Index a varchar column

Question

I asked Google but I am still confused.

1) Is there a problem in indexing a Varchar column. When I shouldn't, and When I should

2) Index a char column VS Varchar column.

Thanks

Answer 1

1 - Index it if you are querying it and it is selective enough. If it is a column where 90% of the values are the same, there won't be much point.

2 - This is not a question, but I will guess you want to know if you should. Yes, if you query it and it meets the criteria above.

Answer 2

• ad 1) Yes, 900 bytes limit, huge keys, lots of index pages, lots of I/O involved, inefficient index operations. Conclusion: DON'T unless your varchar is about 50 chars max.
• ad 2) Same as 1. The real difference between char vs. varchar are fixed size vs. variable size (ie. char(100)) always takes 100 bytes in data page, varchar(100) takes up to 100)