Question

I have a table Com_Main which contains column CompanyName nvarchar(250). It has average length of 19, max length = 250.

To improve performance I want to add a computed column left20_CompanyName which holds the first 20 characters of CompanyName:

alter table Com_main 
add left20_CompanyName as LEFT(CompanyName, 20) PERSISTED

Then I create Index on this column:

create index ix_com_main_left20CompanyName 
on Com_main (LEFT20_CompanyName)

So when I use

select CompanyName from Com_Main
where LEFT20_CompanyName LIKE '122%'

it uses this nonclustered index, but when the query is like:

select CompanyName from Com_Main 
where CompanyName LIKE '122%'

It uses full table scan, and don’t use this index.

So the question:

Is it possible to make SQL Server use this index on computable column in last query?

Answer 1

No. MySQL supports partial indexing of varchar columns but MS SQL Server does not.

You might be able to speed up table scans through partitioning but I don’t know how smart SQL Server is in this regard.