LINQ使用“喜欢"而不是“(((NVL(INSTR(x，y)，0))= 1)"

问题描述

使用.Contains()/.StartsWith()/.EndsWith()时，生成的SQL如下所示:

When using .Contains()/.StartsWith()/.EndsWith() the generated SQL looks like this :

(( NVL(INSTR(x, y), 0) ) = 1)

有没有一种替代方法:

LIKE 'x%' or '%x%' or '%x'

因为在查询的执行计划中这两者之间存在巨大的成本差异(44 000 vs 30).

Because there is a huge cost difference between those two in the execution plan of the query (44 000 vs 30).

推荐答案

当我四处张望时，发现 LINQ中的LIKE运算符是a，其中有一些很好的示例说明了如何执行此操作.我已经测试了下面来自上面链接的那一个

When i look around abit i found LIKE operator in LINQ which were a which have a few good examples of how you could do this. I have tested the one below that was from the link above

这是 adobrzyc

    public static class LinqEx
    {
        private static readonly MethodInfo ContainsMethod = typeof(string).GetMethod("Contains");
        private static readonly MethodInfo StartsWithMethod = typeof(string).GetMethod("StartsWith", new[] { typeof(string) });
        private static readonly MethodInfo EndsWithMethod = typeof(string).GetMethod("EndsWith", new[] { typeof(string) });

        public static Expression<Func<TSource, bool>> LikeExpression<TSource, TMember>(Expression<Func<TSource, TMember>> property, string value)
        {
            var param = Expression.Parameter(typeof(TSource), "t");
            var propertyInfo = GetPropertyInfo(property);
            var member = Expression.Property(param, propertyInfo.Name);

            var startWith = value.StartsWith("%");
            var endsWith = value.EndsWith("%");

            if (startWith)
                value = value.Remove(0, 1);

            if (endsWith)
                value = value.Remove(value.Length - 1, 1);

            var constant = Expression.Constant(value);
            Expression exp;

            if (endsWith && startWith)
            {
                exp = Expression.Call(member, ContainsMethod, constant);
            }
            else if (startWith)
            {
                exp = Expression.Call(member, EndsWithMethod, constant);
            }
            else if (endsWith)
            {
                exp = Expression.Call(member, StartsWithMethod, constant);
            }
            else
            {
                exp = Expression.Equal(member, constant);
            }

            return Expression.Lambda<Func<TSource, bool>>(exp, param);
        }

        public static IQueryable<TSource> Like<TSource, TMember>(this IQueryable<TSource> source, Expression<Func<TSource, TMember>> parameter, string value)
        {
            return source.Where(LikeExpression(parameter, value));
        }

        private static PropertyInfo GetPropertyInfo(Expression expression)
        {
            var lambda = expression as LambdaExpression;
            if (lambda == null)
                throw new ArgumentNullException("expression");

            MemberExpression memberExpr = null;

            switch (lambda.Body.NodeType)
            {
                case ExpressionType.Convert:
                    memberExpr = ((UnaryExpression)lambda.Body).Operand as MemberExpression;
                    break;
                case ExpressionType.MemberAccess:
                    memberExpr = lambda.Body as MemberExpression;
                    break;
            }

            if (memberExpr == null)
                throw new InvalidOperationException("Specified expression is invalid. Unable to determine property info from expression.");


            var output = memberExpr.Member as PropertyInfo;

            if (output == null)
                throw new InvalidOperationException("Specified expression is invalid. Unable to determine property info from expression.");

            return output;
        }
    }

要使用它，您只需在将Contains函数放入的地方添加Like函数即可.您可以在下面查看示例

To use it you you just simply add Like function where you would put the Contains functions. You can see below for a example

            using (CustomerEntities customerContext = new CustomerEntities())
            {
                IQueryable<Customer> customer = customerContext.Customer.Like(x => x.psn, "%1%");
            }

这将创建一个看起来像这样的sql查询.

This will create a sql query that looks something like this.

SELECT
[Extent1].[psn] AS [psn]
FROM [dbo].[Customer] AS [Extent1]
WHERE [Extent1].[psn] LIKE '%1%'

这篇关于LINQ使用“喜欢"而不是“(((NVL(INSTR(x，y)，0))= 1)"的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！