有在需要先整理了一下查詢計劃的一些奧祕。計算標量做什麼以及爲什麼會有一個流聚合。
表值函數返回碎化XML的節點表，每個碎化行一行。當您使用鍵入的XML時，這些列是值，左值，左值和tid。這些列用於計算標量來計算實際值。在那裏的代碼看起來有點奇怪，我不能說我明白爲什麼它是這樣的，但它的要點是函數xsd_cast_to_maybe_large返回值，並且有代碼處理的情況下，當值等於和大於128個字節。
CASE WHEN datalength(
CONVERT_IMPLICIT(sql_variant,
CONVERT_IMPLICIT(nvarchar(64),
xsd_cast_to_maybe_large(XML Reader with XPath filter.[value],
XML Reader with XPath filter.[lvalue],
XML Reader with XPath filter.[lvaluebin],
XML Reader with XPath filter.[tid],(15),(5),(0)),0),0))>=(128)
THEN CONVERT_IMPLICIT(int,CASE WHEN datalength(xsd_cast_to_maybe_large(XML Reader with XPath filter.[value],
XML Reader with XPath filter.[lvalue],
XML Reader with XPath filter.[lvaluebin],
XML Reader with XPath filter.[tid],(15),(5),(0)))<(128)
THEN NULL
ELSE xsd_cast_to_maybe_large(XML Reader with XPath filter.[value],
XML Reader with XPath filter.[lvalue],
XML Reader with XPath filter.[lvaluebin],
XML Reader with XPath filter.[tid],(15),(5),(0))
END,0)
ELSE CONVERT_IMPLICIT(int,CONVERT_IMPLICIT(sql_variant,
CONVERT_IMPLICIT(nvarchar(64),
xsd_cast_to_maybe_large(XML Reader with XPath filter.[value],
XML Reader with XPath filter.[lvalue],
XML Reader with XPath filter.[lvaluebin],
XML Reader with XPath filter.[tid],(15),(5),(0)),0),0),0)
END
用於非類型化XML的相同計算標量非常簡單且實際上可以理解。
CASE WHEN datalength(XML Reader with XPath filter.[value])>=(128)
THEN CONVERT_IMPLICIT(int,XML Reader with XPath filter.[lvalue],0)
ELSE CONVERT_IMPLICIT(int,XML Reader with XPath filter.[value],0)
END
如果有value超過128個字節lvalue獲取來自其他value取。在非類型化XML的情況下，返回的節點表僅輸出列id，值和左值。
當您使用鍵入的XML時，節點值的存儲將根據模式中指定的數據類型進行優化。看起來它可能會以節點表中的值，左值或左值結束，具體取決於它的值是什麼類型，xsd_cast_to_maybe_large可以幫助解決問題。
流聚合對來自計算標量的返回值執行min（）。我們知道，SQL Server（至少有時）確實知道，在value（）函數中指定XPath時，將只有從表值函數返回的一行。解析器確保我們正確地構建XPath，但是當查詢優化器查看估計的行時，它會看到200行。用於解析XML的表值函數的基本估計值爲10000行，然後使用所使用的XPath進行一些調整。在這種情況下，它最終只有200行，只有一行。對我而言，純粹的猜測是流集合在那裏來處理這種差異。它永遠不會聚合任何東西，只發送返回的那一行，但它確實會影響整個分支的基數估計，並確保優化器使用1行作爲該分支的估計值。當優化器選擇加入策略等時，這當然非常重要。
那麼100個屬性如何呢？是的，如果您使用值函數100次，將會有100個分支。但是這裏有一些優化要做。我創建了一個測試裝置，查看使用10行以上的100個屬性，查詢的形狀和形式是最快的。
獲勝者將使用無類型的XML，並且不要使用nodes()函數粉碎r。
select X.value('(/r/@a1)[1]', 'int') as a1,
X.value('(/r/@a2)[1]', 'int') as a2,
X.value('(/r/@a3)[1]', 'int') as a3
from @T
還有避免了100個分支機構使用透視但根據您的實際查詢的樣子，可能是不可能的方式。來自數據透視表的數據類型必須相同。你當然可以將它們作爲字符串提取出來，並轉換爲列表中適當的類型。它還要求您的表具有主鍵/唯一鍵。
select a1, a2, a3
from (
select T.ID, -- primary key of @T
A.X.value('local-name(.)', 'nvarchar(50)') as Name,
A.X.value('.', 'int') as Value
from @T as T
cross apply T.X.nodes('/r/@*') as A(X)
) as T
pivot(min(T.Value) for Name in (a1, a2, a3)) as P
查詢計劃樞軸查詢，10點100的屬性：
下面是結果和測試裝備予使用。我測試了100個屬性和10行以及所有int屬性。
結果：
Test Duration (ms)
-------------------------------------------------- -------------
untyped XML value('/r[1]/@a') 195
untyped XML value('(/r/@a)[1]') 108
untyped XML value('@a') cross apply nodes('/r') 131
untyped XML value('@a') cross apply nodes('/r[1]') 127
typed XML value('/r/@a') 185
typed XML value('(/r/@a)[1]') 148
typed XML value('@a') cross apply nodes('/r') 176
untyped XML pivot 34
typed XML pivot 52
代碼：
drop type dbo.TRABType
drop type dbo.TType;
drop xml schema collection dbo.RAB;
go
declare @NumAtt int = 100;
declare @Attribs nvarchar(max);
with xmlnamespaces('http://www.w3.org/2001/XMLSchema' as xsd)
select @Attribs = (
select top(@NumAtt) 'a'+cast(row_number() over(order by 1/0) as varchar(11)) as '@name',
'sqltypes:int' as '@type',
'required' as '@use'
from sys.columns
for xml path('xsd:attribute')
)
--CREATE XML SCHEMA COLLECTION RAB AS
declare @Schema nvarchar(max) =
'
<xsd:schema xmlns:schema="urn:schemas-microsoft-com:sql:SqlRowSet1" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:sqltypes="http://schemas.microsoft.com/sqlserver/2004/sqltypes" elementFormDefault="qualified">
<xsd:import namespace="http://schemas.microsoft.com/sqlserver/2004/sqltypes" schemaLocation="http://schemas.microsoft.com/sqlserver/2004/sqltypes/sqltypes.xsd" />
<xsd:element name="r" type="r"/>
<xsd:complexType name="r">[ATTRIBS]</xsd:complexType>
</xsd:schema>';
set @Schema = replace(@Schema, '[ATTRIBS]', @Attribs)
create xml schema collection RAB as @Schema
go
create type dbo.TType as table
(
ID int identity primary key,
X xml not null
);
go
create type dbo.TRABType as table
(
ID int identity primary key,
X xml(document rab) not null
);
go
declare @NumAtt int = 100;
declare @NumRows int = 10;
declare @X nvarchar(max);
declare @C nvarchar(max);
declare @M nvarchar(max);
declare @S1 nvarchar(max);
declare @S2 nvarchar(max);
declare @S3 nvarchar(max);
declare @S4 nvarchar(max);
declare @S5 nvarchar(max);
declare @S6 nvarchar(max);
declare @S7 nvarchar(max);
declare @S8 nvarchar(max);
declare @S9 nvarchar(max);
set @X = N'<r '+
(
select top(@NumAtt) 'a'+cast(row_number() over(order by 1/0) as varchar(11))+'="'+cast(row_number() over(order by 1/0) as varchar(11))+'" '
from sys.columns
for xml path('')
)+
'/>';
set @C =
stuff((
select top(@NumAtt) ',a'+cast(row_number() over(order by 1/0) as varchar(11))
from sys.columns
for xml path('')
), 1, 1, '')
set @M =
stuff((
select top(@NumAtt) ',MAX(CASE WHEN name = ''a'+cast(row_number() over(order by 1/0) as varchar(11))+''' THEN val END)'
from sys.columns
for xml path('')
), 1, 1, '')
declare @T dbo.TType;
insert into @T(X)
select top(@NumRows) @X
from sys.columns;
declare @TRAB dbo.TRABType;
insert into @TRAB(X)
select top(@NumRows) @X
from sys.columns;
-- value('/r[1]/@a')
set @S1 = N'
select T.ID'+
(
select top(@NumAtt) ', T.X.value(''/r[1]/@a'+cast(row_number() over(order by 1/0) as varchar(11))+''', ''int'')'
from sys.columns
for xml path('')
)+
' from @T as T
option (maxdop 1)';
-- value('(/r/@a)[1]')
set @S2 = N'
select T.ID'+
(
select top(@NumAtt) ', T.X.value(''(/r/@a'+cast(row_number() over(order by 1/0) as varchar(11))+')[1]'', ''int'')'
from sys.columns
for xml path('')
)+
' from @T as T
option (maxdop 1)';
-- value('@a') cross apply nodes('/r')
set @S3 = N'
select T.ID'+
(
select top(@NumAtt) ', T2.X.value(''@a'+cast(row_number() over(order by 1/0) as varchar(11))+''', ''int'')'
from sys.columns
for xml path('')
)+
' from @T as T
cross apply T.X.nodes(''/r'') as T2(X)
option (maxdop 1)';
-- value('@a') cross apply nodes('/r[1]')
set @S4 = N'
select T.ID'+
(
select top(@NumAtt) ', T2.X.value(''@a'+cast(row_number() over(order by 1/0) as varchar(11))+''', ''int'')'
from sys.columns
for xml path('')
)+
' from @T as T
cross apply T.X.nodes(''/r[1]'') as T2(X)
option (maxdop 1)';
-- value('/r/@a') typed XML
set @S5 = N'
select T.ID'+
(
select top(@NumAtt) ', T.X.value(''/r/@a'+cast(row_number() over(order by 1/0) as varchar(11))+''', ''int'')'
from sys.columns
for xml path('')
)+
' from @TRAB as T
option (maxdop 1)';
-- value('(/r/@a)[1]')
set @S6 = N'
select T.ID'+
(
select top(@NumAtt) ', T.X.value(''(/r/@a'+cast(row_number() over(order by 1/0) as varchar(11))+')[1]'', ''int'')'
from sys.columns
for xml path('')
)+
' from @TRAB as T
option (maxdop 1)';
-- value('@a') cross apply nodes('/r') typed XML
set @S7 = N'
select T.ID'+
(
select top(@NumAtt) ', T2.X.value(''@a'+cast(row_number() over(order by 1/0) as varchar(11))+''', ''int'')'
from sys.columns
for xml path('')
)+
' from @TRAB as T
cross apply T.X.nodes(''/r'') as T2(X)
option (maxdop 1)';
-- pivot
set @S8 = N'
select ID, '[email protected]+'
from (
select T.ID,
A.X.value(''local-name(.)'', ''nvarchar(50)'') as Name,
A.X.value(''.'', ''int'') as Value
from @T as T
cross apply T.X.nodes(''/r/@*'') as A(X)
) as T
pivot(min(T.Value) for Name in ('[email protected]+')) as P
option (maxdop 1)';
-- typed pivot
set @S9 = N'
select ID, '[email protected]+'
from (
select T.ID,
A.X.value(''local-name(.)'', ''nvarchar(50)'') as Name,
cast(cast(A.X.query(''string(.)'') as varchar(11)) as int) as Value
from @TRAB as T
cross apply T.X.nodes(''/r/@*'') as A(X)
) as T
pivot(min(T.Value) for Name in ('[email protected]+')) as P
option (maxdop 1)';
exec sp_executesql @S1, N'@T dbo.TType readonly', @T;
exec sp_executesql @S2, N'@T dbo.TType readonly', @T;
exec sp_executesql @S3, N'@T dbo.TType readonly', @T;
exec sp_executesql @S4, N'@T dbo.TType readonly', @T;
exec sp_executesql @S5, N'@TRAB dbo.TRABType readonly', @TRAB;
exec sp_executesql @S6, N'@TRAB dbo.TRABType readonly', @TRAB;
exec sp_executesql @S7, N'@TRAB dbo.TRABType readonly', @TRAB;
exec sp_executesql @S8, N'@T dbo.TType readonly', @T;
exec sp_executesql @S9, N'@TRAB dbo.TRABType readonly', @TRAB;
通常的方法我做的是'SELECT r.value（ '@一'， '廉政'），r.value（'@ b'，'int'），r.value（'@ c'，'int'）FROM @D CROSS APPLY x.nodes（'/ r'）as ca（r）'，但是這個計劃看起來形狀差不多（TVF節點給出了更高的估計子樹成本） – 2014-11-01 12:06:13