Nonclustered indexes always include a row locator.
For a heap this will be an 8 byte RID (File:Page:Slot). On a table with a clustered index it will be the clustered index key column(s). And it will always be the copied values not a pointer to the values. This duplication of CI key values into all non clustered indexes is why it is often recommended that the CI key be narrow and not frequently updated.
In the table shown in the question the Clustered index key is a 4 byte integer and potentially may also include a 4 byte uniqueifier for any duplicate key values.
In your case as the NCIs are not declared as unique the CI key will be appended to the NCI key.
For unique non clustered indexes the CI key would be added as included column(s) in the leaf pages unless explicitly made part of the key.
See Kalen Delaney: More About Nonclustered Index Keys for some additional information about how you can see this for yourself.
With these 4 rows of data all three indexes only consume a single 8KB data page.
SELECT index_id,
index_level,
page_count,
record_count
FROM sys.dm_db_index_physical_stats(DB_ID(), OBJECT_ID('people'), NULL, NULL, 'DETAILED')
Returns
+----------+-------------+------------+--------------+
| index_id | index_level | page_count | record_count |
+----------+-------------+------------+--------------+
| 1 | 0 | 1 | 4 |
| 2 | 0 | 1 | 4 |
| 3 | 0 | 1 | 4 |
+----------+-------------+------------+--------------+
The additional page shown in use by sys.allocation_units.total_pages
is an IAM page. This is not used for storing data but just for tracking the pages and extents comprising the index.
Why is a Key Lookup required to get A, B and C when they are not referenced in the query at all? I assume they are being used to calculate Comp, but why?
Columns A, B, and C
are referenced in the query plan - they are used by the seek on T2
.
Also, why can the query use the index on t2, but not on t1?
The optimizer decided that scanning the clustered index was cheaper than scanning the filtered nonclustered index and then performing a lookup to retrieve the values for columns A, B, and C.
Explanation
The real question is why the optimizer felt the need to retrieve A, B, and C for the index seek at all. We would expect it to read the Comp
column using a nonclustered index scan, and then perform a seek on the same index (alias T2) to locate the Top 1 record.
The query optimizer expands computed column references before optimization begins, to give it a chance to assess the costs of various query plans. For some queries, expanding the definition of a computed column allows the optimizer to find more efficient plans.
When the optimizer encounters a correlated subquery, it attempts to 'unroll it' to a form it finds easier to reason about. If it cannot find a more effective simplification, it resorts to rewriting the correlated subquery as an apply (a correlated join):
It just so happens that this apply unrolling puts the logical query tree into a form that does not work well with project normalization (a later stage that looks to match general expressions to computed columns, among other things).
In your case, the way the query is written interacts with internal details of the optimizer such that the expanded expression definition is not matched back to the computed column, and you end up with a seek that references columns A, B, and C
instead of the computed column, Comp
. This is the root cause.
Workaround
One idea to workaround this side-effect is to write the query as an apply manually:
SELECT
T1.ID,
T1.Comp,
T1.D,
CA.D2
FROM dbo.T AS T1
CROSS APPLY
(
SELECT TOP (1)
D2 = T2.D
FROM dbo.T AS T2
WHERE
T2.Comp = T1.Comp
AND T2.D > T1.D
ORDER BY
T2.D ASC
) AS CA
WHERE
T1.D IS NOT NULL -- DON'T CARE ABOUT INACTIVE RECORDS
ORDER BY
T1.Comp;
Unfortunately, this query will not use the filtered index as we would hope either. The inequality test on column D
inside the apply rejects NULLs
, so the apparently redundant predicate WHERE T1.D IS NOT NULL
is optimized away.
Without that explicit predicate, the filtered index matching logic decides it cannot use the filtered index. There are a number of ways to work around this second side-effect, but the easiest is probably to change the cross apply to an outer apply (mirroring the logic of the rewrite the optimizer performed earlier on the correlated subquery):
SELECT
T1.ID,
T1.Comp,
T1.D,
CA.D2
FROM dbo.T AS T1
OUTER APPLY
(
SELECT TOP (1)
D2 = T2.D
FROM dbo.T AS T2
WHERE
T2.Comp = T1.Comp
AND T2.D > T1.D
ORDER BY
T2.D ASC
) AS CA
WHERE
T1.D IS NOT NULL -- DON'T CARE ABOUT INACTIVE RECORDS
ORDER BY
T1.Comp;
Now the optimizer does not need to use the apply rewrite itself (so the computed column matching works as expected) and the predicate is not optimized away either, so the filtered index can be used for both data access operations, and the seek uses the Comp
column on both sides:
This would generally be preferred over adding A, B, and C as INCLUDEd
columns in the filtered index, because it addresses the root cause of the problem, and does not require widening the index unnecessarily.
Persisted computed columns
As a side note, it is not necessary to mark the computed column as PERSISTED
, if you don't mind repeating its definition in a CHECK
constraint:
CREATE TABLE dbo.T
(
ID integer IDENTITY(1, 1) NOT NULL,
A varchar(20) NOT NULL,
B varchar(20) NOT NULL,
C varchar(20) NOT NULL,
D date NULL,
E varchar(20) NULL,
Comp AS A + '-' + B + '-' + C,
CONSTRAINT CK_T_Comp_NotNull
CHECK (A + '-' + B + '-' + C IS NOT NULL),
CONSTRAINT PK_T_ID
PRIMARY KEY (ID)
);
CREATE NONCLUSTERED INDEX IX_T_Comp_D
ON dbo.T (Comp, D)
WHERE D IS NOT NULL;
The computed column is only required to be PERSISTED
in this case if you want to use a NOT NULL
constraint or to reference the Comp
column directly (instead of repeating its definition) in a CHECK
constraint.
Best Answer
This assumption is not true - either kind can be indexed. The computed column must be deterministic in either case but when the computed column is persisted, the requirement that the computation is also precise is relaxed (ie it can involve floating point operations).
The result of the function is 'persisted' in the index in either case - the only difference is whether it is persisted in the table.