Multiple non-related dbset counts without filter executes as separate queries

Including multiple non-related dbset counts in one query will perform multiple SQL queries if the dbsets are not filtered by a dummy where clause. AsSingleQuery does not help. I would prefer not having to remember to add the .Where(x => true) part to prevent multiple queries.

See full gist here.

Include your code

                // info: Microsoft.EntityFrameworkCore.Database.Command[20101]
                //     Executed DbCommand (4ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
                //     SELECT COUNT(*)
                //     FROM [Users] AS [u]
                // info: Microsoft.EntityFrameworkCore.Database.Command[20101]
                //     Executed DbCommand (1ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
                //     SELECT COUNT(*)
                //     FROM [Animals] AS [a]
                // info: Microsoft.EntityFrameworkCore.Database.Command[20101]
                //     Executed DbCommand (8ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
                //     SELECT TOP(1) 1
                //     FROM [Users] AS [u]

                var bad = await context.Users.Select(u => new {
                    userCount = context.Users.Count(),
                    animalCount = context.Animals.Count()
                }).FirstAsync();

                // info: Microsoft.EntityFrameworkCore.Database.Command[20101]
                //       Executed DbCommand (13ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
                //       SELECT TOP(1) (
                //           SELECT COUNT(*)
                //           FROM [Users] AS [u0]) AS [userCount], (
                //           SELECT COUNT(*)
                //           FROM [Animals] AS [a]) AS [animalCount]
                //       FROM [Users] AS [u]

                var good = await context.Users.Select(u => new {
                    userCount = context.Users.Where(x => true).Count(),
                    animalCount = context.Animals.Where(x => true).Count()
                }).FirstAsync();

Include provider and version information

EF Core version: 6.0.3 Database provider: Microsoft.EntityFrameworkCore.SqlServer Target framework: .NET 6.0

[roji]

Batching two SELECTs in a single roundtrip is likely to yield very similar performance to having the two selects as subqueries in a single SELECT query, i.e.:

SELECT COUNT(*) FROM [Users];
SELECT COUNT(*) FROM [Animals] AS [a];

… is very likely roughly equivalent to:

SELECT TOP(1) (
    SELECT COUNT(*)
    FROM [Users] AS [u0]) AS [userCount], (
    SELECT COUNT(*)
    FROM [Animals] AS [a]) AS [animalCount]
FROM [Users] AS [u]

(this is quite trivial to check). Batching two selects also has the advantage of possibly reusing query plans when the same SELECT COUNT(*) ... elsewhere (IIRC this isn’t the case with SQL Server semicolon-based batching, but would be the case with DbBatch or on PostgreSQL).

So I do believe #10879 is the right general solution to this. I agree it’s odd for us to do roundtrips only when Where is omitted, but whether that’s worth fixing is an implementation complexity question (so @smitpatel).

[ajcvickers]

@divega Do you know if you ever created an issue for having a general-purpose context.Query method that could execute any query? I’m sure there must be/have been an issue, since it was something we discussed a lot, but I can’t find one!