How to read and interpret the SQL Server log

March 10th, 2014

The SQL Server transaction log contains the history of every action that modified anything in the database. Reading the log is often the last resort when investigating how certain changes occurred. It is one of the main forensic tools at your disposal when trying to identify the author of an unwanted change. Understanding the log and digging through it for information is pretty hard core and definitely not for the faint of heart. And the fact that the output of ::fn_dblog can easily go into millions of rows does not help either. But I’ll try to give some simple practical examples that can go a long way into helping sort through all the information and dig out what you’re interested in.

SQL Server uses Write-ahead logging to maintain transactional consistency

Write-ahead logging (WAL), also referred to as journaling, is best described in the ARIES papers but we can get away with a less academic description: SQL Server will first describe in the log any change is about to make, before modifying any data. There is a brief description of this protocol at SQL Server 2000 I/O Basics and the CSS team has put forward a much referenced presentation at How It Works: Bob Dorr’s SQL Server I/O Presentation. What is important for us in the context of this article is that a consequence of the WAL protocol is that any change that occurred to any data stored in SQL Server must be described somewhere in the log. Remember now that all your database objects (tables, views, stored procedures, users, permissions etc) are stored as data in the database (yes, metadata is still data) so it follows that any change that occurred to any object in the database is also described somewhere in the log. And when I say every operation I really do mean every operation, including the often misunderstood minimally logged and bulk logged operations, and the so called “non-logged” operations, like TRUNCATE. In truth there is no such thing as a non-logged operation. For all practical purposes everything is logged and everything leaves a trace in the log that can be discovered.

The log contains interleaved operations from multiple transactions

The image above shows how 3 concurrent transactions were lay out in the log: the first transaction contains two insert and a delete operation, the second one contains an insert but had rolled back hence it contains a compensating delete and the last operation committed two deletes and an insert operation. Although all three transactions run ‘simultaneous’ the true order of operation is the one in the log, given by the LSN. Usually when analyzing the log we need to look at operations from specific transactions. The key field is the [Transaction ID] column which contains the system degenerate transaction ID for each operation logged. All operations logged by a transaction will have the same [Transaction ID] value.

Lets create a small example with some activity and look at the generated log:


create table demotable (
	id int not null identity(1,1) primary key, 
	data char(20), 
	created_at datetime default getutcdate());
go

insert into demotable (data) values ('standalone xact');
go 5


begin transaction
go

insert into demotable (data) values ('one xact');
go 5

commit;
go

delete from demotable
where id in (2,5,6,9);
go

The first think I will do will be to look at the LOP_BEGIN_XACT operations. This log record marks the beginning of a transaction and is very important in log analysis because is the only log record that contains the date and time when the transaction started, and also contains the SID of the user that had issued the statement. Here is how I query for these operations:


select [Current LSN], [Operation], [Transaction ID], [Parent Transaction ID],
	[Begin Time], [Transaction Name], [Transaction SID]
from fn_dblog(null, null)
where [Operation] = 'LOP_BEGIN_XACT'

We can recognize some of the transactions in our script. Remember that SQL Server will always operate under a transaction, and if one is not explictly started then SQL Server itself will start one for the the statement being executed. At LSN 20:5d:2 we started transaction 2d0 named CREATE TABLE, so this must be the implicit transaction started for the CREATE TABLE statement. At LSN 20:9b:2 the transaction id 2d6 named INSERT is for our first standalone INSERT statement, and similarly LSNs 20:a0:2 starts transaction 2d9, 20:a1:1 starts 2da, LSN 20:a2:1 starts xact 2db, LSN 20:a3:1 starts xact 2db and LSN 20:a3:1 starts xact 2dc respectively. For those of you that detect a certain pattern in the LSN numbering the explanation is given in my What is an LSN: Log Sequence Number article. And if hex number are familiar to you then you’ll also recognize that the transaction ID is basically an incrementing number.

The LSN 20:a4:1 that starts xact 2dd is a little bit different because is named user_transaction. This is how explicit transactions started with BEGIN TRANSACTION show up. There are no more implicit transactions named INSERT for the 5 INSERT statements inside the explicit transaction because the INSERT statement did not have to start an implicit transaction: it used the explicit transaction started by the BEGIN TRANSACTION statement. Finally the LSN 20:aa:1 starts the implicit xact 2df for the DELETE statement.

In addition we also see a number of transactions that we cannot correlate directly with a statement from their name. Transactions 2d2, 2d3, 2d4 and 2d5 are SplitPage transactions. SplitPage transactions are how SQL Server maintains the structural key order constraints of B-Trees and there is ample literature on this subject, see Tracking page splits using the transaction log or How expensive are page splits in terms of transaction log?. Notice that all these SlipPage transactions have the same Parent Transaction ID of 2d0 which is the CREATE TABLE xact. These are page splits that occur in the metadata catalog tables that are being inserted into by the CREATE TABLE operation.

Parent Transaction ID shows when sub-transactions are started internally by SQL Server

We can also see that right after the first INSERT a transaction named Allocate Root was started. Tables are normally created w/o any page and the very first INSERT will trigger the allocation of the first page for the table. the allocation occurs in a separate transaction that is committed imedeatly so it allows other inserts in the table to proceed even if the insert that actually triggered the allocation stalls (does not commit, or even rolls back). This capability of starting and committing transactions in a session independently of the session main transaction exists in the SQL Server, but is not exposed in any way to the T-SQL programming surface.

The Transaction SID column shows the SID of the login that started the transaction. You can use the SUSER_SNAME function to retrieve the actual login username.

Detailed look at one transaction


select [Current LSN], [Operation], 
	[AllocUnitName], [Page ID], [Slot ID], 
	[Lock Information],
	[Num Elements], [RowLog Contents 0], [RowLog Contents 1], [RowLog Contents 2]
from fn_dblog(null, null)
where [Transaction ID]='0000:000002d6'

I choose to look at the xact ID 2d6, which is the first INSERT operation. The result may seem arcane, but there is a lot of useful info if one knows where to look. First the trivial: the xact is 2d6 has 4 operations in the log. Every transaction must have an LOP_BEGIN_XACT and a record to close the xact, usually LOP_COMMIT_XACT. The other two operation are a recording of locks acquired (LOP_LOCK_XACT) and an actual data modification (LOP_INSERT_ROWS). the meat and potatoes lay with the data modification operation, but we’ll see later how LOP_LOCK_XACT can also be extremely useful in analysis.

Operations that modify data, like LOP_INSERT_ROWS will always log the physical details of the operation (page id, slot id) and the object which they modified: the allocation unit id (see sys.allocation_units) and the partition id (see sys.partitions). Remember that even non-partitioned tables are still represented in sys.partitions, you can think at them as being partitioned with a single partition for the whole table. The AllocUnitName column in fn_dblog output shows the is ussualy the simplest way to identify which operations affected a specific table.

The Page ID and Slot ID columns will tell exactly what record, on what page, was modified by this transaction. 4f hex is 79 decimal, my database id is 5:


dbcc traceon(3604,-1);
dbcc page(5,1,79,3);

Slot 0 Offset 0x60 Length 39
Record Type = PRIMARY_RECORD        Record Attributes =  NULL_BITMAP    Record Size = 39
Memory Dump @0x000000000EB0A060
0000000000000000:   10002400 01000000 7374616e 64616c6f 6e652078  ..$.....standalone x
0000000000000014:   61637420 20202020 912ba300 b6a10000 030000    act     ‘+£.¶¡.....
Slot 0 Column 1 Offset 0x4 Length 4 Length (physical) 4
id = 1                              
Slot 0 Column 2 Offset 0x8 Length 20 Length (physical) 20
data = standalone xact              
Slot 0 Column 3 Offset 0x1c Length 8 Length (physical) 8
created_at = 2013-05-06 09:54:05.070
Slot 0 Offset 0x0 Length 0 Length (physical) 0
KeyHashValue = (8194443284a0)       

Using the Page ID and Slot ID is one way to search for log records that affected a row of interest.

Indeed the page 5:1:79 at slot 0 contains the first INSERT from our script. The fn_dblog output matches what we find in the database. Besides the academic interest, this info has practical use: if we want to know what transaction modified a certain record and we know the record position (page, slot) we can search the log for records that updated that particular position. Alas, this approach has its drawbacks. The biggest problem is that in B-Trees the position of a record is very mobile. Page split operations move records and change their physical location, thus making it harder to find the transactions that modified the record.

The Lock Information column is actually my favorite forensic analysis column. Here is the full content for the LOP_INSERT_ROWS operation in my example: HoBt 72057594039042048:ACQUIRE_LOCK_IX OBJECT: 5:245575913:0 ;ACQUIRE_LOCK_IX PAGE: 5:1:79 ;ACQUIRE_LOCK_X KEY: 5:72057594039042048 (8194443284a0). We have here the actual object_id of the table, again the page id, and then the key lock. For B-Trees the key lock is a hash of the key value and thus can be used to locate the row, even if it moved after page splits:


select %%lockres%%, *
from demotable
where %%lockres%% = '(8194443284a0)';


We again located the same row, the first INSERT. Remember that lock resource hashes, by definition, can produce false positives due to hash collision. But the probability is low enough and one must simply exercise common sense if a collision is encountered (eg. the SELECT above returns more than one row for the desired lock resource hash).

The last column(s) I'm going to talk about are the log content columns. These contains the record 'payload' for data operations. The payload varies from operation type to operation type, but it must contain enough information so that recovery can both redo and undo the operation. For an INSERT that means that the full content of the inserted row must be present in the log record. Lets look at the Log Content 0:

0x10002400010000007374616E64616C6F6E6520786163742020202020912BA300B6A10000030000

Is all greek, I know, but just look again at the DBCC PAGE output:


0000000000000000:   10002400 01000000 7374616e 64616c6f 6e652078  ..$.....standalone x
0000000000000014:   61637420 20202020 912ba300 b6a10000 030000    act     ‘+£.¶¡.....

The content is identical. That means that the LOP_INSERT_ROWS operation Log Content 0 will contain the actual image of the row as it was inserted in the page. And this gives us the last, and often most difficult, way to discover the log records that affected a row of interest: analyse the actual log record content for patterns that identify the row. If you are familiar with the Anatomy of a Record you can identify the record parts:

  • 10002400 is the record header
  • 01000000 is the first fixed column (id, int)
  • 0x7374616E64616C6F6E6520786163742020202020 is the second fixed char(20) column
  • 0x912ba300b6a10000 is the datetime created_at column
  • 0x030000 is the NULL bitmap

Lets consider now an example of how to use this newly gained know-how. I did some modifications to the table:


insert into demotable (data) values ('junk'), ('important'), ('do not change!');
update demotable set data='changed' where data = 'do not change!';
update demotable set created_at = getutcdate() where data = 'changed';
delete from demotable where data='important'

Now the table contains a row that should not be there ('junk'), a row that was 'important' is missing and a row that should had not been changed was actually changed. Can we locate the corresponding log operations? Lets start with the easy case: the 'junk' record is still in the table so we can simply search for the log record that inserted it. We'll use the lock resource hash to locate the log record:


declare @lockres nchar(14);
select top (1) @lockres = %%lockres%%
	from demotable
	where data = 'junk';

declare @xact_id nvarchar(14)
select top (1) @xact_id = [Transaction ID]
	from fn_dblog(null, null)
	where charindex(@lockres, [Lock Information]) > 0;

select [Current LSN], [Operation], [Transaction ID], 
	[Transaction SID], [Begin Time], [End Time] 
	from fn_dblog(null, null)
	where [Transaction ID] = @xact_id;

The script finds the lock resource hash value for the row with the 'junk' data value, it then finds the transaction ID that locked such a row and finally it shows all the operations this transaction did. There are couple of leaps-of-faith here: I assume that the lock resource hash is unique and I also assume that the first transaction that locks this row is the transaction we're interested in. In a real life scenario things can get significantly more complex because hash collisions and because there could be multiple transactions that locked the row with the lock resource we're interested in.

The next challenge is to locate the transaction that modified the data that was not supposed to change. We could run the very same script as above, but it would find the transaction that did the original INSERT of the row, not the one that did the UPDATE. So instead lets find all the transactions that locked the row's lock resource hash:


declare @lockres nchar(14);
select top (1) @lockres = %%lockres%%
	from demotable
	where data = 'changed';

declare @xact_id table (id nvarchar(14));
insert into @xact_id (id) 
select [Transaction ID]
	from fn_dblog(null, null)
	where charindex(@lockres, [Lock Information]) > 0;

select [Current LSN], [Operation], [Transaction ID], 
	[Transaction SID], [Begin Time], [End Time],
	[Num Elements], [RowLog Contents 0], [RowLog Contents 1], [RowLog Contents 2],
	[RowLog Contents 3], [RowLog Contents 4], [RowLog Contents 5]
	from fn_dblog(null, null)
	where [Transaction ID] in (select id from @xact_id);

This shows three transactions (2e0, 2e2 and 2e4), the first one being our original INSERT and then two UPDATE transactions: the LOP_MODIFY_ROW is the log record for updating a row. How can we know which UPDATE is the one that modified the data which was not supposed to change? This time I included in the result [RowLog Contents 0] through [RowLog Contents 5], becuase the [Num Elements] is 6 which indicates the log record contains 6 payloads. Remember that the log record must contain sufficient information for recovery to be able to both redo and undo the operation, so these payloads must contain the value being changed, both before and after the change. The LOP_MODIFY_ROWS at LSN 20:b4:2 has payloads like 0x646F206E6F74206368616E676521 0 and 0x6368616E67656420202020202020, which are clearly ASCII strings:


select cast(0x646F206E6F74206368616E676521 as varchar(20)),
	cast(0x6368616E67656420202020202020 as varchar(20));
-------------------- --------------------
do not change!       changed       
(1 row(s) affected)

Even w/o fully understanding the log record internals, we can use some educated guess to figure out which UPDATE did the modification by looking at the log record payloads.

The UPDATE we're interested in is the xact 2e2. From the LOP_BEGIN_XACT log operation for this xact we can, again, figure out when did the change occurred and who did it.

The final challenge was to figure out when was the 'important' record deleted. Since the record is now missing we cannot use again the %%lockres%% lock resource hash trick. If we would know the ID of the deleted record we could do some tricks, because if we recreate a record on the same table with the same key as the deleted row then it will have the same lock resource hash and we can search this hash. But we don't know the ID of the deleted row, only the value in the data column. If the value is distinct enough we can brute force ourselves by searching the log records payload:


select * 
from fn_dblog(null, null)
where charindex(cast('important' as varbinary(20)), [Log Record]) > 0;

We found 3 transactions that had the string 'important' in the log record payload. We know xact 2e0 is the original INSERT and we found xact 2e3 to contain an LOP_DELETE_ROWS, which is the log record for DELETE operations. Using this brute force approach we did find the transaction that deleted the record that contained the 'important' string. A note of caution is warranted here: my example uses a simple ASCII string. One must understand how the values are represented in binary to search the log payload for known values. This is quite tricky for more complex values, as you need to know the internal representation format (eg. the numeric/decimal types) and be able to write the correct Intel platform LSB value for types like int or datetime.

So what's the deal with the LOP_MODIFY_ROWS at xact 2e1, afteral we did not modify the row with the value 'important' in our test script. Again, the log will contain all the details:


select [Current LSN], [Operation], [AllocUnitName], [Transaction Name]
	from fn_dblog(null, null)
	where [Transaction ID] = '0000:000002e1';

The transaction is a stats auto-update. The actual string 'important' was copied into the stats when the automatically generated stats where created and thus the log record for the internal stats blob maintenance update operation will, obviously, contain the string 'important'.

Looking at log after truncation


backup log demolog to disk = 'c:\temp\demolog.trn'

Use fn_dump_dblog to look at the log records in log backup files

I just backed up the database log on my little demo database and this triggered log truncation: since the log records are backed up there is no need to keep them in the active database log, so the space in the LDF file can be reused for new log records. See How to shrink the SQL Server log for more details on how this works. But if I attempt now any of my queries above they will fail to locate anything in the log. To investigate now I have to look at the log backups:


select [Current LSN], [Operation], [AllocUnitName], [Transaction Name]
	from fn_dump_dblog (
    NULL, NULL, 'DISK', 1, 'c:\temp\demolog.trn',
    DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT,
    DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT,
    DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT,
    DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT,
    DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT,
    DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT,
    DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT,
    DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT,
    DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT, DEFAULT)
	where [Transaction ID] = '0000:000002e1';

For a brief introduction into fn_dump_dblog see Using fn_dblog, fn_dump_dblog, and restoring with STOPBEFOREMARK to an LSN. Having the capability to analize database log backups is critical in such forensic investigations. The same techniques apply as the one I showed using fn_dblog, but typically you will have to search through many log backup files.

When Lock Information does not help

You've seen how the [Lock Information] column is of great help to locate the log record of interest, but there are cases when this information is missing from the log. Once case is when the record was never locked individually, eg. when lock escalation occurred or a table lock hint was used and a higher granularity X lock was acquired:


delete top(1) from demotable with (tablockx)

select [Current LSN], [Operation], [Transaction ID],
	[AllocUnitName], [Page ID], [Slot ID], [Lock Information]
	from fn_dblog(null, null);

Because the TABLOCKX hint was used the [Lock Information] does not contain the lock resource hash for the individual row deleted, it contains the record of the table X lock. However the log record payload still contains the deleted row image and we could still search using the [Log Record] or the [Log Contents 0..5] fields. The [Page ID] and [Slot ID] are also present and we could use them to try to identify the log records that affected the row we're interested in.

Another case that makes investigating the log difficult is minimally logged operations. With minimally logging the log contains only a record that describes the fact that a certain page contains minimally logged changes and the SQL Server engine must guarantee that that page is flushed to disk before the transaction commits, see Operations That Can Be Minimally Logged. In such a case the only record youl'll see will be the LOP_FORMAT_PAGE with the [Page ID] of the page that contains minimally logged operations. The good news, for such investigations, is that only INSERT and blob .WRITE updates can be minimally logged (and the legacy, deprecated, WRITETEXT/UPDATETEXT), so the sheer problem space is reduced as most data modifications, and specially accidental modifications, cannot be minimally logged and thus will leave a full blown trace in the log. There is also an opposite case, when the log contain more information about updates: when the table is an article in a transactional replication publication, but I will not enter into details on this topic.

Practical Considerations

My short demo operated on a tiny log. In real cases is not unusual to have to dig out one log record out of millions. And is just very very slow. In my experience is usually better to copy the log first into a temp table, at least the fields of interest and a relevant portion. This table can be then indexed as needed (eg. by [Transaction ID]) and analysis can be done much faster. It also helps if one can restrict the start and end LSN of interest because fn_dblog can return results much faster if is restricted to known LSNs. The shorter the interval (more appropriate LSNs), the faster the investigation. Be careful though as fn_dblog will cause a server core dump if the LSNs passed in are incorrect... on a live production server this can equate several minutes of a complete SQL Server freeze!

DDL Changes

Log analysis can be used for investigating modification to the database schema (DDL changes) using the same methods as one used to analyze data changes. Just remember that databases store their schema inside the database, and every single object is described in one or more catalog (system) tables. All DDL changes end up as data changes done to these catalog tables. Interpreting the catalog changes (insert/update/delete of rows in catalog tables) though is not trivial. The fact that all DDL changes are catalog changes also means that all DDL changes look the same. An LOP_INSER_ROWS into sys.syschhobjs could mean a CREATE TABLE, but it could just as well mean a CREATE VIEW or CREATE PROCEDURE. Furthermore, CREATE TABLE foo will look very similar to CREATE TABLE bar when analyzing the log. One of the main differences from data changes log analysis is using data locks to identify rows: with DDL changes you should not try to follow the actual data locks (locks on catalog table rows acquired as the DDL changes are being applied as basic INSERT/UPDATE/DELETE operation. Instead you should try to locate metadata locks. In general DDL operations will start with a SCH_M lock record which contains the object ID of the object being modified.

Another problem with doing schema modification analysis using the log is that the way SQL Server uses the system catalog to store its metadata changes between releases. So even if you figured out one way to interpret the log to understand what kind of DDL change occurred, you can discover that another SQL Server version uses a very different system catalog structure and changes look completely different on that version. You can rely on the system catalog (and therefore the way DDL operations appear in the log) as being consistent between SQL Server editions and between service packs and cumulative updates of the same version (this later point actually has a couple of exceptions, but is not worth talking about them).

Using object ID

The best way to locate DDL operations that affected an object is to locate transactions that required a SCH_M lock on that object. Use OBJECT_ID to gett he object ID, then look for the pattern N'%SCH%<object id>%' in the Lock Information column. For instance, say I am looking for changes to object id 245575913:


select [Current LSN], Operation, [Lock Information], [Transaction ID], [Description]
from fn_dblog(null, null) 
	where [Lock Information] like N'%SCH%245575913%'

So two transactions have acquired a SCH_M lock on this object: 0000:000002e6 and 0000:000002ea. So lets look at these transactions:


select [Current LSN], Operation, [AllocUnitName], [Lock Information], [Transaction ID], 
	[Description], [Begin Time], [Transaction Name], [Transaction SID]
from fn_dblog(null, null) 
	where [Transaction ID] = N'0000:000002e6';

Most DDL transactions are named after the DDL operation

the first think to notice is that the Transaction Name of the LOP_BEGIN_XACT contains the DDL operation name: CREATE TABLE. Add the Begin Time and the Transaction SID and we found in the log when was the table created and by who. Using the information about System Base Tables the rest of operations for this transactions can be interpreted:

First LOP_INSERT_ROWS into sys.sysschobjs.clst
This is insert of the object record into sys.objects. There are four insert records because there is a clustered index and four non-clustered indexes of syssysschobjs catalog table.
LOP_INSERT_ROWS into sys.syscolpars.clst
These are two insert of column record into sys.columns. Each insert has two records, one for the clustered index one for the non-clustered index.
LOP_INSERT_ROWS into sys.sysidxstats.clst
This is the insert into sys.indexes.
LOP_INSERT_ROWS into sys.sysiscols.clst
This is the insert into sys.index_columns.
Second LOP_INSERT_ROWS into sys.sysschobjs.clst
This is an insert in sys.objects of an object related to the table, but not the table itself. In this case is the primary key constraint, but identifying this sort of information is usually tricky. The best approach is to simply use cast([RowLog Contents 0] as nchar(4000)) and look for some human readable string, that would be the name of the object inserted. In my case the log record content revealed the string PK__test__3213E83F1364FB13 so I could easily deduce that is the default named primary key constraint.

As you can see I did not retrieve exactly the DDL that run, but I have a very good idea what happened: the table was created at 2014/03/10 15:00:08:690, it initially contained two columns and a primary key constraint. Using the same trick of casting the log record content to NCHAR(4000) I could easily retrieve the original column names. My example is for a very simple CREATE TABLE statement, more complex statements can generate more complex log signatures. Partitioning in special adds a lot of LOP_INSERT_ROWS records because every individual partition must be described in sys.sysrowsets and every column for every partition and for every index must be described in sys.sysrscols. And that is before the low level HoBT metadata is being described, see SQL Server table columns under the hood.

Next I'm going to look at the second transaction, 0000:000002ea:


select [Current LSN], Operation, [AllocUnitName], [Lock Information], [Transaction ID], 
	[Description], [Begin Time], [Transaction Name], [Transaction SID]
from fn_dblog(null, null) 
	where [Transaction ID] = N'0000:000002ea';

Again we can see the transaction name that reveals the type of DDL operation (is an ALTER TABLE), the time it occurred and the SID of the user that issued the change. The LOP_INSERT_ROWS into sys.syscolpars.clst tells me this is a DDL change that added a column to the table.

Retrieving a deleted object ID

How about the case when we're investigating an object that was dropped? We cannot use the handy N'%SCH%<object id>%' pattern trick because we don't know the object ID. In this case the we can look for the LOP_DELETE_ROWS record that deleted the object form sys.objects. We know is going to affect the sys.sysschobjs.clst allocation unit and we know the name of the deleted object must be in the log record. In my case I'm looking for a dropped table that was named test:


select [Current LSN], Operation, [AllocUnitName], [Lock Information], [Transaction ID], 
	[Description], [Begin Time], [Transaction Name], [Transaction SID]
from fn_dblog(null, null) 
	where [Operation] = N'LOP_DELETE_ROWS'
	and [AllocUnitName] = N'sys.sysschobjs.clst'
	and CHARINDEX(cast(N'test' as varbinary(4000)), [Log Record]) > 0;

We found two records, from the transaction 0000:000002ee. It shouldn't be a surprise there are two delete operations, after all the CREATE did two INSERT because of the primary key constraint. So lets look at transaction 0000:000002ee:


select [Current LSN], Operation, [AllocUnitName], [Lock Information], [Transaction ID], 
	[Description], [Begin Time], [Transaction Name], [Transaction SID]
from fn_dblog(null, null) 
	where [Transaction ID] = N'0000:000002ee';

Again we have the the transaction time, the user that issued the DROP, and the transaction name DROPOBJ. Notice the LOP_LOCK_XACT operations that is logged right after the transaction start: HoBt 0:ACQUIRE_LOCK_SCH_M OBJECT: 5:245575913:0. This is important, because it gives us the object Id of the object being dropped at the moment it was dropped. Using this object ID we can further dig in the log and locate other DDL changes, just as I showed above.

Object IDs can be recycled

Is important to realize that object IDs can be recycled, after an object was dropped another object can be created using the same object ID (trust me, it can happen). So do pay attention and make sure that when you follow an object ID you are looking at the relevant log records, not the log records of a completely unrelated object that happens to reuse the same object ID. Remember that the log record LSN establishes a clear time ordering of events so it should not be too hard to figure out such a case.

Is also important to realize that the CHARINDEX(cast(N'test' as varbinary(4000)), [Log Record]) > 0 condition could retrieve log records for drops of other objects that simply contain 'test' in their name. This is not the only place where some of these techniques can misfire. These techniques are quite advanced, not meant for everyday users. I expect you to exercise quite advanced judgement. Try to corroborate the information found through multiple methods. Use common sense.

One Response to “How to read and interpret the SQL Server log”

  1. Frank Gill says:

    Great post. The tip about dumping the log to a temp table is a good one. Thanks.