Question

Is it possible to create a primary key or unique index on an NVARCHAR(4000) column used to hold URLs? There appears to be a 900 byte limitation on unique indexes in SQL 2008.

Answer 1

It is better to create a prefixed index on, say, first 50 characters for fast lookups and a UNIQUE index on the MD5 hash (or another hash which is unique enough).

CREATE TABLE urls (url NVARCHAR(4000) NOT NULL, url_prefix AS LEFT(url, 50), url_hash AS HashBytes('MD5', url))

CREATE INDEX ix_urls_prefix ON urls (url_prefix)

CREATE INDEX ix_urls_hash ON urls (url_hash)