Does MySQL automatically peform read / write table locks on MyIsam tables, or do I have to explicitly lock the tables?
Beyond making reasonable efforts to make certain statements atomic, MyISAM doesn't have the concept of transactions and its related row level locking.
Therefore, you should use LOCK TABLES to avoid race conditions or data inconsistencies when you use multiple statements (e.g. a SELECT statement followed by multiple related UPDATE statements).
See here about Pros and Cos Of MyISAM
MyISAM's About Internal Locking
MyISAM uses table-level locking. When a row is inserted or updated, all other changes to that table are held up until that request has been completed.
Correct me if 'm wrong
Related
I have a MYISAM table on which long-running SELECT statements are conducted. Do these statements cause locks on the tables that are being read? If so, how do you stop the locks from occurring?
According to https://dev.mysql.com/doc/refman/5.7/en/internal-locking.html:
MySQL uses table-level locking for MyISAM, MEMORY, and MERGE tables,
permitting only one session to update those tables at a time. This
locking level makes these storage engines more suitable for read-only,
read-mostly, or single-user applications.
Does this also mean that SELECT statements create locks?
Do these [long-running SELECT] statements cause locks on the tables that are being read?
Yes. A MyISAM table cannot be updated while it is being read from. If there is a long-running read (i.e, SELECT) query on the table, any UPDATE or DELETE queries will have to wait until the read query is complete.
(INSERT queries are sometimes an exception. So long as there are no internal holes in the table structure, a writer may be allowed to insert a row at the end of a table without obtaining a lock. However, if any rows are ever updated or deleted, this may create holes, which will require INSERT queries to obtain a write lock as well.)
For more information on the internals, read up on table-level locks. But the short and the long of matters is, you probably shouldn't use MyISAM if concurrency is a concern.
My question is a follow up to this answer. I want to find out how to perform a select statement without locking a table with MyISAM engine.
The answer states the following if you have InnoDB but not MyISAM. What is the equivalent for MyISAM engine?
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED ;
SELECT * FROM TABLE_NAME ;
COMMIT ;
This is the default behaviour with MyISAM tables. If one actually wants to lock a MyISAM table, one must manually acquire a table-level lock. Transaction isolation level, START TRANSACTION, COMMIT, ROLLBACK have no effect on MyISAM tables behaviour since MyISAM does not support transactions.
More about internal locking mechanisms
A READ lock is implicitely acquired before, and released after execution of a SELECT statement. Notice that several concurrent, simultaneous, SELECT statements could be running at the same time, because several sessions may hold a READ lock on the same table.
Conversely, a WRITE lock is implicitely acquired before executing an INSERT or UPDATE or DELETE statement. This means that no read (let alone a concurrent write) can take place as long as a write is in progress*.
The above applies to MyISAM, MEMORY, and MERGE tables only.
You might want to read more about this here:
Internal locking methods
Read vs Write locks
* However, these locks are not always required thanks to this clever trick:
The MyISAM storage engine supports concurrent inserts to reduce contention between readers and writers for a given table: If a MyISAM table has no free blocks in the middle of the data file, rows are always inserted at the end of the data file. In this case, you can freely mix concurrent INSERT and SELECT statements for a MyISAM table without locks.
MyISAM does indeed use a read lock during SELECT. An INSERT at the end of the table can get around that.
But try doing an UPDATE, DELETE, or ALTER TABLE while a long-running SELECT is in progress. Or vice-versa, reading from a table while a change to that table is running. It's first-come, first-serve, and the later thread blocks until the first thread is done.
MyISAM doesn't have any support for transactions, so it must work this way. If a SELECT were reading rows from a table, and a concurrent thread changes some of those rows, you would get a race condition. The SELECT may read some of the rows before the change, and some of the rows after the change, resulting in a completely mixed-up view of the data.
Anything you do with SET TRANSACTION ISOLATION LEVEL has no effect with MyISAM.
For these reasons, it's recommended to use InnoDB instead.
I am using mysql with InnoDB databases.
If all my transactions are Inserts and Selects (no updates), I assume I would not have to worry about SQL deadlocking.
I can't see a scenario where deadlocking would occur. Am I correct to assume deadlocking cannot occur if I only do Inserts and Selects?
May not be relevant but everything transaction is done with PDO
No. You still have to worry about SQL deadlocking.
You can get deadlocks even in the case of a transaction that inserts a single row. This is because the insert operation is not really atomic and locks are set automatically on the (possibly several) index records of the inserted row.
InnoDB MySQL storage engine has row level locks while the MyISAM MySQL storage engine has table level locks. MyISAM simply locks entire tables, and doesn't support transactions, so it's not possible to have database-level deadlocks. Note that an app can lock up another app by sitting on a table lock on the table they are both trying to access, but this is a code error, not a db-level "deadlock".
InnoDB supports transactions and has row-level locks, so db-level deadlocks are possible (and can happen occasionally in a busy system so you do need to code around them). Many of what MySQL will call "deadlocks" aren't "true deadlocks" as much as they're the result of slow UPDATEs causing other queries to time out on row locks.
This is a two part question. When I perform a SELECT on a table, am I locking that table from any usage while the SELECT is running?
If so, what is a way to not lock the table while performing a SELECT statement? I'm using MySQL 4.1.20 MyISAM.
update there is a similar question here Any way to select without causing locking in MySQL? but the answer doesn't work with MyISAM
Yes, with MyISAM tables, the select locks the table for inserts/updates. However several selects can run concurrently (i.e. it applies a read lock). If there's no free space in the middle of the table, inserts will add data to the end of the (internal) storage, and those inserts can still be run concurrently with the selects though.
More info here. Note that MyISAM and InnoDB works very differently in this regard.
There's table locking and row locking. I recommend you read up on database engines MyISAM with Table Locking and InnoDB Row Locking
Think about it: are you changing any table cells when you do your SELECT? If not, there's no need to lock the table and MySQL doesn't. Table locking has to happen at UPDATE time, not at SELECT time.
I need to use a table for a queuing system. The table will be constantly be updated.
For example, multiple users on my website, will add their files for process, and I heard that when updates occur simultaneously from multiple users, the table becomes non responsive or something like that.
so do I need locking tables in this situation ? how do i apply a lock to a mysql table ?
By constantly be updated do you mean it will be appended to 10,000 times per second? Even for middle-range servers, that still presents 10,000 opportunities per second for the table to be shared by other users.
It's only necessary to lock the table when several dependent operations need to occur as a unit. In most cases, it is sufficient to include the series of operations in a database transaction. If the "constant updates" are merely insert sometable values ( ...), then it will be easy to guarantee transaction consistency.
so do I need locking tables in this situation ?
All tables can be locked, there isn't a special type of table. That said, deal with the issue when you run into deadlocks. Isolation levels are a closely related topic as well.
how do i apply a lock to a mysql table ?
There's the LOCK TABLES syntax - this article covers how & why you'd want to lock tables.
When you do several updates at once, you can get into a deadlock situation. Engines such as InnoDB will detect this and fail one of your transactions (You can retry, but only the whole transaction).
You can avoid this by table locking, but it reduces concurrency.
Engines such as MyISAM (which does not support MVCC or transactions anyway) use table locking implicitly anyway. Such table locks exist only for the duration of a query; they're automatically released soonish after the table isn't needed any more (not necessarily as soon as possible, but quite soon)
I recommend you do not use LOCK TABLE unless you feel you really need to; if you get a deadlock, retry the transaction.