XQuery计划复杂性
Chr*_*ith 5 sql-server xquery sqlclr sql-server-2008-r2
我有一个SQLCLR标量函数,它将把我需要根据需要粉碎的XmlReader传入一个内联结果集.这些XML对象是按需生成的,因此我无法使用XML索引.在结果数据集中有超过100列是常见的.考虑以下示例代码:
CREATE XML SCHEMA COLLECTION RAB AS '
<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">
<xsd:attribute name="a" type="sqltypes:int" use="required"/>
<xsd:attribute name="b" type="sqltypes:int" use="required"/>
<xsd:attribute name="c" type="sqltypes:int" use="required"/>
</xsd:complexType>
</xsd:schema>';
GO
DECLARE @D TABLE(x XML(DOCUMENT RAB) NOT NULL);
INSERT INTO @D
VALUES
('<r a="3" b="4" c="34"/>'),
('<r a="5" b="6" c="56"/>'),
('<r a="7" b="8" c="78"/>')
SELECT x.value('/r/@a', 'int') a, x.value('/r/@b', 'int') b, x.value('/r/@c', 'int') c
FROM @d a
这将使用某些XML值填充表变量中的类型化XML列,并将属性分解为单独的列.对此的执行计划似乎过于混乱:
|--Compute Scalar(DEFINE:([Expr1009]=[Expr1008], [Expr1016]=[Expr1015], [Expr1023]=[Expr1022]))
|--Nested Loops(Inner Join, OUTER REFERENCES:([a].[x]))
|--Nested Loops(Inner Join, OUTER REFERENCES:([a].[x]))
| |--Nested Loops(Inner Join, OUTER REFERENCES:([a].[x]))
| | |--Table Scan(OBJECT:(@d AS [a]))
| | |--Stream Aggregate(DEFINE:([Expr1008]=MIN([Expr1024])))
| | |--Compute Scalar(DEFINE:([Expr1024]=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 wi
| | |--Table-valued function
| |--Stream Aggregate(DEFINE:([Expr1015]=MIN([Expr1025])))
| |--Compute Scalar(DEFINE:([Expr1025]=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 XP
| |--Table-valued function
|--Stream Aggregate(DEFINE:([Expr1022]=MIN([Expr1026])))
|--Compute Scalar(DEFINE:([Expr1026]=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 f
|--Table-valued function
每列都有一个嵌套循环!如果我将这些表中的多个表加入100列,那么查询计划将变得过于复杂.另外,我不明白这些StreamAggregate运营商的目的.内容如下:
MIN(
CASE WHEN @d.[x] as [a].[x] IS NULL
THEN NULL ELSE
CASE WHEN datalength(CONVERT_IMPLICIT(sql_variant,
CONVERT_IMPLICIT(nvarchar(64),xsd_cast_to_maybe_large(xrpf.[value],xrpf.[lvalue],xrpf.[lvaluebin],xrpf.[tid],(15),(7))
,0),0))>=(128)
THEN CONVERT_IMPLICIT(int,
CASE WHEN datalength(xsd_cast_to_maybe_large(xrpf.[value],xrpf.[lvalue],xrpf.[lvaluebin],xrpf.[tid],(15),(7)))<(128)
THEN NULL
ELSE xsd_cast_to_maybe_large(xrpf.[value],xrpf.[lvalue],xrpf.[lvaluebin],xrpf.[tid],(15),(7))
END,0)
ELSE CONVERT_IMPLICIT(int,
CONVERT_IMPLICIT(sql_variant,CONVERT_IMPLICIT(nvarchar(64),xsd_cast_to_maybe_large(xrpf.[value],xrpf.[lvalue],xrpf.[lvaluebin],xrpf.[tid],(15),(7)),0),0),0)
END
END)
呸!我认为使用带有类型的类型化XML组sqltype应该避免转换？
或者我高估了这种效率或者我做错了什么.我的问题是如何解决这个问题,因此我没有为每一列添加额外的查询计划运算符,理想情况下避免转换,或者我应该放弃并找到非xpath方式来执行此操作？
参考文献:
sqlTypes http://msdn.microsoft.com/en-us/library/ee320775%28v=sql.105%29.aspx
XML数据类型方法http://technet.microsoft.com/en-us/library/ms190798%28v=sql.105%29.aspx
Mik*_*son 11
查询计划中有一些谜团需要先排序.计算标量是做什么的,为什么有流聚合.
表值函数返回碎片XML的节点表,每个碎片行一行.使用类型化XML时,这些列是value,lvalue,lvaluebin和tid.这些列在计算标量中用于计算实际值.那里的代码看起来有点奇怪,我不能说我明白为什么它是这样但是它的要点是函数xsd_cast_to_maybe_large返回值并且当代码值等于时有代码处理大小写大于128个字节.
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
非类型XML的相同计算标量更简单,实际上是可以理解的.
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value从lvalue提取中获取的提取数超过128个字节value.在非类型化XML的情况下,返回的节点表仅输出列id,value和lvalue.
使用类型化XML时,将根据模式中指定的数据类型优化节点值的存储.看起来它可以在节点表中以值,左值或左值结束,具体取决于它的值类型,并且xsd_cast_to_maybe_large可以帮助排序.
流聚合对计算标量的返回值执行min().我们知道并且SQL Server确实(至少有时)知道在value()函数中指定XPath时,只会从表值函数返回一行.解析器确保我们正确构建XPath,但是当查询优化器查看估计的行时,它会看到200行.解析XML的表值函数的基本估计是10000行,然后使用所使用的XPath进行一些调整.在这种情况下,它最终只有200行,只有一行.我的纯粹推测是,流聚合是为了解决这种差异.它永远不会聚合任何东西,只发送返回的一行,但它确实会影响整个分支的基数估计,并确保优化器使用1行作为该分支的估计值.当优化器选择连接策略等时,这当然非常重要.
那100个属性呢？是的,如果您使用值函数100次,将有100个分支.但是这里有一些优化要做.我创建了一个测试装备,使用超过10行的100个属性来查看查询的形状和形式最快.
获胜者是使用无类型XML而不是使用该nodes()函数来粉碎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
还有一种方法可以避免使用pivot的100个分支,但是根据您的实际查询看起来可能无法实现.从枢轴输出的数据类型必须相同.您当然可以将它们作为字符串提取并在列列表中转换为适当的类型.它还要求您的表具有主/唯一键.
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
数据透视查询的查询计划,10行100个属性:
以下是我使用的结果和测试装备.我测试了100个属性和10行以及所有int属性.
结果:
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
码:
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, '+@C+'
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 ('+@C+')) as P
option (maxdop 1)';
-- typed pivot
set @S9 = N'
select ID, '+@C+'
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 ('+@C+')) 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;
- +1详细和信息丰富的配置替代品!我已经考虑了PIVOT选项,在这种情况下也可以直接从CLR生成密钥/名称/值元组.我希望使用XML来提高效率.这是一个很棒的答案! (2认同)
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