Comparison between A non-clustered primary key and a covering index in terms of performance

Question

I have a table which contains a non-clustered primary key. I intend to remove this primary key and create a unique Covering Index on the same column instead.

Then The table won't have a primary key, but a unique covering index.

I searched google but could not find a relevant topic. Is that fundamentally wrong? or it's ok to be done.

Update:

Why do I need to convert this index to a covering index?

Because this is the primary key and I have many select queries based on this key. At the moment for each select, there will be a need for a RID Lookup because this is a non-clustered key. If I convert it to a covering index, this RID Lookup will be gone for those select queries and hopefully there will be better performance (at least the excecution plan says that).

Of course integrity is also intended and that's why I will make it a unique covering index. The primary key is a uniqueidentifier, generated by the application (which explains why we don't make it clustered)..

What I am wondering is if that has any negative effects on anything else.

Execution Plan Before:

Execution Plan After:

FYI, the primary-key is a uniquidentifier which is generated in the code (application). In load tests we came up with the conclusion of making it non-clustered.

Answer 1

By removing the primary key and adding a unique covering index with more columns, you are fundamentally changing how the table accepts data. It may be that with the covering index in place, the table design no longer fits the purpose it was designed for.

Take for instance:

USE tempdb;

IF OBJECT_ID(N'dbo.T1', N'U') IS NOT NULL
DROP TABLE dbo.T1;
CREATE TABLE dbo.T1
(
    ID1 int NOT NULL
    , ID2 int NOT NULL
    , SomeDate datetime2(7) NOT NULL
    , CONSTRAINT PK_T1
        PRIMARY KEY NONCLUSTERED
        (ID1, ID2)
);

INSERT INTO dbo.T1 (ID1, ID2, SomeDate)
VALUES (1, 1, GETDATE())
WAITFOR DELAY '00:00:00.003';
INSERT INTO dbo.T1 (ID1, ID2, SomeDate)
VALUES (1, 1, GETDATE());

The output:

(1 row(s) affected)
Msg 2627, Level 14, State 1, Line 18
Violation of PRIMARY KEY constraint 'PK_T1'. Cannot insert duplicate key in object 'dbo.T1'. The duplicate key value is (1, 1).
The statement has been terminated.

The table design is only allowing rows where the tuple (ID1, ID2) is unique.

Then look at this, where the primary key does not exist, and has been replaced with a wider unique covering index:

IF OBJECT_ID(N'dbo.T2', N'U') IS NOT NULL
DROP TABLE dbo.T2;
CREATE TABLE dbo.T2
(
    ID1 int NOT NULL
    , ID2 int NOT NULL
    , SomeDate datetime NOT NULL
);

CREATE UNIQUE INDEX IX_T2
ON dbo.T2 (ID1, ID2, SomeDate);

INSERT INTO dbo.T2 (ID1, ID2, SomeDate)
VALUES (1, 1, GETDATE())
WAITFOR DELAY '00:00:00.003';
INSERT INTO dbo.T2 (ID1, ID2, SomeDate)
VALUES (1, 1, GETDATE());

(1 row(s) affected)

(1 row(s) affected)

Table T2 has both rows, where arguably, the second insert should have failed and the table should only have a single row. In this design, the table is now accepting rows where the tuple (ID1, ID2) are not necessarily unique.

I'd think carefully about your table design, and potentially just add the covering index without removing the primary key.

Answer 2

You can change the primary key default to newsequentiaid(), then replace the non-clustered pk with a clustered pk. This solves the non-sequentially increasing primary key issue while retaining unique identifier as the primary key column type.

Compared against a clustered key solution, the biggest downside (so long as you are careful with your unique index logic) is that creating a covering non-clustered index requires you to store the table data twice. inserts and updates have to maintain both copies as well, affecting performance. The additional included columns are stored on leaf pages of the non-clustered index, as well as in original table. They use the same space as additional key columns, but are not considered during seeks and other ops on keys (see this for details).

With a clustered primary key, a single copy of the table data is stored with the clustered key, in index order (so no key lookups).