Declaring an Index as unique in SQL Server

Question

If I know an index will have unique values, how will it affect performance on inserts or selects if I declare it as such.

If the optimiser knows the index is unique how will that affect the query plan?

I understand that specifying uniquenes can serve to preserve integrity, but leaving that discussion aside for the moment, what are the perfomance consequences.

Answer 1

Long story short: if your data are intrinsically UNIQUE, you will benefit from creating a UNIQIE index on them.

See the article in my blog for detailed explanation:

• Making an index UNIQUE

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Now, the gory details.

As @Mehrdad said, UNIQUENESS affects the estimated row count in the plan builder.

UNIQUE index has maximal possible selectivity, that's why:

SELECT  * FROM    table1 t2, table2 t2 WHERE   t1.id = :myid         AND t2.unique_indexed_field = t1.value 

almost surely will use NESTED LOOPS, while

SELECT  * FROM    table1 t2, table2 t2 WHERE   t1.id = :myid         AND t2.non_unique_indexed_field = t1.value 

may benefit from a HASH JOIN if the optimizer thinks that non_unique_indexed_field is not selective.

If your index is CLUSTERED (i. e. the rows theirselves are contained in the index leaves) and non-UNIQUE, then a special hidden column called uniquifier is added to each index key, thus making the key larger and the index slower.

That's why UNIQUE CLUSTERED index is in fact a little more efficicent than a non-UNIQUE CLUSTERED one.

In Oracle, a join on UNIQUE INDEX is required for a such called key preservation, which ensures that each row from a table will be selected at most once and makes a view updatable.

This query:

UPDATE  (         SELECT  *         FROM    mytable t1, mytable t2         WHERE   t2.reference = t1.unique_indexed_field         ) SET     value = other_value 

will work in Oracle, while this one:

UPDATE  (         SELECT  *         FROM    mytable t1, mytable t2         WHERE   t2.reference = t1.non_unique_indexed_field         ) SET     value = other_value 

will fail.

This is not an issue with SQL Server, though.

One more thing: for a table like this,

CREATE TABLE t_indexer (id INT NOT NULL PRIMARY KEY, uval INT NOT NULL, ival INT NOT NULL) CREATE UNIQUE INDEX ux_indexer_ux ON t_indexer (uval) CREATE INDEX ix_indexer_ux ON t_indexer (ival) 

, this query:

/* Sorts on the non-unique index first */ SELECT  TOP 1 * FROM    t_indexer ORDER BY         ival, uval 

will use a TOP N SORT, while this one:

/* Sorts on the unique index first */ SELECT  TOP 1 * FROM    t_indexer ORDER BY         uval, ival 

will use just an index scan.

For the latter query, there is no point in additional sorting on ival, since uval are unique anyway, and the optimizer takes this into account.

On sample data of 200,000 rows (id == uval == ival), the former query runs for 15 seconds, while the latter one is instant.

Answer 2

Of course the optimizer will take uniqueness in consideration. It affects the expected row count in query plans.