在大表中刪除列時的注意事項

Question

我有一個調用數據表，已增長到13億行和173千兆字節的數據我們不再使用兩列，一個是char（15），另一個是varchar（24）。 他們都已經插入NULL一段時間了，我一直推遲刪除列，因為我不確定其含義。 我們在帶有數據庫的驅動器和帶有事務日志的驅動器上的空間有限。

另外我發現這篇帖子說在DBCC REINDEX完成之前這個空間是不可用的。 我認為這既好又壞。 這很好，因為刪除列應該非常快並且不涉及大量日志記錄，但是因為空間不會被回收而很糟糕。 新插入的記錄會占用更少的空間嗎？ 在我的情況下，這將是好的，因為我們在18個月后修剪舊數據，因此空間將逐漸減少。

如果我們做了DBCC REINDEX（或ALTER INDEX REBUILD）那會真的有幫助，因為列不是任何索引的一部分嗎？ 這會占用日志空間還是鎖定表格以使其無法使用？

Answer 1

我發現你的問題很有趣，所以決定在開發數據庫上建模。 SQL Server 2008，數據庫大小400 Mb，日志2.4 Gb。 我假設，從提供的鏈接，您創建了一個具有聚簇索引的表：

CREATE TABLE [dbo].[big_table](
    [recordID] [int] IDENTITY(1,1) NOT NULL,
    [col1] [varchar](50) NOT NULL,
    [col2] [char](15) NULL,
    [col3] [varchar](24) NULL,
 CONSTRAINT [PK_big_table] PRIMARY KEY CLUSTERED 
(
    [recordID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY] 

該表包含1200萬條記錄。

sp_spaceused big_table, true

name-big_table, rows-12031303, reserved-399240 KB, data-397760 KB, index_size-1336 KB, unused-144 KB. 

刪除列

sp_spaceused big_table, true

表大小保持不變。 數據庫和日志大小保持不變。

將300萬行添加到表的其余部分

name-big_table, rows-15031303, reserved-511816 KB, data-509904 KB, index_size-1752 KB, unused-160 KB.

數據庫大小500 Mb，log 3.27 Gb。

后

DBCC DBREINDEX( big_table )

日志大小相同，但數據庫大小增加到866 Mb

name-big_table, rows-12031303, reserved-338376 KB, data-337704  KB, index_size-568 KB, unused-104 KB. 

再次添加300萬行，以查看它們是否進入數據庫中的可用空間。 數據庫大小相同，記錄3.96 Gb，這清楚地表明它們是。

希望它有意義。

Answer 2

不，新插入的記錄不會占用更少的空間。 我在今天早些時候看到了這個確切的問題。

測試表

CREATE TABLE T
(
id int identity primary key,
FixedWidthColToBeDropped char(10),
VariableWidthColToBeDropped varchar(10),
FixedWidthColToBeWidened char(7),
FixedWidthColToBeShortened char(20),
VariableWidthColToBeWidened varchar(7),
VariableWidthColToBeShortened varchar(20),
VariableWidthColWontBeAltered varchar(20)
)

偏移查詢

WITH T
     AS (SELECT ISNULL(LEFT(MAX(name), 30), 'Dropped')  AS column_name,
                MAX(column_id)                          AS column_id,
                ISNULL(MAX(case
                             when column_id IS NOT NULL THEN max_inrow_length
                           END), MAX(max_inrow_length)) AS max_inrow_length,
                leaf_offset,
                CASE
                  WHEN leaf_offset < 0 THEN SUM(CASE
                                                  WHEN column_id IS NULL THEN 2 ELSE 0
                                                END)
                  ELSE MAX(max_inrow_length) - MAX(CASE
                       WHEN column_id IS NULL THEN 0
                       ELSE max_inrow_length
                                                   END)
                END                                     AS wasted_space
         FROM   sys.system_internals_partition_columns pc
                JOIN sys.partitions p
                  ON p.partition_id = pc.partition_id
                LEFT JOIN sys.columns c
                  ON column_id = partition_column_id
                     AND c.object_id = p.object_id
         WHERE  p.object_id = object_id('T')
         GROUP  BY leaf_offset)
SELECT CASE
         WHEN GROUPING(column_name) = 0 THEN column_name
         ELSE 'Total'
       END               AS column_name,
       column_id,
       max_inrow_length,
       leaf_offset,
       SUM(wasted_space) AS wasted_space
FROM   T  
GROUP  BY ROLLUP ((column_name,
                   column_id,
                   max_inrow_length,
                   leaf_offset))
ORDER  BY GROUPING(column_name),
          CASE
            WHEN leaf_offset > 0 THEN leaf_offset
            ELSE 10000 - leaf_offset
          END  

表的初始狀態

column_name                    column_id   max_inrow_length leaf_offset wasted_space
------------------------------ ----------- ---------------- ----------- ------------
id                             1           4                4           0
FixedWidthColToBeDropped       2           10               8           0
FixedWidthColToBeWidened       4           7                18          0
FixedWidthColToBeShortened     5           20               25          0
VariableWidthColToBeDropped    3           10               -1          0
VariableWidthColToBeWidened    6           7                -2          0
VariableWidthColToBeShortened  7           20               -3          0
VariableWidthColWontBeAltered  8           20               -4          0
Total                          NULL        NULL             NULL        0

現在做一些改變

ALTER TABLE T 
ALTER COLUMN FixedWidthColToBeWidened char(12)

ALTER TABLE T 
ALTER COLUMN FixedWidthColToBeShortened char(10)

ALTER TABLE T 
ALTER COLUMN VariableWidthColToBeWidened varchar(12)

ALTER TABLE T 
ALTER COLUMN VariableWidthColToBeShortened varchar(10)

ALTER TABLE T 
DROP COLUMN FixedWidthColToBeDropped, VariableWidthColToBeDropped

再看看桌子

column_name                    column_id   max_inrow_length leaf_offset wasted_space
------------------------------ ----------- ---------------- ----------- ------------
id                             1           4                4           0
Dropped                        NULL        10               8           10
Dropped                        NULL        7                18          7
FixedWidthColToBeShortened     5           10               25          10
FixedWidthColToBeWidened       4           12               45          0
Dropped                        NULL        10               -1          2
VariableWidthColToBeWidened    6           12               -2          0
Dropped                        NULL        20               -3          2
VariableWidthColWontBeAltered  8           20               -4          0
VariableWidthColToBeShortened  7           10               -5          0
Total                          NULL        NULL             NULL        31

插入一行並查看頁面

INSERT INTO T
           ([FixedWidthColToBeWidened]
           ,[FixedWidthColToBeShortened]
           ,[VariableWidthColToBeWidened]
           ,[VariableWidthColToBeShortened])
     VALUES
           ('1','2','3','4')

DECLARE @DBCCPAGE nvarchar(100)

SELECT TOP 1  @DBCCPAGE = 'DBCC PAGE(''tempdb'',' + CAST(file_id AS VARCHAR) + ',' + CAST(page_id AS VARCHAR) + ',3)'
FROM T
CROSS APPLY sys.fn_PhysLocCracker(%%physloc%%) 

DBCC TRACEON(3604)
EXEC (@DBCCPAGE)    

返回

Record Type = PRIMARY_RECORD         Record Attributes =  NULL_BITMAP VARIABLE_COLUMNS
Record Size = 75                     
Memory Dump @0x000000000D5CA060

0000000000000000:   30003900 01000000 26a44500 00000000 †0.9.....&¤E..... 
0000000000000010:   ffffffff ffffff7f 00322020 20202020 †ÿÿÿÿÿÿÿ..2       
0000000000000020:   20202003 00000000 98935c0d 00312020 †   ......\..1   
0000000000000030:   20202020 20202020 200a0080 00050049 †         ......I 
0000000000000040:   004a004a 004a004b 003334†††††††††††††.J.J.J.K.34      

Slot 0 Column 1 Offset 0x4 Length 4 Length (physical) 4

id = 1                               

Slot 0 Column 67108868 Offset 0x8 Length 0 Length (physical) 10

DROPPED = NULL                       

Slot 0 Column 67108869 Offset 0x0 Length 0 Length (physical) 0

DROPPED = NULL                       

Slot 0 Column 67108865 Offset 0x12 Length 0 Length (physical) 7

DROPPED = NULL                       

Slot 0 Column 67108866 Offset 0x19 Length 0 Length (physical) 20

DROPPED = NULL                       

Slot 0 Column 6 Offset 0x49 Length 1 Length (physical) 1

VariableWidthColToBeWidened = 3      

Slot 0 Column 67108867 Offset 0x0 Length 0 Length (physical) 0

DROPPED = NULL                       

Slot 0 Column 8 Offset 0x0 Length 0 Length (physical) 0

VariableWidthColWontBeAltered = [NULL]                                    

Slot 0 Column 4 Offset 0x2d Length 12 Length (physical) 12

FixedWidthColToBeWidened = 1                                              

Slot 0 Column 5 Offset 0x19 Length 10 Length (physical) 10

FixedWidthColToBeShortened = 2                                            

Slot 0 Column 7 Offset 0x4a Length 1 Length (physical) 1

VariableWidthColToBeShortened = 4    

Slot 0 Offset 0x0 Length 0 Length (physical) 0

KeyHashValue = (010086470766)      

您可以看到刪除（和更改）的列仍然占用空間，即使在更改架構時表實際上是空的。

在您的情況下，刪除列的影響將為char 1浪費15個字節， varchar 1浪費2個字節， 除非它是變量部分中不占用空間的最后一列。