Each transaction requests locks of different types on the resources, such as rows, pages, or tables, on which the transaction is dependent. The locks block other transactions from modifying the resources in a way that would cause problems for the transaction requesting the lock. Each transaction frees its locks when it no longer has a dependency on the locked resources.
Kode Cursor Type Dan Lock Type Object Recordset di Visual Basic 6.0
SQL Server supports a range of concurrency control. Users specify the type of concurrency control by selecting transaction isolation levels for connections or concurrency options on cursors. These attributes can be defined using Transact-SQL statements, or through the properties and attributes of database application programming interfaces (APIs) such as ADO, ADO.NET, OLE DB, and ODBC.
A lower isolation level increases the ability of many users to access data at the same time, but increases the number of concurrency effects (such as dirty reads or lost updates) users might encounter. Conversely, a higher isolation level reduces the types of concurrency effects that users may encounter, but requires more system resources and increases the chances that one transaction will block another. Choosing the appropriate isolation level depends on balancing the data integrity requirements of the application against the overhead of each isolation level. The highest isolation level, serializable, guarantees that a transaction will retrieve exactly the same data every time it repeats a read operation, but it does this by performing a level of locking that is likely to impact other users in multi-user systems. The lowest isolation level, read uncommitted, may retrieve data that has been modified but not committed by other transactions. All of the concurrency side effects can happen in read uncommitted, but there is no read locking or versioning, so overhead is minimized.
Applications do not typically request locks directly. Locks are managed internally by a part of the SQL Server Database Engine called the lock manager. When an instance of the SQL Server Database Engine processes a Transact-SQL statement, the SQL Server Database Engine query processor determines which resources are to be accessed. The query processor determines what types of locks are required to protect each resource based on the type of access and the transaction isolation level setting. The query processor then requests the appropriate locks from the lock manager. The lock manager grants the locks if there are no conflicting locks held by other transactions.
The SQL Server Database Engine has multigranular locking that allows different types of resources to be locked by a transaction. To minimize the cost of locking, the SQL Server Database Engine locks resources automatically at a level appropriate to the task. Locking at a smaller granularity, such as rows, increases concurrency but has a higher overhead because more locks must be held if many rows are locked. Locking at a larger granularity, such as tables, are expensive in terms of concurrency because locking an entire table restricts access to any part of the table by other transactions. However, it has a lower overhead because fewer locks are being maintained.
Lock escalation cannot occur if a different SPID is currently holding an incompatible table lock. Lock escalation always escalates to a table lock, and never to page locks. Additionally, if a lock escalation attempt fails because another SPID holds an incompatible TAB lock, the query that attempted escalation does not block while waiting for a TAB lock. Instead, it continues to acquire locks at its original, more granular level (row, key, or page), periodically making additional escalation attempts. Therefore, one method to prevent lock escalation on a particular table is to acquire and to hold a lock on a different connection that is not compatible with the escalated lock type. An IX (intent exclusive) lock at the table level does not lock any rows or pages, but it is still not compatible with an escalated S (shared) or X (exclusive) TAB lock. For example, assume that you must run a batch job that modifies a large number of rows in the mytable table and that has caused blocking that occurs because of lock escalation. If this job always completes in less than an hour, you might create a Transact-SQL job that contains the following code, and schedule the new job to start several minutes before the batch job's start time:
Deadlocks can also occur when a table is partitioned and the LOCK_ESCALATION setting of ALTER TABLE is set to AUTO. When LOCK_ESCALATION is set to AUTO, concurrency increases by allowing the SQL Server Database Engine to lock table partitions at the HoBT level instead of at the table level. However, when separate transactions hold partition locks in a table and want a lock somewhere on the other transactions partition, this causes a deadlock. This type of deadlock can be avoided by setting LOCK_ESCALATION to TABLE; although this setting will reduce concurrency by forcing large updates to a partition to wait for a table lock.
Each user session might have one or more tasks running on its behalf where each task might acquire or wait to acquire a variety of resources. The following types of resources can cause blocking that could result in a deadlock.
For large computer systems, locks on frequently referenced objects can become a performance bottleneck as acquiring and releasing locks place contention on internal locking resources. Lock partitioning enhances locking performance by splitting a single lock resource into multiple lock resources. This feature is only available for systems with 16 or more CPUs, and is automatically enabled and cannot be disabled. Only object locks can be partitioned. Object locks that have a subtype are not partitioned. For more information, see sys.dm_tran_locks (Transact-SQL).
The SQL Server Database Engine supports six data types that can hold large strings up to 2 gigabytes (GB) in length: nvarchar(max), varchar(max), varbinary(max), ntext, text, and image. Large strings stored using these data types are stored in a series of data fragments that are linked to the data row. Row versioning information is stored in each fragment used to store these large strings. Data fragments are a collection of pages dedicated to large objects in a table.
SQL Server does not keep multiple versions of system metadata. Data definition language (DDL) statements on tables and other database objects (indexes, views, data types, stored procedures, and common language runtime functions) change metadata. If a DDL statement modifies an object, any concurrent reference to the object under snapshot isolation causes the snapshot transaction to fail. Read-committed transactions do not have this limitation when the READ_COMMITTED_SNAPSHOT database option is ON.
Locking hints can be specified for individual table references in the SELECT, INSERT, UPDATE, and DELETE statements. The hints specify the type of locking or row versioning the instance of the SQL Server Database Engine uses for the table data. Table-level locking hints can be used when a finer control of the types of locks acquired on an object is required. These locking hints override the current transaction isolation level for the session.
Don't open Dynaset type recordset object on SQL database tables unless you need to add or edit records, or need to see the changes made by other users. Instead, consider using Snapshot recordsets which can be faster to scroll through. Of course, Snapshot recordsets can take longer to open since they require a full read of the source data.
If you need to retrieve a large number of records, use a Dynaset instead of a Snapshot. Snapshot type recordsets must load all records from the data source before becoming available, whereas Dynasets are available as soon as the first 20 or so records are retrieved. Also, when using a Snapshot against large ODBC data sources, you run the risk of running out of disk space on your local computer. This is because all data is downloaded into RAM until the amount of RAM is exhausted. Then, the database engine creates a temporary database to store the contents of the snapshot. In a nutshell, when you open a snapshot, you need at least as much disk space as the largest table you are opening.
Avoid using the Object data type. Instead use the specific data type for the object you are working with. This allows Visual Basic to employ "early binding" which can be substantially faster in many cases.
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With the above configuration, hackers get good information about the source of errors that can help them learn to pick our coding locks. Default ASP.NET error messages show our ASP.NET version and framework, plus the exception type.
Returns an array of ADOFieldObject's, one field object for every column of $table. A field object is a class instance with (name, type, max_length) defined. Currently Sybase does not recognise date types, and ADO cannot identify the correct data type (so we default to varchar).
When an SQL statement successfully is executed by ADOConnection->Execute($sql),an ADORecordSet object is returned. This object contains a virtual cursor so we can move from row to row, functions to obtain information about the columns and column types, and helper functions to deal with formating the results to show to the user. 2ff7e9595c
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