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(Republishing, or using this info in a commercial product/website, is prohibited without permission. Description: This wait type is when a thread is waiting to acquire a Shared lock on a resource and there is at least one other lock in an incompatible mode granted on the resource to a different thread.
In conclusion, the READ_COMMITTED_SNAPSHOT is a database option that changes the behavior of the transactions running under the READ COMMITTED isolation level. By default, it is set OFF in SQL Server databases. In this case, locking is used to eliminate dirty reads in the READ COMMITTED transaction isolation level.
A LCK_M_U is an update lock on a table/index (not on a database), that’s quite normal when you update data in a database.
HADR_SYNC_COMMIT indicates the time between when a transaction ready to commit in the primary replica, and all secondary synchronous-commit replicas have acknowledged the hardening of the transaction commit LSN in an AG.
SQL Server instances with high CPU usage often show the SOS_SCHEDULER_YIELD wait type, which can result from a number of different causes. One cause may be when an active thread voluntarily yields to allow another runnable thread its turn (next from the runnable queue) executing on the CPU.
sp_whoisactive is a comprehensive activity monitoring stored procedure that works for all versions of SQL Server from 2005 through 2017. You can find the most recent versions on the Downloads page. Documentation is available on the Documentation page.
snapshot_isolation_state. State of snapshot-isolation transactions being allowed, as set by the ALLOW_SNAPSHOT_ISOLATION option: 0 = Snapshot isolation state is OFF (default). Snapshot isolation is disallowed.
Because snapshot isolation is enabled, this transaction can read the data that existed before sqlTransaction1 started. It opens a third connection and initiates a transaction using the READ COMMITTED isolation level to attempt to read the data in the table.
ALTER DATABASE enables you to change the overall characteristics of a database. These characteristics are stored in the data dictionary. This statement requires the ALTER privilege on the database. ALTER SCHEMA is a synonym for ALTER DATABASE .
(Republishing, or using this info in a commercial product/website, is prohibited without permission. Description: This wait type is when one of the parallel threads performing redo for an Availability Group replica is waiting for some work to do.
There are two main categories for the SQL Server Wait Statistics; the Single Wait type, where the query is waiting for CPU resource availability and the Resource Wait type, where the query is waiting for a specific resource availability, such as I/O or Memory resources.
So there is one cause for SOS_SCHEDULER_YIELD: a thread exhausting its scheduling quantum and heavily recurring instances can lead to SOS_SCHEDULER_YIELD being the most prevalent wait along with high CPU usage. … But remember that each of these ‘waits’ equals 4ms of CPU time accrued for the query.
“Max Degree of Parallelism” controls the maximum number of CPUs that are assigned to run parallel SQL queries. By default, the value is zero (0) which means that the server can use infinite (all) CPUs for each query.
CXPACKET (Class Exchange Packet) is a wait type that generally reported by SQL Server when SQL Server execute queries using Parallelism. … This is the statement about CXPACKET wait type that we know since many years.
The sp_who is an undocumented system Stored Procedure that is used to get the information of the current state of the SQL instance. The sp_who system Stored Procedure also allows fast monitoring of active and inactive processes.
- Open SQL Server Profiler (in Performance Tools)
- File -> New Trace…
- Connect to your database.
- Click the Events Selection tab.
- Select only events which correspond to SQL queries finishing: …
- Click Column Filters…
- Click Duration in the list.
Sp_WhoIsActive is a useful custom stored procedure and gives important insights from the user sessions in SQL Server with information such as lead blocker, execution plan, wait stats, query text.
A Phantom read occurs when one user is repeating a read operation on the same records, but has new records in the results set: READ UNCOMMITTED. Also called a Dirty read. When this isolation level is used, a transaction can read uncommitted data that later may be rolled back.
In database systems, durability is the ACID property which guarantees that transactions that have committed will survive permanently. … Many DBMSs implement durability by writing transactions into a transaction log that can be reprocessed to recreate the system state right before any later failure.
A database in Microsoft SQL Server can handle thousands of simultaneous requests for selecting data and doing data modifications. … As a workaround, Microsoft introduced for the first time, with Microsoft SQL Server 2005, the READ COMMITTED SNAPSHOT ISOLATION (RCSI) level.
To check the current state of Read-Committed Snapshot, open a query window and execute the following command: select is_read_committed_snapshot_on from sys. databases where name= ‘CitrixSiteDB’; A value of 1 indicates that Read-Committed Snapshot is already enabled and no change is required.
4 Answers. READ COMMITTED SNAPSHOT does optimistic reads and pessimistic writes. In contrast, SNAPSHOT does optimistic reads and optimistic writes. Microsoft recommends READ COMMITTED SNAPSHOT for most apps that need row versioning.
In serializable isolation level, SQL Server acquires key range locks and holds them until the end of the transaction. … In snapshot isolation level, SQL Server does not acquire any locks. Thus, it is possible for a concurrent transaction to modify data that a second transaction has already read.
You can change the collation of any new objects that are created in a user database by using the COLLATE clause of the ALTER DATABASE statement. This statement does not change the collation of the columns in any existing user-defined tables. These can be changed by using the COLLATE clause of ALTER TABLE.
To open a mounted database, use the ALTER DATABASE statement with the OPEN clause: ALTER DATABASE OPEN; After executing this statement, any valid Oracle Database user with the CREATE SESSION system privilege can connect to the database.
If you’re using MySQL version 5.5 (or greater), you are likely using the InnoDB storage engine, which makes the task of renaming databases quite simple. In short, you can use the RENAME TABLE command within a MySQL prompt to effectively change the database name of a particular table while keeping the table name intact.
BROKER_TASK_STOP occurs when the Service Broker queue task handler tries to shut down the task. It should not be a problem and nothingbtonworry about from a performance perspective.
There’s a process called “REDO Thread” running on the secondary replica, that is responsible for applying the changes made in the primary, and this thread cannot be blocked.
The Availability Group Redo Queue Size (KB) alert indicates the amount of log records from log files in KB that need redoing in the secondary replica to complete synchronization. Alert includes affected databases. For more information on configuring alerts and setting thresholds, see Configure alerts.
Distribution statistics are used by SQL Server’s Query Optimiser to determine a good execution plan for your SQL query. … The statistics provide information about the distribution of column values across participating rows, helping the optimizer better estimate the number of rows, or cardinality, of the query results.
Perfmon ObjectPerfmon CounterCounter DescriptionSQLServer:SQL StatisticsBatch Requests/SecNumber of SQL batch requests received by serverSQLServer:SQL StatisticsSQL Compilations/SecNumber of SQL compilationsSQLServer:SQL StatisticsSQL Re-Compilations/SecNumber of SQL re-compilations
A signal wait is accumulated by processes running on SQL Server and waiting for a CPU to become available (so-called because the process has “signaled” it’s ready for time on the CPU).
SOS_SCHEDULER_YIELD means the SQL Operating System (SOS) is waiting for a CPU scheduler to yield more time, but this wait is a little trickier than that.
RESOURCE_SEMAPHORE waits occur when a query is unable to get enough memory to execute. In this post I will describe how to see they are occurring. I will also provide tips on ways to help reduce or eliminate them.
Generally speaking, the CXPACKET wait type is normal for SQL Server and it is an indicator that SQL Server is using a parallel plan in executing a query, which is generally faster comparing to a query executed in a serialized process. …
When Oracle runs SQL statements in parallel, multiple processes work together simultaneously to run a single SQL statement. … Parallelism is the idea of breaking down a task so that, instead of one process doing all of the work in a query, many processes do part of the work at the same time.
The parallelism concept comes from dividing a big task into smaller tasks, where each small task is assigned to a specific person, or processor in the case of a SQL Server query, to accomplish part of the main task. Finally, the partial results taken from each small task will be combined together into one final result.
- In Object Explorer, right-click a server and select Properties.
- Click the Advanced node.
- In the Max Degree of Parallelism box, select the maximum number of processors to use in parallel plan execution.