB is a multiversion concurrency control (MVCC) storage engine
which means many versions of the single row can exist at the same
time. In fact there can be a huge amount of such row versions.
Depending on the isolation mode you have chosen, InnoDB might have to
keep all row versions going back to the earliest active read view, but
at the very least it will have to keep all versions going back to the
start of SELECT query which is currently running
https://www.percona.com/blog/2014/12/17/innodbs-multi-versioning-handling-can-be-achilles-heel/
It depends.
It depends on the client -- some clients allow concurrent access; some will serialize access, thereby losing the expected gain. You have not even specified PHP vs Java vs ... or Apache vs ... or Windows vs ... Many combinations simply do not provide any parallelism.
If different tables, there is only general contention for I/O, CPU, Mutexes on the buffer_pool, etc. A reasonable amount of parallelism is possible.
If same table, it depends on the indexes and access patterns. In some cases the threads will block each other. In some cases it will even "deadlock" and rollback one of the transactions. Deadlocks not only slow you down, but make you retry the inserts.
If you looking for high speed ingestion of a lot of rows, see my blog. It lays out techniques, and points out sever of the ramifications, such as replication, Engine choice, multi-threading.
Multiple threads inserting into the same tables -- It depend a lot on the values you are providing for any PRIMARY or UNIQUE keys. It depends on whether other actions are taken in the same transaction. It depends on how much I/O is involved. It depends on whether you are doing single-row inserts, or batching. It depends on ... (Sorry to be vague, but your question is not very specific.)
If you would like to present specifics on two or three designs, we can discuss the specifics.
I'm starting out with MySQL trnsactions and I have a doubt:
In the documentation it says:
Beginning a transaction causes any pending transaction to be
committed. See Section 13.3.3, “Statements That Cause an Implicit
Commit”, for more information.
I have more or less 5 users on the same web application ( It is a local application for testing ) and all of them share the same MySQL user to interact with the database.
My question is: If I use transactions in the code and two of them start a transaction ( because of inserting, updating or something ) Could it be that the transactions interfere with each other?
I see in the statements that cause an implicit commit Includes starting a transaction. Being a local application It's fast and hard to tell if there is something wrong going on there, every query turns out as expected but I still have the doubt.
The implicit commit occurs within a session.
So for instance you start a transaction, do some updates and then forget to close the transaction and start a new one. Then the first transaction will implicitely committed.
However, other connections to the database will not be affected by that; they have their own transactions.
You say that 5 users use the same db user. That is okay. But in order to have them perform separate operations they should not use the same connection/session.
With MySQl by default each connection has autocommit turned on. That is, each connection will commit each query immediately. For an InnoDb table each transaction is therefore atomic - it completes entirely and without interference.
For updates that require several operations you can use a transaction by using a START TRANSACTION query. Any outstanding transactions will be committed, but this won't be a problem because mostly they will have been committed anyway.
All the updates performed until a COMMIT query is received are guaranteed to be completed entirely and without interference or, in the case of a ROLLBACK, none are applied.
Other transations from other connections see a consistent view of the database while this is going on.
This property is ACID compliance (Atomicity, Consistency, Isolation, Durability) You should be fine with an InnoDB table.
Other table types may implement different levels of ACID compliance. If you have a need to use one you should check it carefully.
This is a much simplified veiw of transaction handling. There is more detail on the MySQL web site here and you can read about ACID compliance here
I was testing transaction support in innoDB tables, and just for the curriosity I tried to run the same transaction on MyIsam table, and surprisingly it worked. I am assuming that on myIsam table queries are executed one after another not in one atomic operation and I don't get any errors from START TRANSACTION and COMMIT and ROLLBACK operations. I am interested, is MyIsam engine just ignoring this operations or does it perform some actions?
MyISAM effectively works in auto-commit mode (as it's not a transactional engine), and it just ignores the commit/rollback.
Actually storage engine is a different layer in the MySQL architecture, separated from the SQL parser, the SQL layer communicates to the storage engine with lower-level API, and that's the reason there is a common SQL and engines, supporting different subset of featured. You can see very high-level overview of the architecture here
MyIsam tabels were not built for this. Not even in the 5+ versions. It only was meant to store data. It gives you no guarantee for transactions or data recovery. You should use InnoDB for this and, if needed, use MyIsam for replication purposes (it's faster to retrieve data with MyIsam tables since there's no cross-table checks).
You may read this post from MySQL
http://forums.mysql.com/read.php?21,68686,69229#msg-69229