Remove forced PK_NewTasks index hint in _LockNextTask; add covering index for lock-next scan

src/DurableTask.SqlServer/Scripts/logic.sql

@@ -1282,7 +1282,6 @@ BEGIN
     -- Table order for this sproc: NewTasks --> Payloads
 
     -- Update (lock) and return a single row.
-    -- The PK_NewTasks hint is specified to help ensure in-order selection.
     -- TODO: Filter out tasks for instances that are in a non-running state (suspended, etc.)
     UPDATE TOP (1) NewTasks WITH (READPAST)
     SET
@@ -1291,7 +1290,7 @@ BEGIN
 	    [LockExpiration] = @LockExpiration,
         [DequeueCount] = [DequeueCount] + 1
     FROM
-        NewTasks WITH (INDEX (PK_NewTasks))
+        NewTasks
     WHERE
         [TaskHub] = @TaskHub AND
 	    ([LockExpiration] IS NULL OR [LockExpiration] < @now) AND

src/DurableTask.SqlServer/Scripts/schema-1.7.0.sql

@@ -0,0 +1,17 @@
+-- Copyright (c) Microsoft Corporation.
+-- Licensed under the MIT License.
+
+-- PERSISTENT SCHEMA OBJECTS (tables, indexes, types, etc.)
+--
+-- The contents of this file must never be changed after
+-- being published. Any schema changes must be done in
+-- new schema-{major}.{minor}.{patch}.sql scripts.
+
+-- Add a covering index on NewTasks to improve the performance of _LockNextTask.
+-- This allows SQL Server to efficiently find unlocked, visible tasks without
+-- performing a full clustered-index scan, which is important under high concurrency
+-- when there may be a backlog of locked or not-yet-visible tasks.
+IF NOT EXISTS (SELECT 1 FROM sys.indexes WHERE object_id = OBJECT_ID('__SchemaNamePlaceholder__.NewTasks') AND name = 'IX_NewTasks_LockNext')
+    CREATE NONCLUSTERED INDEX IX_NewTasks_LockNext ON __SchemaNamePlaceholder__.NewTasks(TaskHub, LockExpiration, VisibleTime)
+        INCLUDE (SequenceNumber, LockedBy, DequeueCount)
+GO

src/common.props

@@ -16,7 +16,7 @@
   <!-- Version settings: https://andrewlock.net/version-vs-versionsuffix-vs-packageversion-what-do-they-all-mean/ -->
   <PropertyGroup>
     <MajorVersion>1</MajorVersion>
-    <MinorVersion>6</MinorVersion>
+    <MinorVersion>7</MinorVersion>
     <PatchVersion>0</PatchVersion>
     <VersionPrefix>$(MajorVersion).$(MinorVersion).$(PatchVersion)</VersionPrefix>
     <VersionSuffix></VersionSuffix>