为什么 Linq To Entities 创建未优化的查询

Question

EF 创建了一个使用索引扫描而不是索引搜索的查询。通过稍微修改查询以不使用参数，而是使用索引查找。索引扫描大约需要 3 秒，而 Seek 是即时的。

生成的查询（使用索引扫描）：

exec sp_executesql N'SELECT TOP (1) 
    [Extent1].[phone_id] AS [phone_id], 
    [Extent1].[phone] AS [phone], 
    [Extent1].[high_usage_flag] AS [high_usage_flag], 
    [Extent1].[cds_flag] AS [cds_flag], 
    [Extent1].[never_call_flag] AS [never_call_flag], 
    [Extent1].[pa_state_dnc_flag] AS [pa_state_dnc_flag], 
    [Extent1].[ma_state_dnc_flag] AS [ma_state_dnc_flag], 
    [Extent1].[national_dnc_flag] AS [national_dnc_flag], 
    [Extent1].[note] AS [note], 
    [Extent1].[pec_never_call_flag] AS [pec_never_call_flag], 
    [Extent1].[nicor_dnc_flag] AS [nicor_dnc_flag], 
    [Extent1].[css_vici_flag] AS [css_vici_flag], 
    [Extent1].[css_pec_flag] AS [css_pec_flag], 
    [Extent1].[css_vici_alt_flag] AS [css_vici_alt_flag], 
    [Extent1].[area_code] AS [area_code], 
    [Extent1].[phone_type_id] AS [phone_type_id], 
    [Extent1].[last_answer_date] AS [last_answer_date], 
    [Extent1].[csr_bad_flag] AS [csr_bad_flag], 
    [Extent1].[to_process_flag] AS [to_process_flag], 
    [Extent1].[deleted_date] AS [deleted_date], 
    [Extent1].[wireless_flag] AS [wireless_flag]
    FROM [dbo].[phone] AS [Extent1]
    WHERE [Extent1].[phone] = @p__linq__0',N'@p__linq__0 nvarchar(4000)',@p__linq__0=N'555555555'

修改为不使用参数（使用索引查找）：

exec sp_executesql N'SELECT TOP (1) 
    [Extent1].[phone_id] AS [phone_id], 
    [Extent1].[phone] AS [phone], 
    [Extent1].[high_usage_flag] AS [high_usage_flag], 
    [Extent1].[cds_flag] AS [cds_flag], 
    [Extent1].[never_call_flag] AS [never_call_flag], 
    [Extent1].[pa_state_dnc_flag] AS [pa_state_dnc_flag], 
    [Extent1].[ma_state_dnc_flag] AS [ma_state_dnc_flag], 
    [Extent1].[national_dnc_flag] AS [national_dnc_flag], 
    [Extent1].[note] AS [note], 
    [Extent1].[pec_never_call_flag] AS [pec_never_call_flag], 
    [Extent1].[nicor_dnc_flag] AS [nicor_dnc_flag], 
    [Extent1].[css_vici_flag] AS [css_vici_flag], 
    [Extent1].[css_pec_flag] AS [css_pec_flag], 
    [Extent1].[css_vici_alt_flag] AS [css_vici_alt_flag], 
    [Extent1].[area_code] AS [area_code], 
    [Extent1].[phone_type_id] AS [phone_type_id], 
    [Extent1].[last_answer_date] AS [last_answer_date], 
    [Extent1].[csr_bad_flag] AS [csr_bad_flag], 
    [Extent1].[to_process_flag] AS [to_process_flag], 
    [Extent1].[deleted_date] AS [deleted_date], 
    [Extent1].[wireless_flag] AS [wireless_flag]
    FROM [dbo].[phone] AS [Extent1]
    WHERE [Extent1].[phone] = ''5555555555'''

谁能告诉我这里发生了什么以及如何让 Linq 使用索引？

Answer 1

问题不是参数嗅探，而是data type precedence。参数类型为 NVarchar，列为 Varchar。 NVarchar 具有较高的数据类型优先级，因此该列将转换为参数类型，并且不能使用索引。

如果您已映射参数，EF Core 将适当地键入参数。因此，您可能缺少模型中的类型配置。 EG

modelBuilder.Entity<Phone>().Property(a => a.Phone).HasColumnType("varchar").HasMaxLength(50);