Review 1.2: scan the version store for its generators at most hourly

sys.dm_tran_top_version_generators reads the whole version store. It ran
every 5 minutes while the version store was over half the threshold,
i.e. exactly when it is large and expensive to scan, and a timeout failed
the whole tempdb collection. It now runs at most once per hour (the
previous answer is kept in between, with its time) and a failed scan
keeps the previous answer.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/SQLInsight.Collectors/Instance/TempdbCollector.cs

@@ -42,6 +42,9 @@ public sealed class TempdbCollector(int versionStoreMegabytes = 1024, int sessio
         ORDER BY t.elapsed_time_seconds DESC
         """;
 
+    /// <summary>How often the version store is scanned for the databases generating it, while it is large.</summary>
+    public static readonly TimeSpan GeneratorsInterval = TimeSpan.FromHours(1);
+
     // Scans the version store: only run when it is large enough to need explaining.
     internal const string GeneratorsSql = """
         SELECT TOP 10 DB_NAME(database_id), SUM(CAST(aggregated_record_length_in_bytes AS bigint))
@@ -68,9 +71,30 @@ public sealed class TempdbCollector(int versionStoreMegabytes = 1024, int sessio
         var oldest = (await Db.QueryAsync(connection, OldestSnapshotSql, r => new SnapshotTransaction(r.Int(0), r.NullableLong(1) ?? 0, r.NullableString(2), r.NullableString(3)), ct)
             .ConfigureAwait(false)).FirstOrDefault();
         var versionMb = space.VersionKb is { } kb ? kb / 1024.0 : space.Version * PageMb;
-        var generators = versionMb >= versionStoreMegabytes / 2.0
+        IReadOnlyList<VersionGenerator> generators = [];
-            ? await Db.QueryAsync(connection, GeneratorsSql, r => new VersionGenerator(r.NullableString(0) ?? "?", (r.NullableLong(1) ?? 0) / 1024.0 / 1024.0), ct).ConfigureAwait(false)
+        DateTimeOffset? generatorsAt = null;
-            : [];
+        if (versionMb >= versionStoreMegabytes / 2.0)
+        {
+            // Scanning a large version store every 5 minutes is not cheap: at most once per GeneratorsInterval, and a
+            // scan that fails (times out) keeps the previous answer instead of failing the whole collection.
+            if (previous is { VersionGeneratorsAt: { } at } && context.Now - at < GeneratorsInterval)
+            {
+                (generators, generatorsAt) = (previous.VersionGenerators, at);
+            }
+            else
+            {
+                try
+                {
+                    generators = await Db.QueryAsync(connection, GeneratorsSql, r => new VersionGenerator(r.NullableString(0) ?? "?", (r.NullableLong(1) ?? 0) / 1024.0 / 1024.0), ct)
+                        .ConfigureAwait(false);
+                    generatorsAt = context.Now;
+                }
+                catch (System.Data.Common.DbException)
+                {
+                    (generators, generatorsAt) = (previous?.VersionGenerators ?? [], previous?.VersionGeneratorsAt);
+                }
+            }
+        }
 
         return TempdbAnalysis.Advance(previous, new TempdbSnapshot
         {
@@ -84,6 +108,7 @@ public sealed class TempdbCollector(int versionStoreMegabytes = 1024, int sessio
             TopSessions = sessions,
             OldestSnapshotTransaction = oldest,
             VersionGenerators = generators,
+            VersionGeneratorsAt = generatorsAt,
             VersionStoreThresholdMegabytes = versionStoreMegabytes,
             SessionThresholdMegabytes = sessionMegabytes,
         });

src/SQLInsight.Core/Data/Tempdb.cs

@@ -32,6 +32,9 @@ public sealed record TempdbSnapshot
     public SnapshotTransaction? OldestSnapshotTransaction { get; init; }
     public IReadOnlyList<VersionGenerator> VersionGenerators { get; init; } = [];
 
+    /// <summary>When <see cref="VersionGenerators"/> was read (the scan is expensive and is repeated at most hourly).</summary>
+    public DateTimeOffset? VersionGeneratorsAt { get; init; }
+
     /// <summary>Version store sizes of the latest samples (MB), oldest first: the trend.</summary>
     public IReadOnlyList<double> VersionStoreHistory { get; init; } = [];