From 0619008f6e1dd62ac2e1b72ff5553a433c0acd14 Mon Sep 17 00:00:00 2001
From: Mark Carrington <31017244+MarkMpn@users.noreply.github.com>
Date: Sat, 14 Sep 2024 17:27:58 +0100
Subject: [PATCH] Fixed folding subquery alias to constant scan Fixes #546

---
 .../AdoProviderTests.cs                       | 25 +++++++++++++++++++
 .../ExecutionPlan/AliasNode.cs                |  4 +--
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/MarkMpn.Sql4Cds.Engine.Tests/AdoProviderTests.cs b/MarkMpn.Sql4Cds.Engine.Tests/AdoProviderTests.cs
index 59a901a2..94ad21d9 100644
--- a/MarkMpn.Sql4Cds.Engine.Tests/AdoProviderTests.cs
+++ b/MarkMpn.Sql4Cds.Engine.Tests/AdoProviderTests.cs
@@ -2441,5 +2441,30 @@ SELECT CAST(CAST(10.6496 AS float) AS int) AS trunc1,
                 }
             }
         }
+
+        [TestMethod]
+        public void QueryDerivedTableWithContradiction()
+        {
+            // https://github.com/MarkMpn/Sql4Cds/issues/546
+            using (var con = new Sql4CdsConnection(_localDataSources))
+            using (var cmd = con.CreateCommand())
+            {
+                cmd.CommandTimeout = 0;
+
+                cmd.CommandText = @"
+SELECT *
+FROM (SELECT a.accountid
+      FROM   account AS a
+      WHERE  1 != 1) AS sub";
+
+                using (var reader = cmd.ExecuteReader())
+                {
+                    var schema = reader.GetSchemaTable();
+
+                    if (reader.Read())
+                        Assert.Fail();
+                }
+            }
+        }
     }
 }
diff --git a/MarkMpn.Sql4Cds.Engine/ExecutionPlan/AliasNode.cs b/MarkMpn.Sql4Cds.Engine/ExecutionPlan/AliasNode.cs
index 614b34b8..3784f724 100644
--- a/MarkMpn.Sql4Cds.Engine/ExecutionPlan/AliasNode.cs
+++ b/MarkMpn.Sql4Cds.Engine/ExecutionPlan/AliasNode.cs
@@ -131,7 +131,7 @@ public override IDataExecutionPlanNodeInternal FoldQuery(NodeCompilationContext
             {
                 // Remove any unused columns
                 var unusedColumns = constant.Schema.Keys
-                    .Where(sourceCol => !ColumnSet.Any(col => col.SourceColumn.SplitMultiPartIdentifier().Last().EscapeIdentifier() == sourceCol))
+                    .Where(sourceCol => !ColumnSet.Any(col => (String.IsNullOrEmpty(constant.Alias) && col.SourceColumn == sourceCol) || (!String.IsNullOrEmpty(constant.Alias) && col.SourceColumn == constant.Alias.EscapeIdentifier() + "." + sourceCol)))
                     .ToList();
 
                 foreach (var col in unusedColumns)
@@ -145,7 +145,7 @@ public override IDataExecutionPlanNodeInternal FoldQuery(NodeCompilationContext
                 // Copy/rename any columns using the new aliases
                 foreach (var col in ColumnSet)
                 {
-                    var sourceColumn = col.SourceColumn.SplitMultiPartIdentifier().Last();
+                    var sourceColumn = constant.Alias == null ? col.SourceColumn : col.SourceColumn.SplitMultiPartIdentifier().Last();
 
                     if (String.IsNullOrEmpty(constant.Alias) && col.OutputColumn != col.SourceColumn ||
                         !String.IsNullOrEmpty(constant.Alias) && col.OutputColumn != constant.Alias.EscapeIdentifier() + "." + col.SourceColumn)