Understanding SQL Server LEFT_SHIFT and RIGHT_SHIFT Functions

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Understanding SQL Server LEFT_SHIFT and RIGHT_SHIFT Functions

Noman072

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March 19, 2025 at 12:00 am

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Comments posted to this topic are about the item Understanding SQL Server LEFT_SHIFT and RIGHT_SHIFT Functions

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Phil Parkin SSC Guru Points: 247137 More actions · Answer 1

The LEFT_SHIFT and RIGHT_SHIFT functions in SQL Server provide an efficient way to manipulate numbers at the binary level. These functions are indispensable in tasks like data encryption, compression, and flag manipulation, where performance is crucial.

While the first sentence may be true, I don't agree that they're 'indispensable'. Until your article, I wasn't aware that these functions even existed, yet somehow I've made it through years of SQL Server development.

It would be interesting to see a real-world demonstration of the use cases you mention (excluding encryption, which is obvious).

Noman072 SSC-Addicted Points: 480 More actions · Answer 2

Thanks for your comment, You are right that these functions are not essential for every SQL developer, and I agree that many developers might not come across these function often. But in certain cases like data encryption, compression, or flag manipulation, the LEFT SHIFT and RIGHT SHIFT functions can really make a difference by working at the binary level. They are not needed in every scenario, but for those working on performance critical tasks or those who need to manipulate data in that way, this can be a really handy tool.

Let me explain with a couple of real world scenario example where these operations can really come handy and can help clarify things further.

1. Data Compression with RIGHT SHIFT:

In this case, we will use bitwise shifts to reduce the size of a number in an example where you need to store a smaller value after shifting bits. This can be useful when you are dealing with large datasets and need to optimize storage by reducing the size of data.

Scenario:

Let’s say you have a large dataset of integers representing certain metrics and you want to reduce these values by shifting bits to the right (effectively dividing by 2). This can help save space when the data doesn't require high precision.

-- Sample data: list of large numbers
DECLARE @Data TABLE (ID INT, Value INT);
INSERT INTO @Data (ID, Value)
VALUES (1, 1024), (2, 2048), (3, 4096), (4, 8192);

-- Use RIGHT_SHIFT to reduce the data by shifting 2 bits to the right (dividing by 4)
SELECT 
    ID, 
    Value,
    RIGHT_SHIFT(Value, 2) AS CompressedValue -- Shifting by 2 bits to reduce the value
FROM 
    @Data;

Explanation:

The RIGHT_SHIFT(Value, 2) divides the values by 4 (since each right shift divides by 2).

This simulates compression by reducing the size of the numbers. It's helpful when you need to store more data in less space without losing precision.

2. Flag Manipulation:

In this case, we wil use bitwise shifts for manipulating flags. Flags are commonly used in databases to represent various permissions or options using individual bits of a number (where each bit corresponds to a specific setting, such as read, write or delete).

Scenario:

Lets say you are working with a system where a User Permissions column stores a number where each bit represents a different permission (e.g., 1 for read, 2 for write, 4 for delete). You can use the LEFT SHIFT to enable certain permissions or RIGHT SHIFT to check or disable them.

-- Sample data: user permissions stored as an integer (bits represent permissions)
DECLARE @Permissions TABLE (UserID INT, Permissions INT);
INSERT INTO @Permissions (UserID, Permissions)
VALUES (1, 5),  -- Binary: 0101 (Read, Delete)
       (2, 3),  -- Binary: 0011 (Read, Write)
       (3, 7);  -- Binary: 0111 (Read, Write, Delete)

-- Use LEFT_SHIFT to add a new permission (e.g., adding a "write" permission to the user)
SELECT 
    UserID, 
    Permissions,
    LEFT_SHIFT(Permissions, 1) AS UpdatedPermissions -- Shifting the bits to the left to simulate adding a permission
FROM 
    @Permissions;

-- Use RIGHT_SHIFT to check for a specific permission (e.g., checking if the 2nd bit for "Write" is set)
SELECT 
    UserID, 
    Permissions,
    CASE 
        WHEN RIGHT_SHIFT(Permissions, 1) & 1 = 1 THEN 'Has Write Permission'
        ELSE 'No Write Permission'
    END AS WritePermissionStatus
FROM 
    @Permissions;

Explanation:

In the first query LEFT SHIFT(Permissions, 1) shifts the permission bits to the left by 1, effectively promoting the permissions, which in practice could be used to add or modify a permission.
In the second query RIGHT SHIFT(Permissions, 1) checks if the second bit (for write permission) is set. If the result of shifting and bitwise AND with 1 equals 1, the user has the write permission.

Real-World Use Cases:

Data Compression: This technique is useful when you want to reduce the size of your data for storage or transmission but can afford to lose some precision, like in storing large numbers in a compact format (e.g., for logs or metrics).
Flag Manipulation: Bitwise operations are commonly used to manage a set of binary flags, where each bit corresponds to a different flag or permission, allowing efficient storage and manipulation of user access or settings.

I hope these examples will give you a better understanding of how you can use the LEFT SHIFT and RIGHT SHIFT functions in real world SQL scenarios.

Thankyou.

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Jeff Moden SSC Guru Points: 1004577 More actions · Answer 3

While I do have an appreciation for anyone that steps up to share information, there are a couple of serious problems with the supposed "Practical Use Cases" in the article...

First, it would be nice if there were examples or at least links to such examples in the article... especially since I believe the examples are bad and should probably be avoided. The following talks to the problems with the suggested uses... and a good number of the uses seem to be quite dangerous...

Exact quotes from the article are in BLUE below. My comments are in RED.

Data Encryption: Bitwise shifts can be used to obscure data temporarily in low-security applications. For instance, before storing data, you might shift the bits of a number to make it less readable. Later, you can reverse the shift to recover the original value.

INCORRECT! Any bits with a value of "1" that rolloff because of the SHIFT are lost forever. See the code example below.

Data Compression: Bitwise shifts are useful for reducing the size of numbers in datasets, allowing for faster storage and processing. Shifting bits to the right can reduce the size of a number, which is particularly helpful in large datasets.

DANGER! Any bits with a value of "1" that roll off because of the SHIFT are lost forever. See the code example below.

Flag Handling: Bitwise shifts allow efficient manipulation of flags. For example, in a system where each bit represents a permission (read, write, delete, etc.), you can use shifts to enable or disable flags efficiently.

Advisory: Prove it. Prove that such a method is faster than setting a bit with a simple add or the new SET_BIT function.

Performance Optimization: Shifting bits is an extremely fast operation compared to multiplication or division. Therefore, bitwise shifts can optimize performance in tasks like large-scale data analysis, image processing, and other computationally expensive applications.

Advisory: Prove it. Prove that such a method is faster than setting a bit with a simple add or the new SET_BIT function.

The following is the code that proves there is a data loss for one of the suggested uses, which should probably be avoided because of the data loss. The other uses will suffer similar data loss.

    SET NOCOUNT ON;
DECLARE @BinDat BINARY(1) = 0xFF;
 SELECT CONVERT(CHAR(29),'Original'),@BinDat;
    SET @BinDat = @BinDat << 1;
 SELECT CONVERT(CHAR(29),'LEFT 1 - Left bit lost'), @BinDat;
    SET @BinDat = @BinDat >> 1;
 SELECT CONVERT(CHAR(29),'RIGHT 1 - Left still bit lost'), @BinDat;

Results:

----------------------------- ----
Original                      0xFF
----------------------------- ----
LEFT 1 - Left bit lost        0xFE
----------------------------- ----
RIGHT 1 - Left still bit lost 0x7F

--Jeff Moden

RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

Change is inevitable... Change for the better is not.

Helpful Links:
How to post code problems
How to Post Performance Problems
Create a Tally Function (fnTally)

Jonathan AC Roberts SSCoach Points: 18406 More actions · Answer 4

RIGHT_SHIFT and LEFT_SHIFT are CPU instructions implemented in hardware using barrel shifters, making them extremely fast, significantly faster than multiplying or dividing by 2 raised to an integer power. However, their efficiency in T-SQL depends on Microsoft's implementation.