SQL Server ->> Computed Column（计算列）

Computed Column（计算列）是自SQL Server 2005开始就有的特性。计算列的定义是一个表达式。表达式可以是非计算列，常量，函数间的组合。但是不可以是子查询。

计算列数据固化

默认情况下计算列的数据是存储在磁盘上，仅当计算列被查询引用是才进行实时计算。只在计算列在定义是添加了PERSISTED关键词是才将数据固化。

计算列上创建索引或者作为分区函数的引用列

计算列上是运行创建索引和作为分区函数的引用列。但是必须指定PERSISTED关键词。

用法其实很简单。那么这里有些问题。使用计算列的代价到底有多大？

INSERT发生时使用计算列和非计算列的性能区别

这里做一个测试。首先创建好两张表

IF EXISTS(SELECT * FROM sys.tables WHERE name = 'computed_column_test_computed')
BEGIN
DROP TABLE dbo.computed_column_test_computed
END

IF EXISTS(SELECT * FROM sys.tables WHERE name = 'computed_column_test_noncomputed')
BEGIN
DROP TABLE dbo.computed_column_test_noncomputed
END

CREATE TABLE dbo.computed_column_test_computed
(
dttm DATETIME,
dttm_year AS YEAR(dttm) PERSISTED,
dttm_month AS MONTH(dttm) PERSISTED,
dttm_nextday AS DATEADD(DAY,1,dttm) PERSISTED,
dttm_previousday AS DATEADD(DAY,-1,dttm) PERSISTED,
dttm_week AS DATEPART(ww,dttm),
dttm_monthname AS CASE DATEPART(mm, dttm)
WHEN 1 THEN 'January'
WHEN 2 THEN 'February'
WHEN 3 THEN 'March'
WHEN 4 THEN 'April'
WHEN 5 THEN 'May'
WHEN 6 THEN 'June'
WHEN 7 THEN 'July'
WHEN 8 THEN 'August'
WHEN 9 THEN 'September'
WHEN 10 THEN 'October'
WHEN 11 THEN 'November'
WHEN 12 THEN 'December'
END PERSISTED,
dttm_quarter AS DATEPART(qq,dttm) PERSISTED
)

CREATE TABLE dbo.computed_column_test_noncomputed
(
dttm DATETIME,
dttm_year SMALLINT,
dttm_month SMALLINT,
dttm_nextday DATETIME,
dttm_previousday DATETIME,
dttm_week INT,
dttm_monthname VARCHAR(30),
dttm_quarter VARCHAR(30)
)

然后开启IO和TIME的统计信息开关，然后分别对两张表进行数据插入。

SET STATISTICS TIME ON
SET STATISTICS IO ON

INSERT dbo.computed_column_test_computed(
dttm
)
SELECT DATEADD(SECOND, Num, GETDATE())
FROM dbo.Numbers
WHERE Num <= 1000000

INSERT dbo.computed_column_test_noncomputed(dttm ,
dttm_year ,
dttm_month ,
dttm_nextday ,
dttm_previousday ,
dttm_week ,
dttm_monthname ,
dttm_quarter)
SELECT     GETDATE(),
YEAR(GETDATE()) PERSISTED,
MONTH(GETDATE()) PERSISTED,
DATEADD(DAY,1,GETDATE()) ,
DATEADD(DAY,-1,GETDATE()),
DATEPART(ww,GETDATE()),
CASE DATEPART(mm, GETDATE())
WHEN 1 THEN 'January'
WHEN 2 THEN 'February'
WHEN 3 THEN 'March'
WHEN 4 THEN 'April'
WHEN 5 THEN 'May'
WHEN 6 THEN 'June'
WHEN 7 THEN 'July'
WHEN 8 THEN 'August'
WHEN 9 THEN 'September'
WHEN 10 THEN 'October'
WHEN 11 THEN 'November'
WHEN 12 THEN 'December'
END,
DATEPART(qq,GETDATE())
FROM dbo.Numbers
WHERE Num <= 1000000

我的例子里面分别进行10万行、30万行、50万行和100万行数据的插入测试。一共有7个计算列。每个例子测试两次。

SQL Server Execution Times:
CPU time = 0 ms,  elapsed time = 0 ms.
Table 'computed_column_test_computed'. Scan count 0, logical reads 100840, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 164, physical reads 2, read-ahead reads 162, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 515 ms,  elapsed time = 1564 ms.

(100000 row(s) affected)

SQL Server parse and compile time:
CPU time = 0 ms, elapsed time = 1327 ms.
Table 'computed_column_test_noncomputed'. Scan count 0, logical reads 100833, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 164, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 265 ms,  elapsed time = 759 ms.

(100000 row(s) affected)

SQL Server parse and compile time:
CPU time = 0 ms, elapsed time = 5 ms.

SQL Server Execution Times:
CPU time = 0 ms,  elapsed time = 0 ms.

SQL Server Execution Times:
CPU time = 0 ms,  elapsed time = 0 ms.
Table 'computed_column_test_computed'. Scan count 0, logical reads 100840, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 164, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 391 ms,  elapsed time = 411 ms.

(100000 row(s) affected)
Table 'computed_column_test_noncomputed'. Scan count 0, logical reads 100833, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 164, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 234 ms,  elapsed time = 273 ms.

(100000 row(s) affected)

SQL Server Execution Times:
CPU time = 0 ms,  elapsed time = 0 ms.
SQL Server parse and compile time:
CPU time = 0 ms, elapsed time = 6 ms.
Table 'computed_column_test_computed'. Scan count 0, logical reads 302521, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 487, physical reads 1, read-ahead reads 330, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 1250 ms,  elapsed time = 1986 ms.

(300000 row(s) affected)
SQL Server parse and compile time:
CPU time = 0 ms, elapsed time = 74 ms.
Table 'computed_column_test_noncomputed'. Scan count 0, logical reads 302499, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 487, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 813 ms,  elapsed time = 966 ms.

(300000 row(s) affected)

SQL Server parse and compile time:
CPU time = 0 ms, elapsed time = 14 ms.
Table 'computed_column_test_computed'. Scan count 0, logical reads 302521, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 487, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 1156 ms,  elapsed time = 1709 ms.

(300000 row(s) affected)
Table 'computed_column_test_noncomputed'. Scan count 0, logical reads 302499, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 487, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 734 ms,  elapsed time = 3089 ms.

(300000 row(s) affected)

SQL Server parse and compile time:
CPU time = 8 ms, elapsed time = 8 ms.
Table 'computed_column_test_computed'. Scan count 0, logical reads 504201, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 809, physical reads 0, read-ahead reads 315, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 2031 ms,  elapsed time = 2080 ms.

(500000 row(s) affected)
Table 'computed_column_test_noncomputed'. Scan count 0, logical reads 504166, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 809, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 1297 ms,  elapsed time = 2915 ms.

(500000 row(s) affected)

SQL Server parse and compile time:
CPU time = 0 ms, elapsed time = 0 ms.
SQL Server parse and compile time:
CPU time = 0 ms, elapsed time = 4 ms.
Table 'computed_column_test_computed'. Scan count 0, logical reads 504201, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 809, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 1984 ms,  elapsed time = 3540 ms.

(500000 row(s) affected)
SQL Server parse and compile time:
CPU time = 0 ms, elapsed time = 2 ms.
Table 'computed_column_test_noncomputed'. Scan count 0, logical reads 504166, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 809, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 1266 ms,  elapsed time = 1341 ms.

(500000 row(s) affected)

SQL Server parse and compile time:
CPU time = 13 ms, elapsed time = 13 ms.
Table 'computed_column_test_computed'. Scan count 0, logical reads 1008359, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 1615, physical reads 0, read-ahead reads 791, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 4500 ms,  elapsed time = 9110 ms.

(1000000 row(s) affected)
Table 'computed_column_test_noncomputed'. Scan count 0, logical reads 1008333, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 1615, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 2531 ms,  elapsed time = 3903 ms.

(1000000 row(s) affected)

SQL Server parse and compile time:
CPU time = 0 ms, elapsed time = 0 ms.
SQL Server parse and compile time:
CPU time = 0 ms, elapsed time = 3 ms.
Table 'computed_column_test_computed'. Scan count 0, logical reads 1008359, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 1615, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 3797 ms,  elapsed time = 6559 ms.

(1000000 row(s) affected)
SQL Server parse and compile time:
CPU time = 0 ms, elapsed time = 3 ms.
Table 'computed_column_test_noncomputed'. Scan count 0, logical reads 1008333, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 1615, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 2422 ms,  elapsed time = 3895 ms.

(1000000 row(s) affected)

可以看到确实使用computed column会对性能有一定的影响。当计算列数量越多的情况下性能的影响越大。但是当计算列数量很少的情况下，影响或者说差别其实很小很小。以我做的实验为例，讲计算列数量减少到只有3个，数据量依旧停留在100万行的情况，两者的性能差异其实已经很小了。

SQL Server parse and compile time:
CPU time = 0 ms, elapsed time = 4 ms.

SQL Server Execution Times:
CPU time = 0 ms,  elapsed time = 0 ms.

SQL Server Execution Times:
CPU time = 0 ms,  elapsed time = 0 ms.
Table 'computed_column_test_computed'. Scan count 0, logical reads 1005813, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 1615, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 3203 ms,  elapsed time = 5058 ms.

(1000000 row(s) affected)
Table 'computed_column_test_noncomputed'. Scan count 0, logical reads 1005319, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 1615, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 2281 ms,  elapsed time = 4747 ms.

(1000000 row(s) affected)

SQL Server parse and compile time:
CPU time = 16 ms, elapsed time = 28 ms.
Table 'computed_column_test_computed'. Scan count 0, logical reads 1005813, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 1615, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 2984 ms,  elapsed time = 3512 ms.

(1000000 row(s) affected)
Table 'computed_column_test_noncomputed'. Scan count 0, logical reads 1005319, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 1615, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
CPU time = 2672 ms,  elapsed time = 2859 ms.

(1000000 row(s) affected)

那么总结下，计算列的使用原则我认为是在表中计算列的数量本身不多，而且一次性数据行插入量不大，计算逻辑固定，计算复杂度大的情况下，推荐使用计算列。

比如像DimDate这种表，表中可能有非常多的属性列用于表示当前日期的一些额外属性，比如下一天的日期，前一天的日期等等。用计算列是一个很好的选择。

参考：

Computed Columns