Addresses a performance issue on the dashboard. (#738)

* Addresses a performance issue on the dashboard.

Using the MI Date table for this query resulted in a cartesian
explosion. The results where correct, but the query plan was awful, it
ended up doing multiple scans (18,000+) of the participant table.

This tweaked query skips the MI table and just scans the converted date.
This is needed to get the full count, but it is better to do it once
rather than tens of thousands of times.

Author: carlsixsmith-moj

src/Server.UI/Pages/Dashboard/Components/SyncComponent.razor.cs

@@ -16,18 +16,15 @@ protected override async Task OnInitializedAsync()
         {
             var uow = GetNewUnitOfWork();
 
-            var query = from d in uow.DbContext.DateDimensions
-                join p in uow.DbContext.Participants
-                    on true equals true// placeholder for where join is done in `where`
-                where p.LastSyncDate >= d.TheDate &&
-                      p.LastSyncDate < d.TheDate.AddDays(1)
-                group p by d.TheDate
-                into g
-                orderby g.Key
+            var query = from p in uow.DbContext.Participants
+                where p.LastSyncDate.HasValue
+                group p by p.LastSyncDate!.Value.Date into g
+                orderby g.Key descending
                 select new SyncRecord(g.Key, g.Count());
 
-        
-            var results = await query.ToArrayAsync();
+            var results = await query.AsNoTracking()
+                .ToArrayAsync();
+
             if (IsDisposed == false)
             {
                 _records = results;
@@ -40,5 +37,5 @@ orderby g.Key
 
     }
 
-    public record SyncRecord(DateTime TheDate, int RecordCount);
+    private record SyncRecord(DateTime TheDate, int RecordCount);
 }