Without concurrent write access
Materialize a selection in a CTE (Common Table Expressions) and join to it in the FROM clause of the UPDATE.
WITH cte AS (
SELECT server_ip -- pk column or any (set of) unique column(s)
WHERE status = 'standby'
LIMIT 1 -- arbitrary pick (cheapest)
When inserting a row, is there a window of opportunity between the generation of a new Identity value and the locking of the corresponding row key in the clustered index, where an external observer could see a newer Identity value inserted by a concurrent transaction?
The allocation of identity values is independent of the containing user transaction. ...
Yes, SQL Server can, under some circumstances read one column's value from the "old" version of the row, and another column's value from the "new" version of the row.
CREATE TABLE Person
Id INT PRIMARY KEY,
CREATE INDEX ix_Name
CREATE INDEX ix_Surname
I use READ_UNCOMMITTED (or NOLOCK) when querying production databases from SSMS but not routinely from application code. This practice (along with a MAXDOP 1 query hint) helps ensure casual queries for data analysis and troubleshooting don't impact the production workload, with the understanding the results might not be correct.
Sadly, I see ...
I have heard of concurrency problems like that in MySQL before. Not so in Postgres.
Built-in row-level locks in the default READ COMMITTED transaction isolation level are enough.
I suggest a single statement with a data-modifying CTE (something that MySQL also doesn't have) because it's convenient to pass values from one table to the other directly (if you ...
I think I probably meant to add that comment on the prior answer, about two separate statements. It was over a year ago, so I'm not totally sure anymore.
The wCTE based query doesn't really solve the problem it's supposed to, but upon reviewing it again over a year later I don't see the possibility of lost updates in the wCTE version.
(Note that all of ...
If I run them serially, one right after the other, I'm expecting it will require 7 minutes to complete on average. Is this reasonable?
If they use unrelated data sets, then yes.
If they share a data set, and the cache is cold for the first query and the query is mostly I/O bound, then the second one might complete in moments. You need to consider caching ...
I'm not a DBA, but a software dev with a few years of DB experience. I'm only "textbook-level" familiar with the inner workings of locking, pages, hints,
Then you should use SNAPSHOT isolation, or set your database to READ COMMITTED SNAPSHOT, because it's fundamentally simpler to write correct, scalable, deadlock-free code.
It's a common ...
It's referring to Azure SQL Database which uses RCSI by default.
Isolation Level SQL
Database default database wide setting is to
enable read committed snapshot isolation (RCSI) by having both the
READ_COMMITTED_SNAPSHOT and ALLOW_SNAPSHOT_ISOLATION database options
set to ON, learn more about isolation levels here. You cannot
change the database default ...
Yes, there is a difference between UPDATE and DELETE + INSERT.
Let's use the pageinspect extension to look at the tuples and the tuple headers.
If you want to repeat my experiment, you have to drop and re-create the table in between. Also, there may be additional flags (hint bits) if you selected the rows before examining them.
The meaning of infomask2 and ...
The only query in what you're showing above appears to be this one, repeated a few times:
select * from [dbo].[FinanceDetail] trd WITH (UPDLOCK, SERIALIZABLE)
where trd.HeaderId = @HeaderId
DELETE from dbo.FinanceDetail
where HeaderId = @...
I have a question regarding inner workings of UPDATE statements with regards to SQL Server.
The statement describes a desired logical change to the database. What happens physically at runtime depends on the execution plan, the state of the database, and what other concurrently running statements/queries are doing.
I mention this because your question ...
The default READ COMMITTED transaction isolation level guarantees that your transaction will not read uncommitted data. It does not guarantee that any data you read will remain the same if you read it again (repeatable reads) or that new data will not appear (phantoms).
These same considerations apply to multiple data accesses within the same statement.
Typically, you have a team table (or similar) with a unique team_id column.
Your FK constraint indicates as much: ... REFERENCES teams(id) - so I'll work with teams(id).
Then, to avoid complications (race conditions or deadlocks) under concurrent write load, it's typically simplest and cheapest to take a write lock on the parent row in team and then, in the ...
Local temporary objects are separated by Session. If you have two queries running concurrently, then they are clearly two completely separate sessions and you have nothing to worry about. The Login doesn't matter. And if you are using Connection Pooling that also won't matter. Local temporary objects (Tables most often, but also Stored Procedures) are safe ...
Would the identity primary key column fail, with a primary key violation in any way, example: processors are trying to input the same identity number?
This is covered in the SQL Server product documentation:
CREATE TABLE (Transact-SQL) IDENTITY (Property)
Identity columns can be used for generating key values. The identity property on a column ...
My first question is, will these update statements work as intended?
Very likely, but not certain.
SQL Server guarantees it will honour the semantics of the query, and the level of ACID compliance determined by the effective isolation level. Beyond that, all is implementation detail (including what type(s) of locks are taken, when, and for how long they ...
The rows that qualify for an update are always stabilized during discovery by using (at least) U locks (Update locks, see Lock Compatibility). When is time to actually update them, the U lock is upgraded to X lock. Because of this stabilization the row cannot disappear nor can it be modified between the discovery and the update. Nor can a second UPDATE ...
The update lock is sufficient, but you can achieve what you want more simply with:
UPDATE dbo.tblOrderNumber WITH (SERIALIZABLE)
SET @NextOrderNumber = NextOrderNumber,
NextOrderNumber = NextOrderNumber + 1
WHERE CustomerID = @CustomerID;
The WITH (SERIALIZABLE) hint is not strictly required if there is a unique index on CustomerID.
A scenario where rows committed before the start of the SELECT query can be missed is when an index key value is updated. Consider this query:
SELECT * FROM dbo.YourTable;
The execution plan will likely perform an ordered clustered scan to return all columns. If a key value is updated and committed during the scan and the new value is less than the ordered ...
If you really need to run multiple threads at the same time you can enable the ignore_dup_key option on the primary key.
This will just give a warning instead of an error when an insert would result in a duplicate key violation. But instead of failing it will discard the row(s) that if inserted would cause the uniqueness violation.
CREATE TABLE [dbo].[...
With plain CREATE INDEX, the table will be locked for writes but not reads.
Use CREATE INDEX CONCURRENTLY to avoid write locks as well.
From the PostgreSQL docs on CREATE INDEX:
When this option is used, PostgreSQL will build the index without
taking any locks that prevent concurrent inserts, updates, or deletes
on the table; whereas a standard ...
SQL Server by default operates under Read Committed isolation.
If you want SQL Server to behave more like other implementations, what you're looking for is either Read Committed Snapshot Isolation, or Snapshot Isolation.
You can read more about them here.
Full disclosure, I am a Brent Ozar Unlimited employee
Using NOLOCK or Read Uncommitted is ...
The UPDLOCK, SERIALIZABLE pattern is all about avoiding incorrect results (including false key violation errors) due to race conditions, when performing a particularly common operation known as an UPSERT - update an existing row if it exists; insert a new row otherwise.
...while avoiding racing conditions, deadlocks, etc?
You seem to be looking for a ...
You could try DROP INDEX [ CONCURRENTLY ] name
Drop the index without locking out concurrent selects, inserts,
updates, and deletes on the index's table. A normal DROP INDEX
acquires exclusive lock on the table, blocking other accesses until
the index drop can be completed. With this option, the command
instead waits until conflicting ...
As Paul White answered absolutely correct there is a possibility for temporarily "skipped" identity rows.
Here is just a small piece of code to reproduce this case for your own.
Create a database and a testtable:
create database IdentityTest
create table dbo.IdentityTest (ID int identity, c1 char(10))
create clustered index ...
Many of the issues you see are being caused by an inefficient execution plan:
Not that the supplied plan and query matches the question, but even so, I'm working with what was provided.
Anyway, you should implement the Name column data type changes (from nvarchar(max)) that I mentioned in your previous question. More importantly, you need to add the ...
The ROW SHARE lock taken by FOR UPDATE prevents concurrent write access that would change the physical location of the row. The manual:
This prevents them from being locked, modified or deleted by other
transactions until the current transaction ends. That is, other
transactions that attempt UPDATE, DELETE, SELECT FOR UPDATE, SELECT FOR NO KEY UPDATE, ...