Clustered index on temp table

Question

I'm trying to optimize a procedure that has code like the following:

CREATE TABLE #t1 (c1 int, c2 varchar(20), c3(varchar(50)...)

CREATE CLUSTERED INDEX ix_t1 ON #t1(c3) ON [PRIMARY]

I wanted to improve that by moving the CLUSTERED index into the table declaration (more caching friendly), but c3 is not unique so this doesn't work:

CREATE TABLE #t1 (c1 int, c2 varchar(20), c3 varchar(50)..., UNIQUE CLUSTERED (c3))

Is there a way to declare a clustered that is not unique in the temp table declaration?

Answer 1

Yes, it is possible in SQL Server 2014 and above, Create table on MSDN. From 2014 you can specify the indexes inline with the create table statement.

 if object_id('tempdb..#t1') is not null drop table #t1;

CREATE TABLE #t1 (
    c1 int, 
    c2 varchar(20), 
    c3 varchar(50), 

    index [CIX_c3] CLUSTERED (c3),
    index [IX_c1] nonclustered (c1)
)

insert #t1(c3) values ('a'), ('a'), ('a')

select * from #t1

Answer 2

No there is not...the existence of the ability to define clustered as an option in table creation is to support declaring primary key and unique column constraints, which themselves create indexes. In other words, CLUSTERED in the CREATE TABLE statement is specifying whether or not the index created by the UNIQUE constraint should be clustered or nonclustered, which is important because a table can only have one clustered index.