Add UUID version 7 as the default guid generator

src/EFCore.PG/ValueGeneration/Internal/NpgsqlValueGeneratorSelector.cs

@@ -104,6 +104,6 @@ public override bool TrySelect(IProperty property, ITypeBase typeBase, out Value
         => property.ClrType.UnwrapNullableType() == typeof(Guid)
             ? property.ValueGenerated == ValueGenerated.Never || property.GetDefaultValueSql() is not null
                 ? new TemporaryGuidValueGenerator()
-                : new GuidValueGenerator()
+                : new UUid7ValueGenerator()
             : base.FindForType(property, typeBase, clrType);
 }

src/EFCore.PG/ValueGeneration/UUid7ValueGenerator.cs

@@ -0,0 +1,59 @@
+using System.Runtime.InteropServices;
+
+namespace Npgsql.EntityFrameworkCore.PostgreSQL.ValueGeneration;
+
+/// <summary>
+///     Generates sequential <see cref="Guid" /> values according to the UUID version 7 specification.
+///     Will be updated to use Guid.CreateVersion7 when available.
+/// </summary>
+public class UUid7ValueGenerator : ValueGenerator<Guid>
+{
+    private const byte Variant10xxValue = 0x80;
+    private const ushort Version7Value = 0x7000;
+    private const ushort VersionMask = 0xF000;
+    private const byte Variant10xxMask = 0xC0;
+
+    /// <summary>
+    ///     Gets a value to be assigned to a property.
+    /// </summary>
+    /// <param name="entry">The change tracking entry of the entity for which the value is being generated.</param>
+    /// <returns>The value to be assigned to a property.</returns>
+    public override Guid Next(EntityEntry entry)
+    {
+        Span<byte> guidBytes = stackalloc byte[16];
+        var succeeded = Guid.NewGuid().TryWriteBytes(guidBytes);
+        var unixms = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
+        Span<byte> counterBytes = stackalloc byte[sizeof(long)];
+        MemoryMarshal.Write(counterBytes, in unixms);
+
+        if (!BitConverter.IsLittleEndian)
+        {
+            counterBytes.Reverse();
+        }
+
+        //unix ts ms - 48 bits (6 bytes)
+        guidBytes[00] = counterBytes[2];
+        guidBytes[01] = counterBytes[3];
+        guidBytes[02] = counterBytes[4];
+        guidBytes[03] = counterBytes[5];
+        guidBytes[04] = counterBytes[0];
+        guidBytes[05] = counterBytes[1];
+
+        //UIDv7 version - first 4 bits (1/2 byte) of the next 16 bits (2 bytes)
+        var _c = BitConverter.ToInt16(guidBytes.Slice(6, 2));
+        _c = (short)((_c & ~VersionMask) | Version7Value);
+        BitConverter.TryWriteBytes(guidBytes.Slice(6, 2), _c);
+
+        //2 bit variant
+        //first 2 bits of the next 64 bits (8 bytes)
+        guidBytes[8] = (byte)((guidBytes[8] & ~Variant10xxMask) | Variant10xxValue);
+        return new Guid(guidBytes);
+    }
+
+    /// <summary>
+    ///     Gets a value indicating whether the values generated are temporary or permanent. This implementation
+    ///     always returns false, meaning the generated values will be saved to the database.
+    /// </summary>
+    public override bool GeneratesTemporaryValues
+        => false;
+}