统计和基数估算 - 为什么我看到这个结果？

Question

当我试图解决一个更复杂的问题时,我遇到了这个小问题,并试图找出优化器.所以,假设我有一个名为`MyTable'的表,可以像这样定义:

CREATE TABLE MyTable (
GroupClosuresID int identity(1,1) not null, 
SiteID int not null, 
DeleteDateTime datetime null
, CONSTRAINT PK_MyTable PRIMARY KEY (GroupClosuresID, SiteID))

该表中有286,685行,运行DBCC SHOW_STATISTICS('MyTable','PK_MyTable')将产生:

Name                                                                                                                             Updated              Rows                 Rows Sampled         Steps  Density       Average key length String Index Filter Expression                                                                                                                                                                                                                                                Unfiltered Rows
-------------------------------------------------------------------------------------------------------------------------------- -------------------- -------------------- -------------------- ------ ------------- ------------------ ------------ ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- --------------------
PK_MyTable                                                                                                                       Aug 10 2011  1:00PM  286685               286685               18     0.931986      8                  NO           NULL                                                                                                                                                                                                                                                             286685

(1 row(s) affected)

All density   Average Length Columns
------------- -------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
3.743145E-06  4              GroupClosuresID
3.488149E-06  8              GroupClosuresID, SiteID

(2 row(s) affected)

RANGE_HI_KEY RANGE_ROWS    EQ_ROWS       DISTINCT_RANGE_ROWS  AVG_RANGE_ROWS
------------ ------------- ------------- -------------------- --------------
1            0             8             0                    1
129          1002          7             127                  7.889764
242          826           6             112                  7.375
531          2010          6             288                  6.979167
717          1108          5             185                  5.989189
889          822           4             171                  4.807017
1401         2044          4             511                  4
1763         1101          3             361                  3.049861
14207        24780         1             12443                1.991481
81759        67071         1             67071                1
114457       31743         1             31743                1
117209       2047          1             2047                 1
179109       61439         1             61439                1
181169       1535          1             1535                 1
229410       47615         1             47615                1
235846       2047          1             2047                 1
275456       39442         1             39442                1
275457       0             1             0                    1

现在,我在此表上运行查询,但没有创建其他索引或统计信息.

SELECT GroupClosuresID FROM MyTable WHERE SiteID = 1397 AND DeleteDateTime IS NULL

现在出现两个新的统计对象,一个用于SiteID列,另一个用于DeleteDateTime列.它们分别在这里(注意:一些不相关的信息已被排除在外):

Name                                                                                                                             Updated              Rows                 Rows Sampled         Steps  Density       Average key length String Index Filter Expression                                                                                                                                                                                                                                                Unfiltered Rows
-------------------------------------------------------------------------------------------------------------------------------- -------------------- -------------------- -------------------- ------ ------------- ------------------ ------------ ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- --------------------
_WA_Sys_00000002_7B0C223C                                                                                                        Aug 10 2011  1:15PM  286685               216605               200    0.03384706    4                  NO           NULL                                                                                                                                                                                                                                                             286685

(1 row(s) affected)

All density   Average Length Columns
------------- -------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
0.0007380074  4              SiteID

(1 row(s) affected)

RANGE_HI_KEY RANGE_ROWS    EQ_ROWS       DISTINCT_RANGE_ROWS  AVG_RANGE_ROWS
------------ ------------- ------------- -------------------- --------------
.
.
.
1397         59.42782      16005.02      5                    11.83174
.
.
.



Name                                                                                                                             Updated              Rows                 Rows Sampled         Steps  Density       Average key length String Index Filter Expression                                                                                                                                                                                                                                                Unfiltered Rows
-------------------------------------------------------------------------------------------------------------------------------- -------------------- -------------------- -------------------- ------ ------------- ------------------ ------------ ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- --------------------
_WA_Sys_00000006_7B0C223C                                                                                                        Aug 10 2011  1:15PM  286685               216605               201    0.7447883     0.8335911          NO           NULL                                                                                                                                                                                                                                                             286685

(1 row(s) affected)

All density   Average Length Columns
------------- -------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
0.0001065871  0.8335911      DeleteDateTime

(1 row(s) affected)

RANGE_HI_KEY            RANGE_ROWS    EQ_ROWS       DISTINCT_RANGE_ROWS  AVG_RANGE_ROWS
----------------------- ------------- ------------- -------------------- --------------
NULL                    0             255827        0                    1
.
.
.

为我上面运行的查询生成的执行计划没有给我带来任何惊喜.它由一个简单的聚集索引扫描与14282.3估计行和15676行实际.根据我对统计学和成本估算的了解,使用上面的两个直方图,我们可以将SiteID的选择性(16005.02/286685)乘以DeleteDateTime(255827/286685)的选择性,得到复合选择性0.0498187307480119.将行总数(286685)乘以该行给出的优化器完全相同:14282.3.

但这里是我感到困惑的地方.我创建了一个索引,用CREATE INDEX IX_MyTable ON Mytable (SiteID, DeleteDateTime)它创建自己的统计对象:

Name                                                                                                                             Updated              Rows                 Rows Sampled         Steps  Density       Average key length String Index Filter Expression                                                                                                                                                                                                                                                Unfiltered Rows
    -------------------------------------------------------------------------------------------------------------------------------- -------------------- -------------------- -------------------- ------ ------------- ------------------ ------------ ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- --------------------
IX_MyTable                                                                                                                       Aug 10 2011  1:41PM  286685               286685               200    0.02749305    8.822645           NO    NULL                                                                                                                            
                                                                                                                             286685

(1 row(s) affected)

All density   Average Length Columns
------------- -------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
0.0007107321  4              SiteID
7.42611E-05   4.822645       SiteID, DeleteDateTime
3.488149E-06  8.822645       SiteID, DeleteDateTime, GroupClosuresID

(3 row(s) affected)

RANGE_HI_KEY RANGE_ROWS    EQ_ROWS       DISTINCT_RANGE_ROWS  AVG_RANGE_ROWS
------------ ------------- ------------- -------------------- --------------
.
.
.
1397         504           15686         12                   42
.
.
.

当我运行与before(SELECT GroupClosuresID FROM MyTable WHERE SiteID = 1397 AND DeleteDateTime IS NULL)相同的查询时,我仍然会返回15676行,但我的估计行数现在是181.82.

我试过操纵数字来试图找出估计来自哪里,但我无法得到它.我必须假设它与IX_MyTable的密度值有关.

任何帮助将不胜感激.谢谢!!

编辑:这是最后一次查询执行的执行计划.

Answer 1

这个挖了一些!

它是以下产品:

• NULL 日期字段中的密度(来自您的第一组统计数据) 255827/286685 = .892363
• ...乘以siteid新索引中第一个field()的密度:0.0007107321

公式是:

.00071017321 * 286685 = 203.7562 
-- est. rows with your value in siteid based on even distribution of values

255827 / 286685 = 0.892363
-- Probability of a NULL across all rows

203.7562 * 0.892363 = 181.8245

我猜测,因为这个实例中的行计数实际上并没有影响任何东西,优化器采用了最简单的路径,只是将概率相乘.