Deadlocks: read system_health files without a temp table

The 2012+ query used SELECT ... INTO #system_health, a write to tempdb.
One SELECT with ROW_NUMBER() now returns the deadlock rows and the newest
row (the read position) in the same pass. SQL Scripts and the read-only
test no longer allow any exception.

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

SQL Scripts/README.md

@@ -61,10 +61,7 @@ Bemærk:
 
 - **Read-only.** Alle scripts læser kun: SELECT mod katalog-views og DMV'er, `DBCC TRACESTATUS`, `DBCC SQLPERF(WAITSTATS)`
   (kun SQL Server 2000), `xp_readerrorlog`, `xp_msver`, `sys.fn_trace_gettable`, lokale variabler og `USE`.
-  **Én undtagelse:** `SQLI-DLK-001` / `_collectors/DeadlockCollector.sql` (SQL Server 2012+) opretter en midlertidig
+  Ingen undtagelser: heller ikke midlertidige tabeller i tempdb.
-  tabel `#system_health` i tempdb og sletter den igen i samme batch. Den er privat for sessionen, og ingen brugerdata,
-  indstillinger eller permanente objekter ændres, men det er strengt taget en skrivning i tempdb. Det står også i
-  filernes header.
 - **Per-database-forespørgsler** (indexes, statistikker, Query Store, database-scoped community-regler) kører agenten i
   hver online brugerdatabase. Her er de vist for `msdb`, som findes på alle instanser, så scriptet kan køres uændret.
   Ret `[msdb]` / `N'msdb'` til den database, du vil se.
@@ -87,7 +84,7 @@ Bemærk:
 - `Every_collector_query_is_registered_and_in_a_collector_script`: hver SQL-konstant og hver `Build…Sql`-metode i
   collectorerne er registreret, og hver registreret forespørgsel står i et `_collectors/`-script.
 - `Community_rule_scripts_contain_the_rule_query_verbatim`: community-reglens `query` fra JSON står ordret i filen.
-- `Scripts_are_read_only_apart_from_the_documented_temp_table`: ingen skrivende sætninger ud over undtagelsen ovenfor.
+- `Scripts_are_read_only`: ingen skrivende sætninger, heller ikke `SELECT … INTO #temp`.
 
 En ny regel skal derfor have sin `SQL Scripts/<id>.sql` (se `CONTRIBUTING.md`). Ændres en collectors SQL, peger
 testen på de filer, der skal opdateres.

SQL Scripts/SQLI-DLK-001.sql

@@ -31,11 +31,8 @@
 --     Delt script: SQL Scripts/_collectors/DeadlockCollector.sql
 -- Regelkode:   src/SQLInsight.Rules/Concurrency/ConcurrencyRules.cs : DeadlockRule.Evaluate
 --
--- Read-only:   IKKE STRENGT READ-ONLY. DeadlockCollector.BuildFileSql (SQL Server 2012+) opretter en midlertidig
+-- Read-only:   Ja: kun SELECT (sys.fn_xe_file_target_read_file og DMV'er) samt SET-sessionsindstillinger. Ingen
---              tabel i tempdb (SELECT ... INTO #system_health) og sletter den igen (DROP TABLE #system_health) i
+--              temp-tabeller; læsepositionen findes med ROW_NUMBER() i samme forespørgsel.
---              samme batch. Den er privat for sessionen; ingen brugerdata, indstillinger eller permanente objekter
---              ændres, men det er en skrivning i tempdb. Ring buffer-varianten (2008/2008 R2) er read-only. Øvrigt:
---              kun SELECT samt SET-sessionsindstillinger.
 -- Kørsel:      Kør i SSMS som agentens login (eller et login med samme rettigheder). Hver variant er sin egen batch
 --              (GO); en variant for en anden SQL Server-version end din kan fejle - det er forventet.
 --              Per-database-forespørgsler er vist for databasen msdb, som findes overalt: ret navnet til den
@@ -59,14 +56,14 @@ GO
 -- Variant: SQL Server 2012 og nyere (system_health-filer; første læsning)
 -- Kilde: DeadlockCollector.BuildFileSql
 -- Kode:  src/SQLInsight.Collectors/Concurrency/DeadlockCollector.cs : DeadlockCollector.BuildFileSql(lastFile: null, lastOffset: null)
-SELECT object_name, file_name, file_offset,
+SELECT e.event_data, e.file_name, e.file_offset, e.rn
-       CASE WHEN object_name = N'xml_deadlock_report' THEN CAST(event_data AS nvarchar(max)) END AS event_data
+FROM (
-INTO #system_health
+    SELECT CASE WHEN object_name = N'xml_deadlock_report' THEN CAST(event_data AS nvarchar(max)) END AS event_data,
-FROM sys.fn_xe_file_target_read_file(N'system_health*.xel', NULL, NULL, NULL);
+           file_name, file_offset,
-
+           ROW_NUMBER() OVER (ORDER BY file_name DESC, file_offset DESC) AS rn
-SELECT event_data FROM #system_health WHERE event_data IS NOT NULL;
+    FROM sys.fn_xe_file_target_read_file(N'system_health*.xel', NULL, NULL, NULL)
-SELECT TOP (1) file_name, file_offset FROM #system_health ORDER BY file_name DESC, file_offset DESC;
+) AS e
-DROP TABLE #system_health;
+WHERE e.event_data IS NOT NULL OR e.rn = 1;
 GO
 
 -- Variant: SQL Server 2008 – 2008 R2, og når event-filerne ikke kan læses

SQL Scripts/_collectors/DeadlockCollector.sql

@@ -12,11 +12,8 @@
 --   GRANT VIEW SERVER SECURITY STATE TO [agent];       -- SQL Server 2022+
 --
 -- Bruges af:   SQLI-DLK-001
--- Read-only:   IKKE STRENGT READ-ONLY. DeadlockCollector.BuildFileSql (SQL Server 2012+) opretter en midlertidig
+-- Read-only:   Ja: kun SELECT (sys.fn_xe_file_target_read_file og DMV'er) samt SET-sessionsindstillinger. Ingen
---              tabel i tempdb (SELECT ... INTO #system_health) og sletter den igen (DROP TABLE #system_health) i
+--              temp-tabeller; læsepositionen findes med ROW_NUMBER() i samme forespørgsel.
---              samme batch. Den er privat for sessionen; ingen brugerdata, indstillinger eller permanente objekter
---              ændres, men det er en skrivning i tempdb. Ring buffer-varianten (2008/2008 R2) er read-only. Øvrigt:
---              kun SELECT samt SET-sessionsindstillinger.
 -- Kørsel:     Kør i SSMS som agentens login. Hver variant er sin egen batch (GO); en variant for en anden SQL Server-version
 --             end din kan fejle - det er forventet. Agenten kører altid med sessionsindstillingerne i _collectors/InstanceProbe.sql.
 -- ===================================================================================================================
@@ -24,14 +21,14 @@
 -- Variant: SQL Server 2012 og nyere (system_health-filer; første læsning)
 -- Kilde: DeadlockCollector.BuildFileSql
 -- Kode:  src/SQLInsight.Collectors/Concurrency/DeadlockCollector.cs : DeadlockCollector.BuildFileSql(lastFile: null, lastOffset: null)
-SELECT object_name, file_name, file_offset,
+SELECT e.event_data, e.file_name, e.file_offset, e.rn
-       CASE WHEN object_name = N'xml_deadlock_report' THEN CAST(event_data AS nvarchar(max)) END AS event_data
+FROM (
-INTO #system_health
+    SELECT CASE WHEN object_name = N'xml_deadlock_report' THEN CAST(event_data AS nvarchar(max)) END AS event_data,
-FROM sys.fn_xe_file_target_read_file(N'system_health*.xel', NULL, NULL, NULL);
+           file_name, file_offset,
-
+           ROW_NUMBER() OVER (ORDER BY file_name DESC, file_offset DESC) AS rn
-SELECT event_data FROM #system_health WHERE event_data IS NOT NULL;
+    FROM sys.fn_xe_file_target_read_file(N'system_health*.xel', NULL, NULL, NULL)
-SELECT TOP (1) file_name, file_offset FROM #system_health ORDER BY file_name DESC, file_offset DESC;
+) AS e
-DROP TABLE #system_health;
+WHERE e.event_data IS NOT NULL OR e.rn = 1;
 GO
 
 -- Variant: SQL Server 2008 – 2008 R2, og når event-filerne ikke kan læses

src/SQLInsight.Collectors/Concurrency/DeadlockCollector.cs

@@ -51,16 +51,17 @@ public sealed class DeadlockCollector : Collector<DeadlockSnapshot>
             ? $"{Db.Literal(lastFile)}, {lastOffset.Value.ToString(CultureInfo.InvariantCulture)}"
             : "NULL, NULL";
 
-        // Only the deadlock rows carry their XML; the rest are kept just to know where reading stopped.
+        // One read of the files and no temp table: the deadlock rows carry their XML, and the newest row
+        // (rn = 1) is returned as well, only to know where reading stopped.
         return $"""
-            SELECT object_name, file_name, file_offset,
+            SELECT e.event_data, e.file_name, e.file_offset, e.rn
-                   CASE WHEN object_name = N'xml_deadlock_report' THEN CAST(event_data AS nvarchar(max)) END AS event_data
+            FROM (
-            INTO #system_health
+                SELECT CASE WHEN object_name = N'xml_deadlock_report' THEN CAST(event_data AS nvarchar(max)) END AS event_data,
-            FROM sys.fn_xe_file_target_read_file(N'system_health*.xel', NULL, {start});
+                       file_name, file_offset,
-
+                       ROW_NUMBER() OVER (ORDER BY file_name DESC, file_offset DESC) AS rn
-            SELECT event_data FROM #system_health WHERE event_data IS NOT NULL;
+                FROM sys.fn_xe_file_target_read_file(N'system_health*.xel', NULL, {start})
-            SELECT TOP (1) file_name, file_offset FROM #system_health ORDER BY file_name DESC, file_offset DESC;
+            ) AS e
-            DROP TABLE #system_health;
+            WHERE e.event_data IS NOT NULL OR e.rn = 1;
             """;
     }
 
@@ -84,14 +85,17 @@ public sealed class DeadlockCollector : Collector<DeadlockSnapshot>
         await using (var reader = await command.ExecuteReaderAsync(ct).ConfigureAwait(false))
         {
             while (await reader.ReadAsync(ct).ConfigureAwait(false))
+            {
+                if (!reader.IsDBNull(0))
                 {
                     events.Add(reader.GetString(0));
                 }
 
-            if (await reader.NextResultAsync(ct).ConfigureAwait(false) && await reader.ReadAsync(ct).ConfigureAwait(false))
+                if (Convert.ToInt64(reader.GetValue(3), CultureInfo.InvariantCulture) == 1)
                 {
-                file = reader.GetString(0);
+                    file = reader.GetString(1);
-                offset = Convert.ToInt64(reader.GetValue(1), CultureInfo.InvariantCulture);
+                    offset = Convert.ToInt64(reader.GetValue(2), CultureInfo.InvariantCulture);
+                }
             }
         }
 

tests/SQLInsight.Collectors.Tests/SqlScripts/SqlScriptTests.cs

@@ -134,7 +134,7 @@ public sealed partial class SqlScriptTests
     }
 
     [Fact]
-    public void Scripts_are_read_only_apart_from_the_documented_temp_table()
+    public void Scripts_are_read_only()
     {
         var problems = new List<string>();
         foreach (var file in AllScripts())
@@ -142,12 +142,6 @@ public sealed partial class SqlScriptTests
             var text = Read(file);
             var code = CommentsAndStrings().Replace(text, " ");
             code = AllowedStatements().Replace(code, " ");
-            if (text.Contains("-- Kilde: DeadlockCollector.BuildFileSql", StringComparison.Ordinal))
-            {
-                // Documented exception: a session-private temp table in tempdb (see the script's Read-only line).
-                code = DeadlockTempTable().Replace(code, " ");
-            }
-
             problems.AddRange(Writes().Matches(code).Select(m => $"{Path.GetRelativePath(Folder, file)}: {m.Value.ToUpperInvariant()}").Distinct());
         }
 
@@ -156,6 +150,7 @@ public sealed partial class SqlScriptTests
 
     [Theory]
     [InlineData("SELECT name INTO dbo.x FROM sys.objects", true)]
+    [InlineData("SELECT name INTO #x FROM sys.objects", true)]
     [InlineData("UPDATE dbo.t SET a = 1", true)]
     [InlineData("EXEC sys.sp_configure N'x', 1", true)]
     [InlineData("SELECT is_auto_update_stats_on, create_date FROM sys.databases -- DROP TABLE x", false)]
@@ -213,9 +208,6 @@ public sealed partial class SqlScriptTests
     [GeneratedRegex(@"\bEXEC\s+master\.dbo\.xp_(readerrorlog|msver)\b|\bDBCC\s+TRACESTATUS\b|\bDBCC\s+SQLPERF\s*\(\s*WAITSTATS\s*\)", RegexOptions.IgnoreCase)]
     private static partial Regex AllowedStatements();
 
-    [GeneratedRegex(@"\bINTO\s+#system_health\b|\bDROP\s+TABLE\s+#system_health\b", RegexOptions.IgnoreCase)]
-    private static partial Regex DeadlockTempTable();
-
     [GeneratedRegex(@"\b(INSERT|UPDATE|DELETE|MERGE|DROP|ALTER|CREATE|TRUNCATE|GRANT|REVOKE|DENY|BACKUP|RESTORE|RECONFIGURE|KILL|SHUTDOWN|BULK|OPENROWSET|OPENQUERY|OPENDATASOURCE|WAITFOR|INTO|EXEC|EXECUTE|DBCC|SETUSER)\b", RegexOptions.IgnoreCase)]
     private static partial Regex Writes();
 }