Question

I have a table with the following definition:

CREATE TABLE url_tracker (
    id int not null identity(1, 1),
    active bit not null,
    install_date int not null,
    partner_url nvarchar(512) not null,
    local_url nvarchar(512) not null,
    public_url nvarchar(512) not null,
    primary key(id)
);

And I have a requirement that these three URLs always be unique – any individual URL can appear many times, but the combination of the three must be unique (for a given day).

Initially I thought I could do this:

CREATE UNIQUE INDEX uniques ON url_tracker 
(install_date, partner_url, local_url, public_url);

However this gives me back the warning:

Warning! The maximum key length is 900 bytes. The index 'uniques' has maximum
length of 3076 bytes. For some combination of large values, the insert/update
operation will fail.

Digging around I learned about the INCLUDE argument to CREATE INDEX, but according to this question converting the command to use INCLUDE will not enforce uniqueness on the URLs.

CREATE UNIQUE INDEX uniques ON url_tracker (install_date)
INCLUDE (partner_url, local_url, public_url);

How can I enforce uniqueness on several relatively large nvarchar fields?

---

Resolution

So from the comments and answers and more research I’m concluding I can do this:

CREATE TABLE url_tracker (
    id int not null identity(1, 1),
    active bit not null,
    install_date int not null,
    partner_url nvarchar(512) not null,
    local_url nvarchar(512) not null,
    public_url nvarchar(512) not null,
    uniquehash AS HashBytes('SHA1',partner_url+local_url+public_url) PERSISTED,
    primary key(id)
);
CREATE UNIQUE INDEX uniques ON url_tracker (install_date,uniquehash);

Thoughts?

Answer 1

I would make a computed column with the hash of the URLs, then make a unique index/constraint on that. Consider making the hash a persisted computed column. It shouldn’t have to be recalculated after insertion.