Improve export performance

Refactored ExportSystem to:
- Only fetch message counts once (instead of a query per member)
- Fetch switches using the newly refactored GetTruncatedSwitchList (gets switches and their members in one shot instead of querying for switch members one switch at a time)

- Added a new MessageCountsPerMember method to MemberStore to support the above
- Modified GetTruncatedSwitchList query to retrieve switches with no members (wasn't important for frontpercent, but is if we're reusing that method here)

This doesn't require any index changes beyond those that support GetTruncatedSwitchList, though we can see a small benefit with an index on messages covering the member column (valuable for other reasons outside of these additions).

PluralKit.Core/DataFiles.cs

@@ -26,11 +26,34 @@ namespace PluralKit.Bot
 
         public async Task<DataFileSystem> ExportSystem(PKSystem system)
         {
+            // Export members
             var members = new List<DataFileMember>();
-            foreach (var member in await _members.GetBySystem(system)) members.Add(await ExportMember(member));
+            var pkMembers = await _members.GetBySystem(system); // Read all members in the system
+            var messageCounts = await _members.MessageCountsPerMember(system); // Count messages proxied by all members in the system
+            members.AddRange(pkMembers.Select(m => new DataFileMember
+            {
+                Id = m.Hid,
+                Name = m.Name,
+                DisplayName = m.DisplayName,
+                Description = m.Description,
+                Birthday = m.Birthday != null ? Formats.DateExportFormat.Format(m.Birthday.Value) : null,
+                Pronouns = m.Pronouns,
+                Color = m.Color,
+                AvatarUrl = m.AvatarUrl,
+                Prefix = m.Prefix,
+                Suffix = m.Suffix,
+                Created = Formats.TimestampExportFormat.Format(m.Created),
+                MessageCount = messageCounts.Where(x => x.Member.Equals(m.Id)).Select(x => x.MessageCount).FirstOrDefault()
+            }));
 
+            // Export switches
             var switches = new List<DataFileSwitch>();
-            foreach (var sw in await _switches.GetSwitches(system, 999999)) switches.Add(await ExportSwitch(sw));
+            var switchList = await _switches.GetTruncatedSwitchList(system, Instant.FromDateTimeUtc(DateTime.MinValue.ToUniversalTime()), SystemClock.Instance.GetCurrentInstant());
+            switches.AddRange(switchList.Select(x => new DataFileSwitch
+            {
+                Timestamp = Formats.TimestampExportFormat.Format(x.TimespanStart),
+                Members = x.Members.Select(m => m.Hid).ToList() // Look up member's HID using the member export from above
+            }));
 
             return new DataFileSystem
             {
@@ -47,28 +70,6 @@ namespace PluralKit.Bot
             };
         }
 
-        private async Task<DataFileMember> ExportMember(PKMember member) => new DataFileMember
-        {
-            Id = member.Hid,
-            Name = member.Name,
-            DisplayName = member.DisplayName,
-            Description = member.Description,
-            Birthday = member.Birthday != null ? Formats.DateExportFormat.Format(member.Birthday.Value) : null,
-            Pronouns = member.Pronouns,
-            Color = member.Color,
-            AvatarUrl = member.AvatarUrl,
-            Prefix = member.Prefix,
-            Suffix = member.Suffix,
-            Created = Formats.TimestampExportFormat.Format(member.Created),
-            MessageCount = await _members.MessageCount(member)
-        };
-
-        private async Task<DataFileSwitch> ExportSwitch(PKSwitch sw) => new DataFileSwitch
-        {
-            Members = (await _switches.GetSwitchMembers(sw)).Select(m => m.Hid).ToList(),
-            Timestamp = Formats.TimestampExportFormat.Format(sw.Timestamp)
-        };
-
         public async Task<ImportResult> ImportSystem(DataFileSystem data, PKSystem system, ulong accountId)
         {
             // TODO: make atomic, somehow - we'd need to obtain one IDbConnection and reuse it