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Query null semantics

  • Artikkeli

Introduction

SQL databases operate on 3-valued logic (true, false, null) when performing comparisons, as opposed to the boolean logic of C#. When translating LINQ queries to SQL, EF Core tries to compensate for the difference by introducing additional null checks for some elements of the query. To illustrate this, let's define the following entity:

public class NullSemanticsEntity
{
    public int Id { get; set; }
    public int Int { get; set; }
    public int? NullableInt { get; set; }
    public string String1 { get; set; }
    public string String2 { get; set; }
}

and issue several queries:

var query1 = context.Entities.Where(e => e.Id == e.Int);
var query2 = context.Entities.Where(e => e.Id == e.NullableInt);
var query3 = context.Entities.Where(e => e.Id != e.NullableInt);
var query4 = context.Entities.Where(e => e.String1 == e.String2);
var query5 = context.Entities.Where(e => e.String1 != e.String2);

The first two queries produce simple comparisons. In the first query, both columns are non-nullable so null checks are not needed. In the second query, NullableInt could contain null, but Id is non-nullable; comparing null to non-null yields null as a result, which would be filtered out by WHERE operation. So no additional terms are needed either.

SELECT [e].[Id], [e].[Int], [e].[NullableInt], [e].[String1], [e].[String2]
FROM [Entities] AS [e]
WHERE [e].[Id] = [e].[Int]

SELECT [e].[Id], [e].[Int], [e].[NullableInt], [e].[String1], [e].[String2]
FROM [Entities] AS [e]
WHERE [e].[Id] = [e].[NullableInt]

The third query introduces a null check. When NullableInt is null the comparison Id <> NullableInt yields null, which would be filtered out by WHERE operation. However, from the boolean logic perspective this case should be returned as part of the result. Hence EF Core adds the necessary check to ensure that.

SELECT [e].[Id], [e].[Int], [e].[NullableInt], [e].[String1], [e].[String2]
FROM [Entities] AS [e]
WHERE ([e].[Id] <> [e].[NullableInt]) OR [e].[NullableInt] IS NULL

Queries four and five show the pattern when both columns are nullable. It's worth noting that the <> operation produces more complicated (and potentially slower) query than the == operation.

SELECT [e].[Id], [e].[Int], [e].[NullableInt], [e].[String1], [e].[String2]
FROM [Entities] AS [e]
WHERE ([e].[String1] = [e].[String2]) OR ([e].[String1] IS NULL AND [e].[String2] IS NULL)

SELECT [e].[Id], [e].[Int], [e].[NullableInt], [e].[String1], [e].[String2]
FROM [Entities] AS [e]
WHERE (([e].[String1] <> [e].[String2]) OR ([e].[String1] IS NULL OR [e].[String2] IS NULL)) AND ([e].[String1] IS NOT NULL OR [e].[String2] IS NOT NULL)

Treatment of nullable values in functions

Many functions in SQL can only return a null result if some of their arguments are null. EF Core takes advantage of this to produce more efficient queries. The query below illustrates the optimization:

var query = context.Entities.Where(e => e.String1.Substring(0, e.String2.Length) == null);

The generated SQL is as follows (we don't need to evaluate the SUBSTRING function since it will be only null when either of the arguments to it is null.):

SELECT [e].[Id], [e].[Int], [e].[NullableInt], [e].[String1], [e].[String2]
FROM [Entities] AS [e]
WHERE [e].[String1] IS NULL OR [e].[String2] IS NULL

The optimization can also be used for user-defined functions. See user defined function mapping page for more details.

Writing performant queries

  • Comparing non-nullable columns is simpler and faster than comparing nullable columns. Consider marking columns as non-nullable whenever possible.

  • Checking for equality (==) is simpler and faster than checking for non-equality (!=), because query doesn't need to distinguish between null and false result. Use equality comparison whenever possible. However, simply negating == comparison is effectively the same as !=, so it doesn't result in performance improvement.

  • In some cases, it is possible to simplify a complex comparison by filtering out null values from a column explicitly - for example when no null values are present or these values are not relevant in the result. Consider the following example:

var query1 = context.Entities.Where(e => e.String1 != e.String2 || e.String1.Length == e.String2.Length);
var query2 = context.Entities.Where(
    e => e.String1 != null && e.String2 != null && (e.String1 != e.String2 || e.String1.Length == e.String2.Length));

These queries produce the following SQL:

SELECT [e].[Id], [e].[Int], [e].[NullableInt], [e].[String1], [e].[String2]
FROM [Entities] AS [e]
WHERE ((([e].[String1] <> [e].[String2]) OR ([e].[String1] IS NULL OR [e].[String2] IS NULL)) AND ([e].[String1] IS NOT NULL OR [e].[String2] IS NOT NULL)) OR ((CAST(LEN([e].[String1]) AS int) = CAST(LEN([e].[String2]) AS int)) OR ([e].[String1] IS NULL AND [e].[String2] IS NULL))

SELECT [e].[Id], [e].[Int], [e].[NullableInt], [e].[String1], [e].[String2]
FROM [Entities] AS [e]
WHERE ([e].[String1] IS NOT NULL AND [e].[String2] IS NOT NULL) AND (([e].[String1] <> [e].[String2]) OR (CAST(LEN([e].[String1]) AS int) = CAST(LEN([e].[String2]) AS int)))

In the second query, null results are filtered out from String1 column explicitly. EF Core can safely treat the String1 column as non-nullable during comparison, resulting in a simpler query.

Using relational null semantics

It's possible to disable the null comparison compensation and use relational null semantics directly. This can be done by calling UseRelationalNulls(true) method on the options builder inside OnConfiguring method:

new SqlServerDbContextOptionsBuilder(optionsBuilder).UseRelationalNulls();

Warning

When using relational null semantics, your LINQ queries no longer have the same meaning as they do in C#, and may yield different results than expected. Exercise caution when using this mode.