Question

I’m working under refactoring of database (SQL Server 2008) scheme and gather arguments to change NCHAR(1) columns (which keep Y|N values) to BIT. Everybody understand this is necessary and don’t know why does it take place but this change affects production database so weighty arguments are required. Table keeps address catalog (up to 1m of records).

First argument I found – each nchar fields take 2 bytes, each 8 bit fields – 1 byte (next 8 – additional 1 byte).

What’s next? Maybe some indexes performance issues?

Answer 1

A bit field helps your logic by automatically enforcing what is currently an implicit business rule (i.e., this column can only contain ‘Y’ or ‘N’). If you’re enforcing that rule programmatically, you can save by eliminating that overhead. Indexing a bit column on its own has little value due to the low cardinality, but it could be useful as part of a composite index.

See also:

• Bit vs. Char(1) in SQL Server
• Should I index a bit field in SQL
  Server?