如果查询超时，会发生什么情况？

Question

Let's say I have a query that is sent to my SQL-Server database, it takes more than 30 seconds, and my program throws an SQL Query Timeout exception. Is the query still chugging along on my database or does it get terminated as soon as the exception is thrown?

Answer 1

A client signals a query timeout to the server using an attention event. An attention event is simply a distinct type of TDS packet a SQL Server client can send to it. In addition to connect/disconnect, T-SQL batch, and RPC events, a client can signal an attention to the server. An attention tells the server to cancel the connection's currently executing query (if there is one) as soon as possible. An attention doesn't rollback open transactions, and it doesn't stop the currently executing query on a dime -- the server aborts whatever it was doing for the connection at the next available opportunity. Usually, this happens pretty quickly, but not always.

SourceThere's no such thing as a query timeout...

Answer 2

When the client decides that the command has run long enough, it issues an "Abort". The query simply stops running in the database.

Any CATCH block won't be hit, transactions will be left open and locks can still remain allocated after this, even if the connection is closed because "close" means "return to connection pool".

If you expect a lot of Command Timeouts then consider using SET XACT_ABORT ON ( and this too ) that will release locks and rollback transactions. or fix the code...

Answer 3

Before executing a query, SQL Server estimates how much memory it needs to run and tries to reserve this amount of memory from the buffer pool. If the reservation succeeds the query is executed immediately. If there is not enough memory readily available from the buffer pool, then the query is put into a queue with a timeout value, where the timeout value is guided by the query cost. The basic rule is: higher the estimated cost is, larger the time out value is. When the waiting time of this query exceeds the timeout value, a time out error is thrown and the query is removed from the queue.

Source

Answer 4

If you get a SQL timeout then SQL has stopped, however web applications can time out and the SQL query can continue.

Answer 5

Usually when it times out it means the connection has died, meaning the query has not been sent to the database, most database support Transactions where you can start a transaction, run your queries, and if your happy you can commit them.

Example:

BEGIN TRAN

UPDATE  authors
   SET  au_fname = 'John'
WHERE   au_id = '172-32-1176'

UPDATE  authors
   SET  au_fname = 'Marg'
WHERE   au_id = '213-46-8915'

COMMIT TRAN