如何使用 FireDAC 和 SQL Server 避免不需要的事务和锁

Question

From analyzing table locks in SQL Server, my Win32 application built in RAD Studio XE7 starts numerous transactions while each FDQuery is active. Sometimes this causes application problems and locks with dozens of users. Especially with triggered tables.

For my test, I used simple FDConnection and FDQuery as Select * from Customer with default settings, and concluded that FDQuery1.Active:=True causes the start of a Customer table transaction. The transaction disappears when FDQuery1.Active:=false .

I would like to inhibit the starting of transactions in FDQuery for read-only, as lists of data for grid or reports.

But I can't find a way to find the appropriate tuning of FDQuery.

Answer 1

By default, SQL Server does not implement versioning of data blocks. So, to return a consistent set of rows, it guarantee that no other sessions makes changes do data during execution of a query, using shared locks.

Using "WITH(NOLOCK)" disable shared locks, but can result in an inconsistent result set.

The only one solution is to use READ_COMMITED_SNAPSHOT isolation level, which store changed data to temp, used to return consistent result sets without locking updates.

Answer 2

Look at the NOLOCK keyword for tables. https://www.mssqltips.com/sqlservertip/2470/understanding-the-sql-server-nolock-hint/