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/Stores.cs

@@ -172,6 +172,25 @@ namespace PluralKit {
                 return await conn.QuerySingleAsync<int>("select count(*) from messages where member = @Id", member);
         }
 
+        public struct MessageBreakdownListEntry
+        {
+            public int Member;
+            public int MessageCount;
+        }
+
+        public async Task<IEnumerable<MessageBreakdownListEntry>> MessageCountsPerMember(PKSystem system)
+        {
+            using (var conn = await _conn.Obtain())
+                return await conn.QueryAsync<MessageBreakdownListEntry>(
+                    @"SELECT messages.member, COUNT(messages.member) messagecount
+                        FROM members
+                        JOIN messages
+                        ON members.id = messages.member
+                        WHERE members.system = @System
+                        GROUP BY messages.member",
+                    new { System = system.Id });
+        }
+
         public async Task<int> MemberCount(PKSystem system)
         {
             using (var conn = await _conn.Obtain())
@@ -362,7 +381,7 @@ namespace PluralKit {
                 var switchMembersEntries = await conn.QueryAsync<SwitchMembersListEntry>(
                     @"SELECT switch_members.member, switches.timestamp
                         FROM switches
-                        JOIN switch_members
+                        LEFT JOIN switch_members
                         ON switches.id = switch_members.switch
                         WHERE switches.system = @System
                         AND (
@@ -451,7 +470,7 @@ namespace PluralKit {
                 select new SwitchListEntry
                 {
                     TimespanStart = g.Key,
-                    Members = g.Select(x => memberObjects[x.Member]).ToList()
+                    Members = g.Where(x => x.Member != 0).Select(x => memberObjects[x.Member]).ToList()
                 };
 
             // Loop through every switch that overlaps the range and add it to the output list