Improve some sparse arrays

src/NHibernate/Loader/Loader.cs

@@ -82,7 +82,8 @@ public sealed class QueryCacheInfo
 		/// <summary>
 		/// Caches subclass entity aliases for given persister index in <see cref="EntityPersisters"/>  and subclass entity name
 		/// </summary>
-		private readonly ConcurrentDictionary<Tuple<int, string>, string[][]> _subclassEntityAliasesMap = new ConcurrentDictionary<Tuple<int, string>, string[][]>();
+		private readonly Lazy<ConcurrentDictionary<Tuple<int, string>, string[][]>> _subclassEntityAliasesMap =
+			new(() => new ConcurrentDictionary<Tuple<int, string>, string[][]>());
 
 		protected Loader(ISessionFactoryImplementor factory)
 		{
@@ -1322,7 +1323,7 @@ private void LoadFromResultSet(DbDataReader rs, int i, object obj, ILoadable per
 		private string[][] GetSubclassEntityAliases(int i, ILoadable persister)
 		{
 			var cacheKey = System.Tuple.Create(i, persister.EntityName);
-			return _subclassEntityAliasesMap.GetOrAdd(
+			return _subclassEntityAliasesMap.Value.GetOrAdd(
 				cacheKey,
 				k => EntityAliases[i].GetSuffixedPropertyAliases(persister));
 		}

src/NHibernate/Mapping/Constraint.cs

@@ -15,7 +15,7 @@ namespace NHibernate.Mapping
 	public abstract class Constraint : IRelationalModel
 	{
 		private string name;
-		private readonly List<Column> columns = new List<Column>();
+		private readonly List<Column> columns = new List<Column>(1);
 		private Table table;
 
 		/// <summary>

src/NHibernate/Mapping/ForeignKey.cs

@@ -2,6 +2,7 @@
 using System.Text;
 using NHibernate.Util;
 using System;
+using System.Linq;
 
 namespace NHibernate.Mapping
 {
@@ -14,7 +15,7 @@ public class ForeignKey : Constraint
 		private Table referencedTable;
 		private string referencedEntityName;
 		private bool cascadeDeleteEnabled;
-		private readonly List<Column> referencedColumns = new List<Column>();
+		private List<Column> referencedColumns;
 
 		/// <summary>
 		/// Generates the SQL string to create the named Foreign Key Constraint in the database.
@@ -26,29 +27,26 @@ public class ForeignKey : Constraint
 		/// <returns>
 		/// A string that contains the SQL to create the named Foreign Key Constraint.
 		/// </returns>
-		public override string SqlConstraintString(Dialect.Dialect d, string constraintName, string defaultCatalog, string defaultSchema)
-		{
-			string[] cols = new string[ColumnSpan];
-			string[] refcols = new string[ColumnSpan];
-			int i = 0;
-			IEnumerable<Column> refiter;
-			if (IsReferenceToPrimaryKey)
-				refiter = referencedTable.PrimaryKey.ColumnIterator;
-			else
-				refiter = referencedColumns;
-			foreach (Column column in ColumnIterator)
-			{
-				cols[i] = column.GetQuotedName(d);
-				i++;
-			}
+		public override string SqlConstraintString(
+			Dialect.Dialect d,
+			string constraintName,
+			string defaultCatalog,
+			string defaultSchema)
+		{
+			var refiter = IsReferenceToPrimaryKey
+				? referencedTable.PrimaryKey.Columns
+				: referencedColumns;
+
+			var cols = Columns.ToArray(column => column.GetQuotedName(d));
+			var refcols = refiter.ToArray(column => column.GetQuotedName(d));
+
+			string result = d.GetAddForeignKeyConstraintString(
+				constraintName,
+				cols,
+				referencedTable.GetQualifiedName(d, defaultCatalog, defaultSchema),
+				refcols,
+				IsReferenceToPrimaryKey);
 
-			i = 0;
-			foreach (Column column in refiter)
-			{
-				refcols[i] = column.GetQuotedName(d);
-				i++;
-			}
-			string result = d.GetAddForeignKeyConstraintString(constraintName, cols, referencedTable.GetQualifiedName(d, defaultCatalog, defaultSchema), refcols, IsReferenceToPrimaryKey);
 			return cascadeDeleteEnabled && d.SupportsCascadeDelete ? result + " on delete cascade" : result;
 		}
 
@@ -172,40 +170,24 @@ public virtual void AddReferencedColumns(IEnumerable<Column> referencedColumnsIt
 
 		private void AddReferencedColumn(Column column)
 		{
+			referencedColumns ??= new List<Column>(1);
 			if (!referencedColumns.Contains(column))
 				referencedColumns.Add(column);
 		}
 
 		internal void AddReferencedTable(PersistentClass referencedClass)
 		{
-			if (referencedColumns.Count > 0)
-			{
-				referencedTable = referencedColumns[0].Value.Table;
-			}
-			else
-			{
-				referencedTable = referencedClass.Table;
-			}
+			referencedTable = IsReferenceToPrimaryKey ? referencedClass.Table : referencedColumns[0].Value.Table;
 		}
 
 		public override string ToString()
 		{
-			if (!IsReferenceToPrimaryKey)
-			{
-				var result = new StringBuilder();
-				result.Append(GetType().FullName)
-					.Append('(')
-					.Append(Table.Name)
-					.Append(string.Join(", ", Columns))
-					.Append(" ref-columns:")
-					.Append('(')
-					.Append(string.Join(", ", ReferencedColumns))
-					.Append(") as ")
-					.Append(Name);
-				return result.ToString();
-			}
+			if (IsReferenceToPrimaryKey)
+				return base.ToString();
 
-			return base.ToString();
+			var columns = string.Join(", ", Columns);
+			var refColumns = string.Join(", ", referencedColumns);
+			return $"{GetType().FullName}({Table.Name}{columns} ref-columns:({refColumns}) as {Name}";
 		}
 
 		public bool HasPhysicalConstraint
@@ -218,7 +200,11 @@ public bool HasPhysicalConstraint
 
 		public IList<Column> ReferencedColumns
 		{
-			get { return referencedColumns; }
+			get
+			{	
+				referencedColumns ??= new List<Column>(1);
+				return referencedColumns;
+			}
 		}
 
 		public string ReferencedEntityName
@@ -228,10 +214,7 @@ public string ReferencedEntityName
 		}
 
 		/// <summary>Does this foreignkey reference the primary key of the reference table </summary>
-		public bool IsReferenceToPrimaryKey
-		{
-			get { return referencedColumns.Count == 0; }
-		}
+		public bool IsReferenceToPrimaryKey => referencedColumns == null || referencedColumns.Count == 0;
 
 		public string GeneratedConstraintNamePrefix => "FK_";
 
@@ -242,12 +225,7 @@ public override bool IsGenerated(Dialect.Dialect dialect)
 			if (dialect.SupportsNullInUnique || IsReferenceToPrimaryKey)
 				return true;
 
-			foreach (var column in ReferencedColumns)
-			{
-				if (column.IsNullable)
-					return false;
-			}
-			return true;
+			return referencedColumns.All(column => !column.IsNullable);
 		}
 	}
 }

src/NHibernate/Mapping/Index.cs

@@ -14,7 +14,7 @@ namespace NHibernate.Mapping
 	public class Index : IRelationalModel
 	{
 		private Table table;
-		private readonly List<Column> columns = new List<Column>();
+		private readonly List<Column> columns = new List<Column>(1);
 		private string name;
 
 		public static string BuildSqlCreateIndexString(Dialect.Dialect dialect, string name, Table table,

src/NHibernate/Mapping/SimpleValue.cs

@@ -15,7 +15,7 @@ namespace NHibernate.Mapping
 	[Serializable]
 	public class SimpleValue : IKeyValue
 	{
-		private readonly List<ISelectable> columns = new List<ISelectable>();
+		private readonly List<ISelectable> columns = new List<ISelectable>(1);
 		private IType type;
 		private IDictionary<string, string> typeParameters;
 

src/NHibernate/Mapping/Table.cs

@@ -1064,9 +1064,12 @@ internal ForeignKeyKey(IEnumerable<Column> columns, string referencedClassName,
 			{
 				this.referencedClassName = referencedClassName;
 				this.columns = new List<Column>(columns);
+				this.columns.TrimExcess();
+
 				if (referencedColumns != null)
 				{
 					this.referencedColumns = new List<Column>(referencedColumns);
+					this.referencedColumns.TrimExcess();
 				}
 				else
 				{

src/NHibernate/SqlCommand/SqlString.cs

@@ -215,6 +215,9 @@ internal SqlString(IEnumerable<object> parts)
 			_firstPartIndex = _parts.Count > 0 ? 0 : -1;
 			_lastPartIndex = _parts.Count - 1;
 			_length = sqlIndex;
+
+			_parts.TrimExcess();
+			_parameters.TrimExcess();
 		}
 
 		#endregion