Understanding VARCHAR and NVARCHAR Data Type Sizes in SQL Server

Discover the reasons for unexpected reductions in table size when converting `VARCHAR` to `NVARCHAR` in SQL Server. Learn practical solutions for managing data efficiently.
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Understanding VARCHAR and NVARCHAR Data Type Sizes in SQL Server

The Problem

If you are using SQL Server and have ever worked with varchar and nvarchar data types, you might have encountered some puzzling scenarios regarding database size allocation. One such situation arose from an SQL Server 2016 user who was confused about the size differences after converting a varchar column to nvarchar. According to SQL Server documentation, varchar takes n + 2 bytes, while nvarchar requires 2 * n + 2 bytes. Thus, when converting a varchar(2000) to nvarchar(2000), one would typically expect a doubling in size. However, the user reported a decrease in total size after the conversion, raising several questions about database management and size increments related to other data types.

Exploring the Sizes of VARCHAR and NVARCHAR

What are VARCHAR and NVARCHAR?

VARCHAR(n): This data type stores non-Unicode characters and is suited for English and other languages that do not require multilingual support. It can store up to n characters and requires n + 2 bytes for storage.

NVARCHAR(n): A Unicode data type that can store characters from multiple languages, accommodating special characters and symbols. It stores up to n characters requiring 2 * n + 2 bytes for storage.

Understanding Size Discrepancies

Given the definitions:

When converting a varchar(2000) to nvarchar(2000), we would expect the size to double.

However, some users have noticed unexpected sizes, such as a reduction after conversion.

Factors Contributing to Size Reduction

Reindexing Issues: Many users misunderstand the difference between rebuilding an index and updating it as part of the maintenance plan. Sometimes size discrepancies arise from not fully rebuilding the index correctly.

Database Maintenance: Regular database maintenance activities, including updates and indexing, can impact total size reporting.

The Solution

After reaching out for professional help from an MSSQL specialist, the user discovered a crucial detail: the maintenance task labeled "rebuild index" in the SQL Server maintenance plan does not always execute a full rebuild, which can lead to size miscalculations.

Recommended Steps:

Run Manual Index Rebuild Query:

Execute a manual rebuild query to ensure the index and table are optimized:

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Monitor Database Size: After executing the proper rebuild, check the size of your database again. In the user's case, the size shrunk by up to 80%.

Consideration for Data Types:

The user also reported that changing int to tinyint unexpectedly increased the database size by the indicated percentages. This further emphasizes the importance of understanding how changes in data types interact with indexing and storage.

Conclusion

Understanding the intricacies of varchar and nvarchar sizes is pivotal for efficient SQL database management. It’s essential to ensure that maintenance tasks are working as intended, especially when dealing with data conversions. Always consider reaching out to a database specialist for complexities that regular documentation might not clarify. By following the recommendations provided, users can achieve better size optimization and overall performance in their SQL Server databases.

By staying informed and taking proactive maintenance measures, you can avoid pitfalls commonly associated with data type conversions and storage size management.