TempDB 中表变量和局部临时表的compare
参考资料来源：
https://blogs.msdn.com/b/sqlserverstorageengine/archive/tags/tempdb/
我们都知道，tempdb是用来为应用程序和SQL Server临时储存运行的中间结果的。由用户和应用程序创建的对象叫做用户对象，由SQL
Server引擎产生的对象叫做内部对象，在这篇博文中，我们主要讨论用户对象中的临时表（#，##）和表变量。大家可能对##表（全局临时表）和#表（局部临时表）的区别比较了解，但对临时表和表变量却不是很清楚，下面我们详述两者的主要区别。
和其他变量一样，表变量是一种非常有用的程序构造。表变量的有效范围和其他程序变量的有效范围是一样的。例如，如果你在存储过程中定义了一个变量，那么它就不能在存储过程外被访问。巧合的是，临时表也是这样的。那为什么我们还要创建表变量呢？因为表变量在存储过程中可以作为输出/输入参数（此功能从SQL
Server2008开始可用）或者用来存储函数的返回结果。以下是表变量和临时表的相同和不同之处：
• 首先，表变量不一定常驻内存。在内存压力大的时候，属于表变量的页可以被放入tempdb。以下是一个例子描述表变量在tempdb中所占空间。
use tempdb
go
drop table #tv_source
go
create table #tv_source(c1 int, c2 char(8000))
go
declare @i int
select @i = 0
while (@i < 1000)
begin
insert into #tv_source values (@i, replicate ('a', 100))
select @i = @i + 1
end
DECLARE @tv_target TABLE (c11 int, c22 char(8000))
INSERT INTO @tv_target (c11, c22)
SELECT c1, c2
FROM #tv_source
-- checking the size through DMV.
-- The sizes here are in 8k pages. This shows the
allocated space
-- to user objects to be 2000 pages (1000 pages
for #tv_source and
-- 1000 pages for @tv_target
Select total_size = SUM (unallocated_extent_page_count) +
SUM (user_object_reserved_page_count) +
SUM (internal_object_reserved_page_count) +
SUM (version_store_reserved_page_count) +
SUM (mixed_extent_page_count),
SUM (unallocated_extent_page_count) as freespace_pgs,
SUM (user_object_reserved_page_count) as user_obj_pgs,
SUM (internal_object_reserved_page_count) as internal_obj_pgs,
SUM (version_store_reserved_page_count) as version_store_pgs,
SUM (mixed_extent_page_count) as mixed_extent_pgs
from sys.dm_db_file_space_usage
• 其次，如果您创建了一个表变量，它会像一个常规的DDL操作一样将元数据储存在系统目录中，以下示例说明了这一点：
declare @ttt TABLE(c111 int, c222 int)
select name from sys.columns where object_id > 100 and name
like 'c%'
结果会返回两行，包含列C111和C222。这表明如果遇到定义冲突时，把临时表改成表变量不能解决问题。
• 第三，事务处理和锁定语句。表变量不能参与事务处理和锁定，以下示例说明了这一点
-- create a source table
create table
tv_source(c1 int, c2 char(100))
go
declare @i int
select @i = 0
while (@i < 100)
begin
insert into tv_source values (@i, replicate ('a', 100))
select @i = @i + 1
end
-- using #table
create table #tv_target (c11 int, c22 char(100))
go
BEGIN TRAN
INSERT INTO #tv_target (c11, c22)
SELECT c1, c2
FROM
tv_source
--
using table variable
DECLARE @tv_target TABLE (c11 int, c22 char(100))
BEGIN TRAN
INSERT INTO @tv_target (c11, c22)
SELECT c1, c2
FROM tv_source
-- Now if I look at the locks, you will see that
only
-- #table takes locks. Here is the query that
used
-- to check the locks
select
t1.request_session_id as spid,
t1.resource_type as type,
t1.resource_database_id as dbid,
(case
resource_type
WHEN 'OBJECT' then object_name(t1.resource_associated_entity_id)
WHEN 'DATABASE' then ' '
ELSE (select object_name(object_id)
from sys.partitions
where hobt_id=resource_associated_entity_id)
END) as objname,
t1.resource_description as description,
t1.request_mode as mode,
t1.request_status as status,
t2.blocking_session_id
from sys.dm_tran_locks as t1 left outer join sys.dm_os_waiting_tasks as t2
ON t1.lock_owner_address = t2.resource_address
另一个有趣的现象是，如果回滚的事务里涉及表变量，表变量的数据不会被回滚。
Rollback
-- this
query will return 100 for table variable but 0 for #table.
SELECT COUNT(*) FROM @tv_target
• 第四，表变量上的操作不被日志文件记录。请看下面这个例子：
--
create a table variable, insert bunch of rows and update
DECLARE @tv_target TABLE (c11 int, c22 char(100))
INSERT INTO @tv_target (c11, c22)
SELECT c1, c2
FROM tv_source
-- update all the rows
update @tv_target set c22 = replicate ('b', 100)
-- look at the top 10 log records. I get no
records for this case
select top 10 operation,context, [log record fixed length], [log record length],
AllocUnitId, AllocUnitName
from fn_dblog(null, null)
where AllocUnitName like '%tv_target%'
order by [Log Record Length] Desc
-- create a local temptable
drop table #tv_target
go
create table #tv_target (c11 int, c22 char(100))
go
INSERT INTO #tv_target (c11, c22)
SELECT c1, c2
FROM tv_source
--
update all the rows
update #tv_target set c22 = replicate ('b', 100)
-- look
at the log records. Here I get 100 log records for update
select
operation,context, [log
record fixed length], [log record length], AllocUnitName
from fn_dblog(null, null)
where AllocUnitName like '%tv_target%'
order by [Log
Record Length] Desc
• 第五，表变量中不允许DDL运行，所以，如果你有一个大的行集需要经常进行查询，您可能要使用临时表并创建合适的索引。你可以在声明表变量时创建唯一约束来解决这个问题。
• 第六，表变量不维护统计数据。这意味着任何表变量数据更改都不会引起相关查询语句进行重编译。
• 最后， 涉及表变量的查询不能生成并行的查询计划，因此我们认为对于庞大的临时数据集最好使用临时表来发挥并行查询的优势。