Translate COALESCE as ISNULL

[ranma42]

COALESCEis a syntactic shortcut for the CASE expression. As such, the input values are evaluated multiple times.

ISNULL does not have this shortcoming.

Fixes #32519.

[ranma42]

At least a test for the type propagation is needed

[ranma42]

It looks like the type mapping computed in Coalesce does not match the actual expression type :(
At least the test was helpful in identifying this issue 😅
This problem could be related to #14719 or #15586

[raymens]

Another application for this is that COALESCE cannot be used for Indexed Views in SQL Server, however ISNULL is supported in case of aggregations.

This makes it difficult/impossible to make EF Core generated queries use the indexed views as I suppose the query processor thinks they're different.

For example:

CREATE VIEW [dbo].[vAggregation]
   WITH SCHEMABINDING
   AS
      SELECT a, SUM(ISNULL(b, 0.0)) AS bSum, COUNT_BIG(*) AS Count 
      FROM dbo.c
      GROUP BY a
GO

CREATE UNIQUE CLUSTERED INDEX IDX_vAggregation
   ON vAggregation(a)
GO

SELECT SUM(b) FROM c -- SLOW: not using the indexed view
SELECT SUM(COALESCE(b, 0)) FROM c -- SLOW: not using the indexed view
SELECT SUM(ISNULL(b, 0)) FROM c -- FAST: uses the indexed view