Don't simplify filter with custom functions over projection

[kosinsky]

Don't simplify filter with custom functions over projection to avoid suboptimal SQL plans and allow window functions (these function not allowed in WHERE clause)

The change helps following cases:

• Suboptimal SQL performance with custom functions

For IQueryable that looks like:

context.Items.Select(i => new {i.Id, i.Title, Result = MyFuncs.MyFunc(i.Title)})
                      .Where(i => i.Result > 10)

EF6 simplifies expression and keeps only one SELECT with call to the function in SELECT and WHERE parts:

SELECT
       Id,
       Title,
       dbo.UDF_Test(Title) as L
FROM Items
WHERE dbo.UDF_Test(Title) > 10

Not simplified version should look like:

SELECT *
FROM (
     SELECT
           Id,
           Title,
           dbo.UDF_Test(Title) as L
     FROM Items
) T
WHERE L > 10

For table with 100K records.
Simplified query execution takes: 1100 ms of SQL CPU time
Not simplified: 600 ms of SQL CPU time

If I replace dbo.UDF_Test by LEN (build in function), execution plans are the same and no performance difference are observed.

Custom UDF are block box and database have to call function twice for each row and couldn't optimize it.

• Window functions
  When instead of Custom UDF, window functions are used (ROW_NUMBER, LEAD, LAG, PERCENTILE_CONT etc.), SQL SERVER (and I assume every DB that supports window functions) will return error that window function supported only in SELECT or ORDER BY clauses

[dnfclas]

All CLA requirements met.

[ajcvickers]

Thanks @kosinsky. Sorry for the long delay.

[divega]

+1 thanks for this @kosinsky!