Translate 'case when X is null...' to 'coalesce(X,...)'

Author: bgurmendi

src/EntityFramework6.Npgsql/SqlGenerators/SqlBaseGenerator.cs

@@ -829,6 +829,23 @@ protected string GetDbType(EdmType edmType)
 
         public override VisitedExpression Visit([NotNull] DbCaseExpression expression)
         {
+            // Check for COALESCE like CASE
+            if (expression.When.Count == 1 &&
+                expression.When[0].ExpressionKind == DbExpressionKind.IsNull)
+            {
+                var is_null = expression.When[0] as DbIsNullExpression;
+                if (is_null.Argument.Equals(expression.Else))
+                {
+                    LiteralExpression coalesceExpression = new LiteralExpression(" COALESCE( ");
+                    coalesceExpression.Append(expression.Else.Accept(this));
+                    coalesceExpression.Append(",");
+                    coalesceExpression.Append(expression.Then[0].Accept(this));
+                    coalesceExpression.Append(") ");
+                    return coalesceExpression;
+                }
+            }
+
+            // General CASE
             var caseExpression = new LiteralExpression(" CASE ");
             for (var i = 0; i < expression.When.Count && i < expression.Then.Count; ++i)
             {