给 EF Core 查询增长 With NoLock

给 EF Core 查询增长 With NoLock

Intro

EF Core 在 3.x 版本中增长了 Interceptor，使得咱们能够在发生低级别数据库操做时做为 EF Core 正常运行的一部分自动调用它们。 例如，打开链接、提交事务或执行命令时。

因此咱们能够自定义一个 Interceptor 来记录执行的 sql 语句，也能够经过 Interceptor 来实现 sql 语句的执行。

这里咱们能够借助 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，这样会形成必定的脏读，可是从效率上而言，是比较高效的，不会由于别的事务长时间未提交致使查询阻塞，因此对于大数据场景下，查询 SQL 加 NOLOCK 仍是比较有意义的

NoLockInterceptor

继承 DbCommandInterceptor，重写查询 sql 执行以前的操做，在执行 sql 以前增长 WITH(NOLOCK)，实现代码以下：

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，能够配置 Interceptor，配置以下：

var services = new ServiceCollection();
services.AddDbContext<TestDbContext>(options =>
{
    options
        .UseLoggerFactory(loggerFactory)
        .UseSqlServer(DbConnectionString)
        .AddInterceptors(new QueryWithNoLockDbCommandInterceptor())
        ;
});

使用效果

经过 loggerFactory 记录的日志查看查询执行的 sql 语句

能够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