sql-server – 为什么DELETE会对性能产生影响?

最后是一个测试脚本,用于比较@table变量和#temp表之间的性能.我想我已经正确设置了 – 性能时间是在DELETE / TRUNCATE命令之外进行的.我得到的结果如下(以毫秒为单位的时间).

@Table Variable  #Temp (delete)  #Temp (truncate)
---------------  --------------  ----------------
5723             5180            5506
15636            14746           7800
14506            14300           5583
14030            15460           5386
16706            16186           5360

为了确保我的理智,这表明CURRENT_TIMESTAMP(又名GetDate())是在语句时而不是批处理中进行的,因此TRUNCATE / DELETE与SET @StartTime = CURRENT_TIMESTAMP之间不应该有任何交互.声明.

select current_timestamp
waitfor delay '00:00:04'
select current_timestamp

-----------------------
2012-10-21 11:29:20.290

-----------------------
2012-10-21 11:29:24.290

当使用DELETE清除表时,第一次运行和后续运行之间的跳转非常一致.我对DELETE的理解中缺少什么?我重复了这么多次,交换了订单,调整tempdb大小以不需要增长等.

CREATE TABLE #values (
  id int identity primary key, -- will be clustered
  name varchar(100) null,
  number int null,
  type char(3) not null,
  low int null,
  high int null,
  status smallint not null
);
GO
SET NOCOUNT ON;

DECLARE @values TABLE (
  id int identity primary key clustered,
  name varchar(100) null,
  number int null,
  type char(3) not null,
  low int null,
  high int null,
  status smallint not null
);
DECLARE  @ExecutionTime  TABLE(      Duration bigINT    ) 
DECLARE  @StartTime DATETIME,  @i INT = 1; 
WHILE (@i <= 5) 
  BEGIN 
    DELETE @values;
    DBCC freeproccache With NO_InfoMSGS;
    DBCC DROPCLEANBUFFERS With NO_InfoMSGS;
    SET @StartTime = CURRENT_TIMESTAMP -- alternate getdate() 
    /****************** measured process ***********************/ 

    INSERT @values SELECT a.* FROM master..spt_values a join master..spt_values b on b.type='P' and b.number < 1000;

    /**************** end measured process *********************/ 
    INSERT @ExecutionTime 
    SELECT DurationInMilliseconds = datediff(ms,@StartTime,CURRENT_TIMESTAMP) 
    SET @i +=  1 
  END -- WHILE 

SELECT DurationInMilliseconds = Duration FROM   @ExecutionTime 
GO 

-- Temporary table
DECLARE  @ExecutionTime  TABLE(      Duration bigINT    ) 
DECLARE  @StartTime DATETIME,  @i INT = 1; 
WHILE (@i <= 5) 
  BEGIN 
    delete #values;
    -- TRUNCATE TABLE #values;
    DBCC freeproccache With NO_InfoMSGS;
    DBCC DROPCLEANBUFFERS With NO_InfoMSGS;
    SET @StartTime = CURRENT_TIMESTAMP -- alternate getdate() 
    /****************** measured process ***********************/ 

    INSERT #values SELECT a.* FROM master..spt_values a join master..spt_values b on b.type='P' and b.number < 1000;

    /**************** end measured process *********************/ 
    INSERT @ExecutionTime 
    SELECT DurationInMilliseconds = datediff(ms,@StartTime,CURRENT_TIMESTAMP) 
    SET @i +=  1 
  END -- WHILE 

SELECT DurationInMilliseconds = Duration FROM   @ExecutionTime 
GO

DROP TABLE  #values 
SET NOCOUNT OFF;
最佳答案
当对象是B树时,这种差异似乎只适用.删除表变量上的主键,因此它是一个堆,我得到了以下结果

2560
2120
2080
2130
2140

但是在PK中,我在测试中发现了类似的模式以及下面的典型结果.

+--------+--------+---------+-------------------+
| @table | #table | ##table | [permanent_table] |
+--------+--------+---------+-------------------+
|   2670 |   2683 |    9603 |              9703 |
|   6823 |   6840 |    9723 |              9790 |
|   6813 |   6816 |    9626 |              9703 |
|   6883 |   6816 |    9600 |              9716 |
|   6840 |   6856 |    9610 |              9673 |
+--------+--------+---------+-------------------+

我的理论是,当对本地临时B树进行批量插入时,存在一些可用的优化,仅在它尚未分配任何页面时才适用.

我基于以下观察结果.

>运行各种版本的测试代码时,我只看到了@table_variables和#temp表的这种模式.不是tempdb中的永久表,也不是##表.
>为了获得较慢的性能,不必先从表中添加和删除大量行.只需添加一行并将其留在那里即可.
> TRUNCATE从表中取消分配所有页面. DELETE不会导致表中的最后一页被释放.
>使用VS 2012探查器显示,在更快的情况下,SQL Server使用不同的代码路径. 36%的时间花在sqlmin.dll!RowsetBulk :: InsertRow vs sqlmin.dll花费的时间的61%!RowsetNewSS :: InsertRow用于较慢的情况.

运行

SELECT * 
FROM sys.dm_db_index_physical_stats(2,OBJECT_ID('tempdb..#values'),1,NULL, 'DETAILED')

删除后返回

+-------------+------------+--------------+--------------------+
| index_level | page_count | record_count | ghost_record_count |
+-------------+------------+--------------+--------------------+
|           0 |          1 |            0 |                  1 |
|           1 |          1 |            1 |                  0 |
|           2 |          1 |            1 |                  0 |
+-------------+------------+--------------+--------------------+

我发现有可能在enabling trace flag 610之间减少时间差异.

这样可以大大减少后续插入的日志记录量(从350 MB减少到103 MB,因为它不再记录单个插入的行值),但这对第二次和后续的@table的时间只有很小的改进. ,#table案例和差距仍然存在.跟踪标志显着提高了插入到其他两种表类型的一般性能.

+--------+--------+---------+-------------------+
| @table | #table | ##table | [permanent_table] |
+--------+--------+---------+-------------------+
|   2663 |   2670 |    5403 |              5426 |
|   5390 |   5396 |    5410 |              5403 |
|   5373 |   5390 |    5410 |              5403 |
|   5393 |   5410 |    5406 |              5433 |
|   5386 |   5396 |    5390 |              5420 |
+--------+--------+---------+-------------------+

通过查看事务日志,我注意到针对空本地临时表的初始插入似乎记录得更少(96 MB).

值得注意的是,这些更快的插入只有657个事务(LOP_BEGIN_XACT / LOP_COMMIT_XACT对),而在较慢的情况下超过10,000.特别是LOP_FORMAT_PAGE操作似乎大大减少了.对于表中的每个页面(约10,270),较慢的情况具有针对此的事务日志条目,而在快速情况下仅有4个这样的条目.

在所有三种情况下使用的日志如下(我已删除日志记录以更新系统基表以减少文本量,但它们仍包含在总计中)

记录针对@table_var的第一个插入(96.5 MB)

+-----------------------+----------+----------------------------------------------+---------------+---------+
|       Operation       | Context  |                AllocUnitName                 | Size in Bytes |   Cnt   |
+-----------------------+----------+----------------------------------------------+---------------+---------+
| LOP_BEGIN_XACT        | LCX_NULL | NULL                                         |         83876 |     658 |
| LOP_COMMIT_XACT       | LCX_NULL | NULL                                         |         34164 |     657 |
| LOP_CREATE_ALLOCCHAIN | LCX_NULL | NULL                                         |           120 |       3 |
| LOP_FORMAT_PAGE       | LCX_HEAP | dbo.#531856C7                                |            84 |       1 |
| LOP_FORMAT_PAGE       | LCX_IAM  | dbo.#4F47C5E3.PK__#4F47C5E__3213E83F51300E55 |            84 |       1 |
| LOP_FORMAT_PAGE       | LCX_IAM  | dbo.#531856C7                                |            84 |       1 |
| LOP_FORMAT_PAGE       | LCX_IAM  | Unknown Alloc Unit                           |            84 |       1 |
| LOP_HOBT_DDL          | LCX_NULL | NULL                                         |           216 |       6 |
| LOP_HOBT_DELTA        | LCX_NULL | NULL                                         |           320 |       5 |
| LOP_IDENT_NEWVAL      | LCX_NULL | NULL                                         |     100240000 | 25
2560
2120
2080
2130
2140

|
| LOP_INSERT_ROWS | LCX_HEAP | dbo.#531856C7 | 72 | 1 |
| LOP_MODIFY_ROW | LCX_IAM | dbo.#531856C7 | 88 | 1 |
| LOP_MODIFY_ROW | LCX_PFS | dbo.#4F47C5E3.PK__#4F47C5E__3213E83F51300E55 | 158592 | 1848 |
| LOP_MODIFY_ROW | LCX_PFS | dbo.#531856C7 | 80 | 1 |
| LOP_MODIFY_ROW | LCX_PFS | Unknown Alloc Unit | 216016 | 2455 |
| LOP_SET_BITS | LCX_GAM | dbo.#4F47C5E3.PK__#4F47C5E__3213E83F51300E55 | 84360 | 1406 |
| LOP_SET_BITS | LCX_GAM | Unknown Alloc Unit | 147120 | 2452 |
| LOP_SET_BITS | LCX_IAM | dbo.#4F47C5E3.PK__#4F47C5E__3213E83F51300E55 | 84360 | 1406 |
| LOP_SET_BITS | LCX_IAM | Unknown Alloc Unit | 147120 | 2452 |
| Total | NULL | NULL | 101209792 | 2519475 |
+-----------------------+----------+----------------------------------------------+---------------+---------+

记录后续插入TF 610关闭(350 MB)

+-----------------------+--------------------+----------------------------------------------+---------------+---------+
|       Operation       |      Context       |                AllocUnitName                 | Size in Bytes |   Cnt   |
+-----------------------+--------------------+----------------------------------------------+---------------+---------+
| LOP_BEGIN_CKPT        | LCX_NULL           | NULL                                         |            96 |       1 |
| LOP_BEGIN_XACT        | LCX_NULL           | NULL                                         |       1520696 |   12521 |
| LOP_COMMIT_XACT       | LCX_NULL           | NULL                                         |        651040 |   12520 |
| LOP_CREATE_ALLOCCHAIN | LCX_NULL           | NULL                                         |            40 |       1 |
| LOP_DELETE_SPLIT      | LCX_INDEX_INTERIOR | dbo.#4F47C5E3.PK__#4F47C5E__3213E83F51300E55 |          2160 |      36 |
| LOP_END_CKPT          | LCX_NULL           | NULL                                         |           136 |       1 |
| LOP_FORMAT_PAGE       | LCX_HEAP           | dbo.#4F47C5E3.PK__#4F47C5E__3213E83F51300E55 |        859236 |   10229 |
| LOP_FORMAT_PAGE       | LCX_IAM            | Unknown Alloc Unit                           |            84 |       1 |
| LOP_FORMAT_PAGE       | LCX_INDEX_INTERIOR | dbo.#4F47C5E3.PK__#4F47C5E__3213E83F51300E55 |          3108 |      37 |
| LOP_HOBT_DDL          | LCX_NULL           | NULL                                         |           648 |      18 |
| LOP_HOBT_DELTA        | LCX_NULL           | NULL                                         |        657088 |   10267 |
| LOP_IDENT_NEWVAL      | LCX_NULL           | NULL                                         |     100239960 | 2505999 |
| LOP_INSERT_ROWS       | LCX_CLUSTERED      | dbo.#4F47C5E3.PK__#4F47C5E__3213E83F51300E55 |     258628000 | 25
2560
2120
2080
2130
2140

|
| LOP_INSERT_ROWS | LCX_HEAP | dbo.#531856C7 | 72 | 1 |
| LOP_INSERT_ROWS | LCX_INDEX_INTERIOR | dbo.#4F47C5E3.PK__#4F47C5E__3213E83F51300E55 | 1042776 | 10302 |
| LOP_MODIFY_HEADER | LCX_HEAP | dbo.#4F47C5E3.PK__#4F47C5E__3213E83F51300E55 | 859236 | 10229 |
| LOP_MODIFY_HEADER | LCX_INDEX_INTERIOR | dbo.#4F47C5E3.PK__#4F47C5E__3213E83F51300E55 | 3192 | 38 |
| LOP_MODIFY_ROW | LCX_IAM | dbo.#4F47C5E3.PK__#4F47C5E__3213E83F51300E55 | 704 | 8 |
| LOP_MODIFY_ROW | LCX_PFS | dbo.#4F47C5E3.PK__#4F47C5E__3213E83F51300E55 | 934264 | 11550 |
| LOP_MODIFY_ROW | LCX_PFS | Unknown Alloc Unit | 783984 | 8909 |
| LOP_SET_BITS | LCX_GAM | dbo.#4F47C5E3.PK__#4F47C5E__3213E83F51300E55 | 76980 | 1283 |
| LOP_SET_BITS | LCX_GAM | Unknown Alloc Unit | 534480 | 8908 |
| LOP_SET_BITS | LCX_IAM | dbo.#4F47C5E3.PK__#4F47C5E__3213E83F51300E55 | 76980 | 1283 |
| LOP_SET_BITS | LCX_IAM | Unknown Alloc Unit | 534480 | 8908 |
| LOP_SHRINK_NOOP | LCX_NULL | NULL | 32 | 1 |
| LOP_XACT_CKPT | LCX_NULL | NULL | 92 | 1 |
| Total | NULL | NULL | 367438748 | 5119297 |
+-----------------------+--------------------+----------------------------------------------+---------------+---------+

记录后续插入TF 610(103 MB)

+-------------------------+-------------------------+----------------------------------------------+---------------+---------+
|        Operation        |         Context         |                AllocUnitName                 | Size in Bytes |   Cnt   |
+-------------------------+-------------------------+----------------------------------------------+---------------+---------+
| LOP_BEGIN_CKPT          | LCX_NULL                | NULL                                         |           192 |       2 |
| LOP_BEGIN_XACT          | LCX_NULL                | NULL                                         |       1339796 |   11099 |
| LOP_BULK_EXT_ALLOCATION | LCX_NULL                | NULL                                         |         20616 |     162 |
| LOP_COMMIT_XACT         | LCX_NULL                | NULL                                         |        577096 |   11098 |
| LOP_CREATE_ALLOCCHAIN   | LCX_NULL                | NULL                                         |            40 |       1 |
| LOP_DELETE_SPLIT        | LCX_INDEX_INTERIOR      | dbo.#6DCC4D03.PK__#6DCC4D0__3213E83F6FB49575 |          2160 |      36 |
| LOP_END_CKPT            | LCX_NULL                | NULL                                         |           272 |       2 |
| LOP_FORMAT_PAGE         | LCX_BULK_OPERATION_PAGE | dbo.#6DCC4D03.PK__#6DCC4D0__3213E83F6FB49575 |        863520 |   10280 |
| LOP_FORMAT_PAGE         | LCX_IAM                 | Unknown Alloc Unit                           |            84 |       1 |
| LOP_FORMAT_PAGE         | LCX_INDEX_INTERIOR      | dbo.#6DCC4D03.PK__#6DCC4D0__3213E83F6FB49575 |          3108 |      37 |
| LOP_HOBT_DELTA          | LCX_NULL                | NULL                                         |        666496 |   10414 |
| LOP_IDENT_NEWVAL        | LCX_NULL                | NULL                                         |     100239960 | 2505999 |
| LOP_INSERT_ROWS         | LCX_CLUSTERED           | dbo.#6DCC4D03.PK__#6DCC4D0__3213E83F6FB49575 |         23544 |     218 |
| LOP_INSERT_ROWS         | LCX_HEAP                | dbo.#719CDDE7                                |            72 |       1 |
| LOP_INSERT_ROWS         | LCX_INDEX_INTERIOR      | dbo.#6DCC4D03.PK__#6DCC4D0__3213E83F6FB49575 |       1042776 |   10302 |
| LOP_MODIFY_HEADER       | LCX_BULK_OPERATION_PAGE | dbo.#6DCC4D03.PK__#6DCC4D0__3213E83F6FB49575 |        780216 |   10266 |
| LOP_MODIFY_HEADER       | LCX_HEAP                | dbo.#6DCC4D03.PK__#6DCC4D0__3213E83F6FB49575 |       1718472 |   20458 |
| LOP_MODIFY_HEADER       | LCX_INDEX_INTERIOR      | dbo.#6DCC4D03.PK__#6DCC4D0__3213E83F6FB49575 |          3192 |      38 |
| LOP_MODIFY_ROW          | LCX_IAM                 | dbo.#6DCC4D03.PK__#6DCC4D0__3213E83F6FB49575 |           704 |       8 |
| LOP_MODIFY_ROW          | LCX_PFS                 | dbo.#6DCC4D03.PK__#6DCC4D0__3213E83F6FB49575 |        114832 |    1307 |
| LOP_MODIFY_ROW          | LCX_PFS                 | Unknown Alloc Unit                           |        231696 |    2633 |
| LOP_RANGE_INSERT        | LCX_NULL                | NULL                                         |            48 |       1 |
| LOP_SET_BITS            | LCX_GAM                 | dbo.#6DCC4D03.PK__#6DCC4D0__3213E83F6FB49575 |         77100 |    1285 |
| LOP_SET_BITS            | LCX_GAM                 | Unknown Alloc Unit                           |        157920 |    2632 |
| LOP_SET_BITS            | LCX_IAM                 | dbo.#6DCC4D03.PK__#6DCC4D0__3213E83F6FB49575 |         77100 |    1285 |
| LOP_SET_BITS            | LCX_IAM                 | Unknown Alloc Unit                           |        157920 |    2632 |
| LOP_XACT_CKPT           | LCX_NULL                | NULL                                         |            92 |       1 |
| Total                   | NULL                    | NULL                                         |     108102960 | 2602218 |
+-------------------------+-------------------------+----------------------------------------------+---------------+---------+

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