给 EF Core 查询增加 With NoLock
Intro
EF Core 在 3.x 版本中增加了 Interceptor，使得我们可以在发生低级别数据库操作时作为 EF Core 正常运行的一部分自动调用它们。例如，打开连接、提交事务或执行命令时。
所以我们可以自定义一个 Interceptor 来记录执行的 sql 语句，也可以通过 Interceptor 来实现 sql 语句的执行。
这里我们可以借助 Interceptor 实现对于查询语句的修改，自动给查询语句加 (WITH NOLOCK)， WITH NOLOCK 等效于 READ UNCOMMITED（读未提交）的事务级别，这样会造成一定的脏读，但是从效率上而言，是比较高效的，不会因为别的事务长时间未提交而导致查询阻塞，所以对于大数据场景下，查询 SQL 加 NOLOCK 还是比较有意义的
NoLockInterceptor
继承 DbCommandInterceptor，重写查询 sql 执行之前的操作，在执行 sql 之前增加 WITH(NOLOCK)，实现代码如下：
public class QueryWithNoLockDbCommandInterceptor : DbCommandInterceptor
{private static readonly Regex TableAliasRegex =new Regex(@"(?<tableAlias>AS \[[a-zA-Z]\w*\](?! WITH \(NOLOCK\)))",RegexOptions.Multiline | RegexOptions.Compiled | RegexOptions.IgnoreCase);public override InterceptionResult<object> ScalarExecuting(DbCommand command, CommandEventData eventData, InterceptionResult<object> result){command.CommandText = TableAliasRegex.Replace(command.CommandText,"${tableAlias} WITH (NOLOCK)");return base.ScalarExecuting(command, eventData, result);}public override Task<InterceptionResult<object>> ScalarExecutingAsync(DbCommand command, CommandEventData eventData, InterceptionResult<object> result,CancellationToken cancellationToken = new CancellationToken()){command.CommandText = TableAliasRegex.Replace(command.CommandText,"${tableAlias} WITH (NOLOCK)");return base.ScalarExecutingAsync(command, eventData, result, cancellationToken);}public override InterceptionResult<DbDataReader> ReaderExecuting(DbCommand command, CommandEventData eventData, InterceptionResult<DbDataReader> result){command.CommandText = TableAliasRegex.Replace(command.CommandText,"${tableAlias} WITH (NOLOCK)");return result;}public override Task<InterceptionResult<DbDataReader>> ReaderExecutingAsync(DbCommand command, CommandEventData eventData, InterceptionResult<DbDataReader> result,CancellationToken cancellationToken = new CancellationToken()){command.CommandText = TableAliasRegex.Replace(command.CommandText,"${tableAlias} WITH (NOLOCK)");return base.ReaderExecutingAsync(command, eventData, result, cancellationToken);}
}
Interceptor 的使用
在注册 DbContext 服务的时候，可以配置 Interceptor，配置如下：
var services = new ServiceCollection();
services.AddDbContext<TestDbContext>(options =>
{options.UseLoggerFactory(loggerFactory).UseSqlServer(DbConnectionString).AddInterceptors(new QueryWithNoLockDbCommandInterceptor());
});
使用效果
通过 loggerFactory 记录的日志查看查询执行的 sql 语句
可以看到查询语句自动加上了 WITH(NOLOCK)
Reference
https://docs.microsoft.com/en-us/sql/t-sql/queries/hints-transact-sql-table?view=sql-server-ver15
https://docs.microsoft.com/en-us/ef/core/what-is-new/ef-core-3.0/#interception-of-database-operations
https://docs.microsoft.com/zh-cn/ef/core/what-is-new/ef-core-3.0/#interception-of-database-operations
https://github.com/WeihanLi/WeihanLi.EntityFramework/blob/dev/src/WeihanLi.EntityFramework/Interceptors/QueryWithNoLockDbCommandInterceptor.cs
