我有一个调用数据表，已经增长到13亿行和173千兆字节的数据我们不再使用两列，一个是char（15），另一个是varchar（24）。他们都已经插入NULL一段时间了，我一直推迟删除列，因为我不确定其含义。我们在带有数据库的驱动器和带有事务日志的驱动器上的空间有限。
此外，我发现this post表示在DBCC REINDEX完成之前该空间将无法使用。我认为这既好又坏。这很好，因为删除列应该非常快并且不涉及大量日志记录，但是因为空间不会被回收而很糟糕。新插入的记录会占用更少的空间吗？在我的情况下这很好，因为我们在18个月之后修剪旧数据，因此空间将逐渐减少。
如果我们做了一个DBCC REINDEX（或ALTER INDEX REBUILD）那会真的有帮助，因为这些列不是任何索引的一部分吗？这会占用日志空间还是锁定表格以便无法使用？
答案 0 :(得分：3)
我发现你的问题很有趣，所以决定在开发数据库上建模。 SQL Server 2008，数据库大小400 Mb，日志2.4 Gb。 我假设，从提供的链接，您创建了一个具有聚集索引的表：
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]
此表包含1,200万条记录。
sp_spaceused big_table, true
name-big_table, rows-12031303, reserved-399240 KB, data-397760 KB, index_size-1336 KB, unused-144 KB.
删除列
sp_spaceused big_table, true
表格大小保持不变。数据库和日志大小保持不变。
向表的其余部分添加300万行
name-big_table, rows-15031303, reserved-511816 KB, data-509904 KB, index_size-1752 KB, unused-160 KB.
数据库大小500 Mb，日志3.27 Gb。
之后
DBCC DBREINDEX( big_table )
日志大小相同，但数据库大小增加到866 Mb
name-big_table, rows-12031303, reserved-338376 KB, data-337704 KB, index_size-568 KB, unused-104 KB.
再次添加300万行，以查看它们是否进入数据库中的可用空间。 数据库大小相同，记录3.96 Gb，清楚地显示它们是。
希望它有意义。
答案 1 :(得分：0)
不，新插入的记录不会占用更少的空间。我在今天早些时候看到了这个确切的问题。
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)
)
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
表的初始状态
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
现在进行一些更改
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
再次查看表格
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
插入一行并查看页面
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)
您可以看到删除（和更改）的列仍然占用空间，即使在更改架构时表实际为空。
在您的情况下，已删除列的影响将为15一个字节浪费char个字节，而varchar一个会浪费2个字节，除非它是变量部分中的最后一列，它不占用空间。