Internals

What is deferred compilation?

When I talked about row estimations for table variables, I mentioned ‘deferred compile’, but didn’t give a whole lot of details. What, then, is a deferred compilation? Let’s start with how batches work normally.

T-SQL is an interpreted language. While we talk about compiles, they’re not compilations in the sense of what happens to C++. There’s no conversion of the script to a machine language or intermediate language which is used from that point onwards. Every time a batch executes, it has to be parsed, bound and have an execution plan generated or fetched from cache.

When a batch is parsed, the entire batch is parsed.

SET STATISTICS TIME ON
GO

SELECT * FROM dbo.Clients

SELECT * FROM dbo.StarSystems ss 
INNER JOIN dbo.Stations s ON s.StarSystemID = ss.StarSystemID

SELECT GETDATE()
GO

Ignoring the SET STATISTICS, we’ve got there one batch with two queries in it. Two queries, two execution times but only one parse and compile time. The entire batch was compiled and then the batch was executed.

If we set up an XE session to track compiles, it shows similar

The XE session shows that the compilations for both queries were completed before either started execution.

But what happens if we reference a table that doesn’t exist?

SET STATISTICS TIME ON
GO

SELECT * FROM dbo.Clients

SELECT * FROM dbo.StarSystems_DoesNotExist ss 
INNER JOIN dbo.Stations s ON s.StarSystemID = ss.StarSystemID

SELECT GETDATE()
GO

The first of the queries is still compiled, but not the second. The second can’t be compiled because it references an object that doesn’t exist. The first query then executes, and right before when the second query would execute, it’s sent back to be bound and optimised. In this case, the object still doesn’t exist and so we get an error.

Msg 208, Level 16, State 1, Line 6
Invalid object name 'dbo.StarSystems_DoesNotExist'
.

But if the object was created between the start of the batch and the query that uses it, we get a still different result

SET STATISTICS TIME ON
GO

SELECT * FROM dbo.Clients

SELECT * INTO #StarSystems FROM dbo.StarSystems

SELECT * FROM #StarSystems ss 
INNER JOIN dbo.Stations s ON s.StarSystemID = ss.StarSystemID

SELECT GETDATE()
GO

This time we have three queries. Two of them get an execution plan generated when the batch starts, but the third can’t, because the table it references doesn’t exist. Instead, the statement starts executing but can’t execute because there’s no plan. It gets sent back to the optimiser to be compiled, then the query executes.

This is a deferred compile (also called deferred resolution). A compile that does not happen when the batch starts, but is rather deferred until the point that the query itself executes, usually because the table does not exist at the point the batch starts..

On table variable row estimations

At first glance, the question of how many rows are estimated from a table variable is easy.

But, is it really that simple? Well, not really. To dig into the why, first we need to identify why table variables estimate 1 row. The obvious answer is because they don’t have statistics. However…

ALTER DATABASE CURRENT SET AUTO_CREATE_STATISTICS OFF
GO

CREATE TABLE Test (SomeCol INT);

INSERT INTO Test (SomeCol)
VALUES (1),(22),(37),(45),(55),(67),(72),(86),(91)

SELECT SomeCol FROM Test

SELECT * FROM sys.stats WHERE object_id = OBJECT_ID('Test')

DROP TABLE dbo.Test

That table has no statistics, but it still estimates rows correctly.

So it’s not just the absence of statistics. Hmmm… Where else is there a difference with a table variable?

It has to do with when the plans are generated. The XE event results are from an event tracking statement start and end and the post-compilation event for the plan. For the query using the table variable, the entire batch is compiled before the execution starts. For the permanent table, there are multiple compilation events.

And this is because of something called ‘deferred compile’. For the table variable, the entire batch is compiled at the start, at a time where the table variable does not exist, and because there are no statistics, no recompile is triggered after the insert. Hence, there cannot be any row estimation other than 1 row, because the table did not exist when the estimate was made.

For the permanent table, the compilation of the query that uses the table is deferred until the query starts, not when the batch starts. Hence the plan for the query is generated after the table exists, after it’s been populated. That’s the difference here.

Now, there’s still no statistics, and so there’s no way to get data distribution, but that’s not the only way to get information on the rows in the table. The Storage Engine knows how many rows are in the table, though data distribution isn’t known.

Hence, with a table variable we can expect to see an estimated row count other than 1 any time the table variable exists before the query that uses it is compiled.

That will happen when the table variable is a table-type parameter, when the query using it has the RECOMPILE option, and when SQL 2019’s deferred compile for table variables is in play.

CREATE OR ALTER PROCEDURE TestRowEstimations @Input TestTableType READONLY AS

SELECT SomeCol FROM @Input;

DECLARE @Test TABLE (SomeCol INT);

INSERT INTO @Test (SomeCol)
VALUES (1),(22),(37),(45),(55),(67),(72),(86),(91);

SELECT SomeCol FROM @Test;

SELECT SomeCol FROM @Test OPTION (RECOMPILE);
GO
Table-valued parameter
Normal select on compatibility mode 140
Normal select on compatibility mode 150
Select with OPTION(RECOMPILE)

Stop using Task Manager to check SQL’s memory usage!

There’s two fairly common questions I see on the forums around SQL Server’s memory usage. Either the question asks why SQL’s using too much memory, or why it’s using too little.

Too much memory isn’t usually a real problem, it’s often due to max server memory being left at its default of 2048TB, along with a lack of understanding of how SQL uses memory.

Too little memory used is also often not a real problem, rather it’s usually from using the wrong tools to check SQL Server’s memory usage.

Let’s start by looking at an example.

This particular server has 16 GB of memory, and SQL Server’s max server memory is set to 10GB. Since the last restart of the instance, I’ve run SELECT * FROM .. against every table in a 30GB database. That should have warmed the cache up nicely.

MemoryTaskManager

Err, what? SQL Server’s not even using 100MB? I’ve just read 30GB of data and SQL Server’s not even using 1% of it’s allowed memory!!!

Or is it?

TotalServerMemory

A look at perfmon shows a completely different picture. Total and target server memory are both at 10GB. So why is Task Manager showing such a low figure?

LockedPagesInErrorLog

The service account that SQL’s running under has been granted the Lock Pages in Memory permission. This means that SQL’s not using the normal Windows memory routines to allocate memory.

Normally, SQL Server and other Windows applications allocate memory using the VirtualAlloc API call. This allocates virtual memory which is pageable. When SQL Server has been given the Lock Pages in Memory permission, it doesn’t use VirtualAlloc to allocate memory. Instead it uses the API call AllocateUserPhysicalPages. Memory allocated with this API call is not pageable, it has to remain in physical memory.

Task Manager’s memory counters (the Working Set ones) only show memory that’s been allocated using VirtualAlloc. Hence, when SQL Server has the Lock Pages in Memory permission and is allocating most of its memory using AllocateUserPhysicalPages, its memory usage in Task Manager will appear to be extraordinarily low. As far as I recall, in that case it’s only the non-buffer memory which is still allocated with VirtualAlloc, and that’s just things like the thread stacks, the CLR memory, backup buffers, and similar. It’s not the buffer pool. The buffer pool gets allocated with AllocateUserPhysicalPages.

If I remove the Lock Pages in Memory and re-run the test, Task Manager shows completely different values

MemoryTaskManager_NoLockedPages

Now the buffer pool is being allocated with VirtualAlloc and so Task Manager shows the full 10GB of memory usage.

In summary, Task Manager can show a completely incorrect value for SQL Server’s memory usage if the SQL service account has the Lock Pages in Memory permission. This can lead to a lot of wasted time if it is concluded that Task Manager is correct and SQL Server is using little memory.

Rather leave Task Manager alone and use perfmon and the DMVs to check what SQL Server’s memory allocation actually is. They’ll both be correct whether Lock Pages are being used or not.

Of clustered indexes and ordering

There is a particularly irritating and persistent belief that indexes (usually it’s the clustered that gets picked on) are always physically ordered within the data file by the key columns. That is, that the data within the database file is always ordered by the key column.

It doesn’t help that official documentation states this ‘fact’.

I’m going to diverge from my usual methodology of first proving (or disproving) a statement and then explaining it in this case.

Do indexes (clustered or non-clustered) define the physical storage order of the rows?

No, absolutely not.

What indexes do is provide a logical ordering, a collection of pointers, that allow the storage engine to retrieve data from an index ordered by the index key, but that’s logical ordering, it specified nothing regarding the physical ordering.

The index structure is such that the page with key values 4, 5 and 6 will appear earlier in the index’s logical ordering than the page with key values 10,11 and 12. Where these pages are in the file is not defined at all. The page with key values 10,11 and 12 could be page 240 in the database file while the page with key values 4, 5 and 6 could be page 655.

On the data pages themselves there’s no guarantee that the row with the key value 4 will appear earlier on the page than the row with the key value of 6. 6 could be the first row on the page and 4 last and that would be just fine.

Let’s prove this. Time for DBCC page and some undocumented commands.

First up, the order of rows on the page. I’m going to create a table in a nice new database (so that there are no other tables around messing things up) and populate it with some data.

CREATE TABLE OddandEven (
SomeNumber INT,
Filler CHAR(500) DEFAULT ' '
) ;
GO

CREATE UNIQUE CLUSTERED INDEX idx_SomeNumber ON OddandEven (SomeNumber);
GO

INSERT INTO OddandEven (SomeNumber)
SELECT TOP (50) (ROW_NUMBER() OVER (ORDER BY object_id))*2 - 1 FROM sys.objects;

INSERT INTO OddandEven (SomeNumber)
SELECT TOP (50) (ROW_NUMBER() OVER (ORDER BY object_id))*2 FROM sys.objects;

So what I’m doing there is simply inserting 50 odd numbers first and 50 even numbers second

A quick check with DBCC IND shows me that page 89 of this database is a data page for this table. I’m going to use dump style 2 for DBCC Page, because I want a raw binary dump with no interpretation (I’m removing the portions that are just the filler, as that’s just intentionally wasted space)

000000000EB6AC50:   20020000 1000fb01 37332020 20202020 † …..û.73
000000000EB6AE40:   20202020 20202020 20202020 20202002 †               .
000000000EB6AE50:   00001000 fb013735 20202020 20202020 †….û.75
000000000EB6AE60:   20202020 20202020 20202020 20202020 †
000000000EB6B040:   20202020 20202020 20202020 20020000 †             …
000000000EB6B050:   1000fb01 36342020 20202020 20202020 †..û.64 
000000000EB6B060:   20202020 20202020 20202020 20202020 †
000000000EB6B240:   20202020 20202020 20202002 00001000 †           …..
000000000EB6B250:   fb013636 20202020 20202020 20202020 †û.66
000000000EB6B260:   20202020 20202020 20202020 20202020 †

Hmm… 73, 75, 64, 66. That’s not the correct physical ordering… What happened here is that I inserted the odd values first, they were written to the pages then when I wrote the even numbers the pages had to split (firstly) leaving them probably around 50% full, then the even numbers were added in the empty space. SQL doesn’t reorder the rows on the page (that would be expensive).

What keeps track of the logical ordering, what rows should be read first, second, etc. to get the results back in logical ordering, is the slot array at the end of the page

OFFSET TABLE:
Row - Offset
 14 (0xe) - 7236 (0x1c44)
 13 (0xd) - 3666 (0xe52)
 12 (0xc) - 6726 (0x1a46)
 11 (0xb) - 3156 (0xc54)
 10 (0xa) - 6216 (0x1848)
 9 (0x9) - 2646 (0xa56)
 8 (0x8) - 5706 (0x164a)
 7 (0x7) - 2136 (0x858)
 6 (0x6) - 1626 (0x65a)
 5 (0x5) - 5196 (0x144c)
 4 (0x4) - 1116 (0x45c)
 3 (0x3) - 4686 (0x124e)
 2 (0x2) - 606 (0x25e)
 1 (0x1) - 4176 (0x1050)
 0 (0x0) - 96 (0x60)

That tells me that the row with the lowest key value is found at offset 0x60, the next lowest at offset 0x1050, then 0x25e, etc. The rows are not stored on this page in physical order, the slot array defines the logical order so that anything needing the rows in logical order of the index, can read them off the page that way.

That answers the question about rows on a page. Let’s now look at whether pages are always stored in physical order within the data file.

I’m going to drop the OddandEven table and create a new table with the rows sized so that only a few rows fit onto a page.

CREATE TABLE PagePhysicalOrder (
  SomeNumber INT,
  Filler CHAR(800) DEFAULT ' '
);

CREATE UNIQUE CLUSTERED INDEX idx_TestingPhysicalOrder ON PagePhysicalOrder (SomeNumber)

DECLARE @i INT = 9;
WHILE @i >= 0
  BEGIN
    INSERT INTO dbo.PagePhysicalOrder (SomeNumber, Filler)
    SELECT TOP (10)
      ROW_NUMBER() OVER (ORDER BY (SELECT 1)) [email protected]*10,''
      FROM sys.objects;

    SET @i = @i - 1;
  END

That gets me 100 rows in the table, written in groups of 10, with the higher values for SomeNumber being inserted first. Now, to find where the rows are stored, I’m going to use the sys.fn_PhysLocFormatter function and the %%physloc%% virtual column. See http://www.sqlskills.com/blogs/paul/sql-server-2008-new-undocumented-physical-row-locator-function/ for more details on these.

SELECT SomeNumber,
sys.fn_PhysLocFormatter(%%physloc%%) AS RowLocation
FROM dbo.PagePhysicalOrder

RowPhysicalLocations

The output of the PhysLocFormatter is FileID : Page Number : Slot Index. The output shows the rows with SomeNumber 75, 76, 77 and a few others are on page 197 while rows with a lower SomeNumber (65-70) are on page 248, further into the data file than the page containing the larger values of SomeNumber.

Hence we can say that the clustered index doesn’t enforce the physical order of the pages in the data file either.

The only thing that the clustered index (or nonclustered indexes) enforce is what values belong on a page together. If we have a table with an index on an integer column, we cannot have a situation where rows with a key value of 1, 2, 4, 8, 9 are on one page and rows with a key value of 3, 5, 6, 7 and 10 are on another. If only 5 rows fit onto a page, one page will have 1, 2, 3, 4 and 5 and another page will have 6, 7, 8, 9 and 10.  The physical order of the rows on those pages is irrelevant, as is the physical order of those two pages in the data file.

I suspect this myth came about because, when SQL creates or rebuilds an index, it will try as best as possible to put the pages of the index down in physical order of the index key. Doing so reduces logical fragmentation and allows the read-ahead reads to work as efficiently as possible. This applies only when the index is created, rebuilt or reorganised, not during regular operations.

Are all updates split into delete-insert?

This should be another quick one.

Earlier I saw a forum post where someone asserted that SQL always executes an update as a delete of the row and then an insert of the row with the new values. Now I need another excuse to play with undocumented features, so let’s see if that’s true (or at least true for all the common cases I try)

First up, a heap, no indexes at all, an update that does not change the size of the row.

CREATE TABLE TestingUpdate1 (
ID INT IDENTITY,
SomeString CHAR(50)
)

INSERT INTO TestingUpdate1 (SomeString)
VALUES
('One'),('Two'),('Three'),('Four'),('Five'),('Six'),('Seven'),('Eight'),('Nine')

CHECKPOINT -- truncate the log, DB is in simple recovery.

UPDATE TestingUpdate1
SET SomeString = 'NotFour'
WHERE ID = 4 -- one row

SELECT Operation, Context, AllocUnitName, [Transaction Name], Description FROM fn_dblog(NULL, NULL) AS TranLog

DROP TABLE TestingUpdate1

UpdateHeapFixedSize

The log operation here is Modify Row. so in this case, the update was done as an in-place update.

Second test, a heap, no indexes at all, and an update that changes the row size (however plenty free space on the page, so no issues with forwarding pointers here)

(more…)

On the exorcism of Ghost Records

There was a question earlier on one of the SQL forums as to whether or not Ghost Cleanup overwrote the deleted rows when it ran so as to ensure no one could read that data again.

Now I could just reference Paul Randal‘s blog post where he described Ghost Cleanup in depth and leave it there, but where would the fun be in that? Smile

So, now that everyone’s read Paul’s blog post, time for some fun with undocumented commands to prove (again) what we already know (and what Paul’s already shown).

Setup code:

CREATE TABLE TestingCleanup (
ID INT IDENTITY PRIMARY KEY,
Description VARCHAR(20),
Filler CHAR(50) DEFAULT ''
);
GO

INSERT INTO TestingCleanup (Description)
VALUES ('One'), ('Two'), ('Three'), ('Four'), ('Five'), ('Six'), ('Seven'), ('Eight'), ('Nine'), ('Ten')

-- Find the page number that the table is on

SELECT OBJECT_ID('TestingCleanup') -- 1399012065
DBCC IND(11,1399012065,1)
-- The IAM is 1:309 and the data page is 1:308

Now I’m going to delete half the table and then go and look at the page. The ghost cleanup can’t run until the transaction commits, so running this within a transaction gives a chance to go and look at how things work.

BEGIN TRANSACTION
DELETE FROM TestingCleanup WHERE ID%2 = 0 -- delete the even rows
SELECT ID, Description FROM TestingCleanup -- 5 rows
DBCC TRACEON (3604)
DBCC PAGE(11,1,310,1)
DBCC TRACEOFF (3604)
COMMIT TRANSACTION

The delete removes all the rows with even identity values and the select returns only 5 rows, as expected

Dump type 1 for DBCC Page gives the header and then each row separately in binary. I’m editing out uninteresting parts of the output to keep things manageable.

(more…)