I know that you would normally apply WITH (NOLOCK) at the table level but let's say hypothetically you'd like to join 15 tables together. Is there an easier way to apply WITH (NOLOCK) to all of the tables without having to write it after each table name?
You can set the transaction isolation level to read uncommitted:
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
SELECT ...
This effectively treats the entire transaction as a WITH (NOLOCK)
From MSDN:
READ UNCOMMITTED
Specifies that statements can read rows that have been modified by other transactions but not yet committed.
Transactions running at the READ UNCOMMITTED level do not issue shared locks to prevent other transactions from modifying data read by the current transaction. READ UNCOMMITTED transactions are also not blocked by exclusive locks that would prevent the current transaction from reading rows that have been modified but not committed by other transactions. When this option is set, it is possible to read uncommitted modifications, which are called dirty reads. Values in the data can be changed and rows can appear or disappear in the data set before the end of the transaction. This option has the same effect as setting NOLOCK on all tables in all SELECT statements in a transaction. This is the least restrictive of the isolation levels.
Related
What is the relation between isolation levels of relational databases and select for update?
If I use the plain vanilla JDBC connectivity with SQL Server and set isolation level to READ_REPEATABLE and use a simple select, would I see inconsistency in repeatable reads or not? Or should I use select for update always to avoid inconsistent repeatable reads in transactions? If so, what is the deal with isolation levels, how they would come into play?
SQL Server doesn't have the select ... for update syntax. The equivalent in SQL Server is to use the UPDLOCK table hint.
This hint is used when reading a row and immediately updating the same row in an atomic transaction. EG
declare #balance = (select balance from account where accountId = #id)
update account set balance = #balance + #amount where accountId = #id
At READ COMMITTED, or in any isolation level without a mult-statement transaction, multiple sessions can run the first query, and have lost updates when updating the balance.
Using the REPEATABLE READ or SERIALIZABLE isolation level will prevent this update anomaly, but they do it by blocking the first writer if there are any concurrent transactions that have read the row, and if one of the other transactions attempts to update the row, causing a deadlock.
Mostly this behavior is not worth the performance cost and annoyance of handling deadlocks. So you use 'select for update' aka UPDLOCK to place a U lock on the row while reading and block subsequent readers from acquiring a conflicting lock.
eg
declare #balance = (select balance from account with (updlock) where accountId = #id)
update account set balance = #balance + #amount where accountId = #id
Not exactly sure what you mean by "inconsistent repeatable reads" but with the REPEATABLE READ isolation level in SQL Server, shared locks will be held until the end of the transaction. Only committed data will be returned and other sessions cannot modify the rows read until the transaction is committed (or auto-committed in the case of no explict transaction).
REPEATABLE READ does not prevent other sessions from inserting new rows so rerunning the same query in the same transaction may return new rows (phantom reads) that were not returned originally.
The documentation describes this in more detail along with the other transaction isolation levels.
There is no need to intervene with the default behavior using hints or otherwise.
SQL Server guarantees transaction isolation levels per your declared required level.
REPEATABLE READ guarantees that within a transaction, your statements will see a consistent view of the data that was read, but doesn't guarantee there will be no phantom rows. To avoid the latter you will need to use SERIALIZABLE.
The higher the isolation, the higher the concurrency penalty of course.
See the JDBC Page for isolation levels and the general page for SET TRANSACTION ISOLATION LEVEL for a somewhat more in-depth discussion.
HTH
I'm defining the isolation level as READ UNCOMMITTED because this is a long running process on a few tables and there's no risk for dirty read because I'm just inserting new data.
Based on my understanding, because I'm using this isolation level, I should be able to execute SELECT statements from the table where I'm inserting rows but I can't, it gets blocked.
Why is this isolation level blocking the SELECT statement?
It is supposed to allow to query those tables and, in the worst case scenario, retrieve dirty data.
Just in case this helps, I'm working with a CURSOR (I know, I hate them too but I did not write this code) over really big data and multiple tables.
When you do an insert (regardless of what the isolation level is) locks occur. There is no way to insert rows without doing some locking. The isolation levels affect reading.
If you do a select * from tablea with (nolock) you will read uncommitted rows but for a brief period (because of the insert) a lock will occur.
If you have the following sql, is it possible that if it is run multiple times by many different processes at exactly the same time, that two or more processes may update the table?
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
UPDATE table
SET Column1 = 1
WHERE Column1 = 0
No other locks etc are specified in the sql, other that Read Uncommitted.
I'm trying to track down an issue, and I'm now clutching at straws...
Got this from MSDN.
Transactions running at the READ UNCOMMITTED level do not issue shared locks to prevent other transactions from modifying data read by the current transaction. READ UNCOMMITTED transactions are also not blocked by exclusive locks that would prevent the current transaction from reading rows that have been modified but not committed by other transactions. When this option is set, it is possible to read uncommitted modifications, which are called dirty reads. Values in the data can be changed and rows can appear or disappear in the data set before the end of the transaction. This option has the same effect as setting NOLOCK on all tables in all SELECT statements in a transaction. This is the least restrictive of the isolation levels.
So basically, this is equivalent to SQL Server , NOLOCK hint. This might result in dirty reads, i.e. if some process in updated 1000 records and updated 500 till now, and other process read that data, then data might be in inconsistent form. This also helps in executing update without getting blocked (shared lock) by multiple select queries.
Hope this make some sense to your question. For reference -- MSDN
So I've got a query that keeps deadlocking on me. People who know the system well can't figure out why the sproc is deadlocking, but they tell me that I should just add this to it:
SET NOCOUNT ON
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
Is this really a valid solution? What does that do?
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
This will cause the system to return inconsitent data, including duplicate records and missing records. Read more at Previously committed rows might be missed if NOLOCK hint is used, or here at Timebomb - The Consistency problem with NOLOCK / READ UNCOMMITTED.
Deadlocks can be investigated and fixed, is not a big deal if you follow the proper procedure. Of course, throwing a dirty read may seem easier, but down the road you'll be sitting long hours staring at your general ledger and wondering why the heck it does not balance debits and credits. So read again until you really grok this: DIRTY READs ARE INCONSISTENT READS.
If you want a get-out-of-jail card, turn on snapshot isolation:
ALTER DATABASE MyDatabase
SET READ_COMMITTED_SNAPSHOT ON
But keep in mind that snapshot isolation does not fix the deadlocks, it only hides them. Proper investigation of the deadlock cause and fix is always the appropriate action.
NOCOUNT will keep your query from returning rowcounts back to the calling application (i.e. 1000000 rows affected).
TRANSACTION ISOLATION LEVEL READ UNCOMMITTED will allow for dirty reads as indicated here.
The isolation level may help, but do you want to allow dirty reads?
Randomly adding SET options to the query is unlikely to help I'm afraid
SET NOCOUNT ON
Will have no effect on the issue.
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
will prevent your query taking out shared locks. As well as reading "dirty" data it also can lead to your query reading the same rows twice, or not at all, dependant upon what other concurrent activity is happening.
Whether this will resolve your deadlock issue depends upon the type of deadlock. It will have no effect at all if the issue is 2 writers deadlocking due to non linear ordering of lock requests. (transaction 1 updating row a, transaction 2 updating row b then tran 1 requesting a lock on b and tran 2 requesting a lock on a)
Can you post the offending query and deadlock graph? (if you are on SQL 2005 or later)
The best guide is:
http://technet.microsoft.com/es-es/library/ms173763.aspx
Snippet:
Specifies that statements can read rows that have been modified by other
transactions but not yet committed.
Transactions running at the READ
UNCOMMITTED level do not issue shared
locks to prevent other transactions
from modifying data read by the
current transaction. READ UNCOMMITTED
transactions are also not blocked by
exclusive locks that would prevent the
current transaction from reading rows
that have been modified but not
committed by other transactions. When
this option is set, it is possible to
read uncommitted modifications, which
are called dirty reads. Values in the
data can be changed and rows can
appear or disappear in the data set
before the end of the transaction.
This option has the same effect as
setting NOLOCK on all tables in all
SELECT statements in a transaction.
This is the least restrictive of the
isolation levels.
In SQL Server, you can also minimize
locking contention while protecting
transactions from dirty reads of
uncommitted data modifications using
either:
The READ COMMITTED isolation level
with the READ_COMMITTED_SNAPSHOT
database option set to ON. The
SNAPSHOT isolation level
.
On a different tack, there are two other aspects to consider, that may help.
1) Indexes and the indexes used by the SQL. The indexing strategy used on the tables will affect how many rows are affected. If you make the data modifications using a unique index, you may reduce the chance of deadlocks.
One algorithm - of course it will not work it all cases. The use of NOLOCK is targeted rather than being global.
The "old" way:
UPDATE dbo.change_table
SET somecol = newval
WHERE non_unique_value = 'something'
The "new" way:
INSERT INTO #temp_table
SELECT uid FROM dbo.change_table WITH (NOLOCK)
WHERE non_unique_value = 'something'
UPDATE dbo.change_table
SET somecol = newval
FROM dbo.change_table c
INNER JOIN
#temp_table t
ON (c.uid = t.uid)
2) Transaction duration
The longer a transaction is open the more likely there may be contention. If there is a way to reduce the amount of time that records remain locked, you can reduce the chances of a deadlock occurring.
For example, perform as many SELECT statements (e.g. lookups) at the start of the code instead of performing an INSERT or UPDATE, then a lookup, then an INSERT, and then another lookup.
This is where one can use the NOLOCK hint for SELECTs on "static" tables that are not changing reducing the lock "footprint" of the code.
I remember an example where reads in a transaction then writing back the data is not safe because another transaction may read/write to it in the time between. So i would like to check the date and prevent the row from being modified or read until my transaction is finish. Would this do the trick? and are there any sql variants that this will not work on?
update tbl set id=id where date>expire_date and id=#id
Note: date>expire_date happens to be my condition. It could be anything. Would this prevent other transaction from reading the row until i commit or rollback?
In a lot of cases, your UPDATE statement will not prevent other transactions from reading the row.
ziang mentioned transaction isolation levels.
Depending on the isolation level, databases use different types of locking. At the highest level, locking can be divided into two categories:
- pessimistic,
- optimistic
MS SQL 2008, for example, has 6 isolation levels, 4 of them are pessimistic, 2 are optimistic. By default , it uses READ COMMITTED isolation level, which falls into the pessimistic category.
Oracle, on another note, uses optimistic locking by default.
The statement that will lock your record for writing is
SELECT * FROM TBL WITH UPDLOCK WHERE id=#id
From that point on, no other transaction will be able to update your record with id=#id
And only transactions running in isolation level READ UNCOMMITTED will be able to read it.
With the default transaction level, READ COMMITTED, no other thansaction will be able to read or write into this record until you either commit or roll back your entire transaction.
It depends on the transaction isolation level you set on your transaction control. There are 4 types of read
READ UNCOMMITTED: this allows the dirty read
READ COMMITTED
REPEATABLE READ
SERIALIZABLE
for more info, you can check msdn.
You should be able to do this in a normal select using a combination of
HOLDLOCK/ROWLOCK
It very well may work. Different platforms offer different services. For instance, in T-SQL, you can simply set the isolation level of the transaction and, as a result, force a lock to be obtained. I don't know what platform you are using so I cannot answer your question definitively.