The dark side of the GUID

In this article we are going to deep dive in a very old battle: GUID vs INT and try to understand what are the benefits from each one.

?? BEFORE WE START

All the points in this article doesn't mean you can't use GUID anymore, by the way I'm a big fan of using GUIDs, I use it all time, including in my product.

You are most welcome to express your opinion, so please, let a comment, tell your experiences and feel free to correct or complement any point of this article.

Only this way we can grow together! ??

Introduction

When a CPU processes data, the size of the data being processed—such as a GUID (128-bit) versus an INT (32-bit)—has a significant impact on the efficiency and performance of the operations.

Data Size and processing efficiency

INT (32-bit):

Most modern CPUs are optimized to handle 32-bit and 64-bit operations efficiently.

Processing a 32-bit INT typically requires a single CPU cycle or a minimal number of cycles, depending on the operation and the CPU architecture.

The data fits well within the CPU's registers, caches, and memory bus, which are often optimized for 32-bit or 64-bit data.

Accessing a 32-bit INT from memory or caches is generally faster because it's smaller and fits well within typical cache lines (usually 64 bytes).

GUID

Processing a 128-bit GUID requires handling four times the data compared to a 32-bit INT. If the CPU is 64-bit, processing a 128-bit value generally requires two 64-bit operations, or four operations if the CPU is optimized for 32-bit processing.

This increases the number of cycles required to process each GUID-related operation.

Accessing a 128-bit GUID involves transferring more data from memory or caches, which can be slower.

It requires more cache space and might lead to more cache misses, especially if working with large datasets where memory and cache efficiency are critical.

Indexes

Clustered Index

A clustered index determines the physical order of data in a table. It is essentially the way the data is stored on disk.

When you create a clustered index on a table, SQL Server sorts and stores the data rows in the table based on the indexed column(s).

This means that the data in the table is organized in a specific order according to the clustered index key.

• Only one per table: You can only have one clustered index per table because the data rows can only be stored in one order.
• Direct access: Since the data is stored in the same order as the clustered index, searching for data using the clustered index can be very fast. When you query a table based on its clustered index, SQL Server can quickly locate the row(s) you're looking for without needing to perform additional lookups.
• Default primary key: By default, when you define a primary key on a table, SQL Server creates a clustered index on that column (unless specified otherwise).

Example: If you have a table of Employees with a clustered index on the EmployeeID column, SQL Server will store the rows in order of EmployeeID.

So, when you query based on EmployeeID, the database engine can quickly locate the relevant rows because they are stored in sequence.

Non-Clustered Index

A non-clustered index does not alter the physical order of the data in the table. Instead, it creates a separate structure within the database that points to the physical data stored in the table.

Think of it as a lookup table that helps the database find the rows more quickly without having to scan the entire table.

• Multiple per table: You can have multiple non-clustered indexes on a table, each one potentially on different columns or combinations of columns.
• Index lookups: When you query a table based on a non-clustered index, SQL Server first searches the index to find the references to the data rows, and then it uses those references to fetch the actual data. This can involve an extra step, which might be slower than using a clustered index if the data isn’t directly available in the index (this is known as a "key lookup").
• Include columns: Non-clustered indexes can also include non-key columns to avoid the need to go back to the clustered index (or the actual table) for additional data.

Example: Suppose you create a non-clustered index on the LastName column of the Employees table. The database will create an index structure that stores the LastName values along with pointers to where the full row of data is stored.

When you search by LastName, the database engine uses the non-clustered index to find the row locations and then retrieves the data.

Differences between Clustered and Non-Clustered indexes

• Physical vs. Logical Order: The clustered index dictates the physical order of rows in the table, while a non-clustered index maintains a logical order separate from the actual data storage.
• Number per Table: There can only be one clustered index per table, but there can be multiple non-clustered indexes.
• Performance Impact: Clustered indexes generally offer faster data retrieval for range queries or searches that can benefit from the sorted order. Non-clustered indexes are more flexible and can improve performance for queries involving columns that aren’t part of the clustered index.
• Storage Requirements: Clustered indexes may result in larger tables (due to the need to maintain order), while non-clustered indexes require additional storage space to maintain the index structures.

Usage Scenarios

• Clustered Index: Ideal for primary keys, columns frequently used for sorting, and columns that are often involved in range queries.
• Non-Clustered Index: Suitable for columns used in frequent search queries, especially those that are not unique, or for optimizing specific queries that involve WHERE clauses on non-primary key columns.

GUID vs INT on Database

When deciding between using an INT or a GUID (also known as UNIQUEIDENTIFIER) as a primary key in SQL Server, there are several important differences to consider:

Size and Storage

An INT is a 4-byte integer, which can store values from -2,147,483,648 to 2,147,483,647.

This makes it a very space-efficient option, especially when dealing with large tables.

A GUID is a 16-byte value (128 bits), which is significantly larger than an INT. This larger size increases the storage requirements for the primary key and any associated indexes.

Performance

Due to its smaller size, using an INT as a primary key generally results in faster performance, especially for large datasets.

Indexes on INT columns are smaller, and operations like sorting and searching are faster.

Because a GUID is larger, it consumes more memory and can lead to slower performance for indexing and querying.

GUIDs are also not naturally sequential, which can lead to fragmented indexes and slower insert performance.

Uniqueness and Distribution

Typically, INT primary keys are sequential (e.g., auto-incrementing), which can make them predictable.

This predictability can be a security concern in some scenarios but is generally not an issue for most applications.

GUIDs are globally unique, which makes them an excellent choice when you need to ensure uniqueness across multiple databases or systems.

GUIDs are particularly useful in distributed systems where different nodes or instances might generate keys independently.

Fragmentation

Sequential INT keys lead to minimal index fragmentation since new rows are typically added at the end of the table.

Randomly generated GUIDs (using NEWID(), for example) can cause significant index fragmentation because new rows can be inserted anywhere in the table.

However, GUIDs generated sequentially (NEWSEQUENTIALID()) can mitigate this issue.

Index Size

Indexes on INT columns are smaller, leading to more efficient use of memory and faster index operations.

Indexes on GUID columns are larger, which can lead to increased storage requirements and slower index operations.

Portability

Using INT as a primary key is straightforward and works well within a single database or system.

However, ensuring uniqueness across multiple databases can be challenging.

GUIDs are portable and ensure uniqueness across multiple systems, which is advantageous in distributed environments or when merging data from different sources.

Natural Partitioning

Sequential INT keys don’t provide any inherent partitioning advantages.

If GUIDs are generated sequentially (using NEWSEQUENTIALID()), they can help distribute inserts more evenly across partitions or clustered indexes, reducing the hotspot effect.

Conclusion

In summary, the choice between using an INT or a GUID as a primary key in SQL Server has significant implications for performance, storage efficiency, and system design.

INT keys, with their smaller size and sequential nature, offer better performance in terms of indexing and querying, making them ideal for scenarios where efficiency and speed are crucial.

On the other hand, GUIDs provide global uniqueness and are particularly advantageous in distributed systems, though they come with trade-offs in terms of larger storage requirements and potential for index fragmentation.

Additionally, the use of clustered and non-clustered indexes further influences how data is organized and accessed, with clustered indexes offering faster retrieval for sorted data, while non-clustered indexes provide flexibility for complex queries.

Ultimately, the decision should be guided by the specific needs of the application, balancing factors like performance, scalability, and data distribution.