调整查询以解析 SQL Server 2014 上的 XML 数据

Question

我在 SQL Server 2014 数据库上有一个 table，它在 VARCHAR(MAX) 列（可怜的 CDC）中存储记录更改的审核信息。

此数据采用以下格式：

<span class="fieldname">Assigned To</span>
   changed from <span class="oldvalue">user1</span>
   to <span class="newvalue">user2</span><br />
<span class="fieldname">Status</span>
   changed from <span class="oldvalue">QA</span>
   to <span class="newvalue">Development</span><br />
<span class="fieldname">Progress</span>
   changed from <span class="oldvalue">Yes</span>
   to <span class="newvalue">No</span><br />
...

我需要解析该信息以检索转置的数据，使其看起来像这样：

Record    FieldName      OldValue   NewValue
------    ---------      --------   --------
1234      Assigned To    user1      user2
1234      Status         QA         Development
1234      Progress       Yes        No

存储过程尝试通过将数据转换为 XML 然后使用 XPath 检索必要的部分来执行此操作：

;WITH TT AS (
   SELECT TransId,
      CAST('<root><rec>' + REPLACE(REPLACE(TransDescription, 'Ticket reopened... Status', 'Status'), '<br />', '</rec><rec>') + '</rec></root>' AS XML) TransXml
   FROM dbo.Trans
   WHERE TransDate >= '11/1/2016'
      AND (TransDescription LIKE '%Ticket reopened... Status%' OR TransDescription LIKE '%Status%'))
SELECT TransId,
   TransXml,
   FieldName = T.V.value('span[@class="fieldname"][1]', 'varchar(255)'),
   OldValue = NULLIF(T.V.value('span[@class="oldvalue"][1]', 'varchar(255)'), 'nothing'),
   NewValue = NULLIF(T.V.value('span[@class="newvalue"][1]', 'varchar(255)'), 'nothing')
INTO #tmp
FROM TT
   CROSS APPLY TT.TransXml.nodes('root/rec') T(V);

执行计划如下：https://www.brentozar.com/pastetheplan/?id=rJF2GRB7g

对应的IO统计：

Table 'Trans'. Scan count 9, logical reads 27429, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 0, logical reads 2964994, physical reads 0, read-ahead reads 0, lob logical reads 2991628, lob physical reads 0, lob read-ahead reads 0.

此查询速度极慢（该示例仅针对 10 天的数据），并且随着数据的增加而逐渐变慢。

调优此查询的选项是什么？

Answer 1

您真正需要加快速度的是一些 xml 索引。但是，由于您正在动态创建 XML，因此不会发生这种情况。实际上，这相当于 CROSS JOIN，并且会随着时间的推移呈指数级变慢。

有关详细讨论以及索引如何提供帮助，请参阅 cross apply xml query performs exponentially worse as xml document grows。如果你想通过 XML 执行此操作，你确实需要存储 XML 以便你可以索引 XML.

Answer 2

CROSS JOIN 是不能很好扩展的事情之一，随着您的表变大，"nested loops" 上的 "number of executions" 呈指数级增长。在您提交的执行计划中，每个循环的数字都超过 60 万。您的逻辑读取量确实很低，但页面会一遍又一遍地处理。 （如果您的查询大小超过缓冲区大小并假脱机到磁盘上，那么您将遭受真正的伤害。）

这是一个允许您利用 XML 索引的解决方案，它可能对您的情况有所帮助。

--PREPARE SAMPLE DATA
DROP TABLE #Trans
CREATE TABLE #Trans(TransID INT
                    ,TransDate DATE
                    ,TransDescription VARBINARY(MAX)
                    )
INSERT INTO #Trans VALUES
(
1, '20160101'
,CAST('<span class="fieldname">Assigned To</span>
   changed from <span class="oldvalue">user1</span>
   to <span class="newvalue">user2</span><br />
<span class="fieldname">Status</span>
   changed from <span class="oldvalue">QA</span>
   to <span class="newvalue">Development</span><br />
<span class="fieldname">Progress</span>
   changed from <span class="oldvalue">Yes</span>
   to <span class="newvalue">No</span><br />' AS varbinary(MAX)))
,(2, '20160101'
,CAST('<span class="fieldname">Assigned To</span>
   changed from <span class="oldvalue">user1</span>
   to <span class="newvalue">user2</span><br />
<span class="fieldname">Status</span>
   changed from <span class="oldvalue">QA</span>
   to <span class="newvalue">Development</span><br />
<span class="fieldname">Progress</span>
   changed from <span class="oldvalue">Yes</span>
   to <span class="newvalue">No</span><br />' AS varbinary(MAX)))
,(3, '20160101'
,CAST('<span class="fieldname">Assigned To</span>
   changed from <span class="oldvalue">user1</span>
   to <span class="newvalue">user2</span><br />
<span class="fieldname">Status</span>
   changed from <span class="oldvalue">QA</span>
   to <span class="newvalue">Development</span><br />
<span class="fieldname">Progress</span>
   changed from <span class="oldvalue">Yes</span>
   to <span class="newvalue">No</span><br />' AS varbinary(MAX)))

---------------------------------------------------------------------------------------------------
--RUN BELOW THIS LINE COLLECTIVELY, THE ORIGINAL QUERY IS SHOWING UP WITH APPROX 93% OR OVERALL COST

--BUILD A TEMP TABLE TO RECIEVE XML FORMATTED DATA
DROP TABLE #XmlData
CREATE TABLE #XmlData (
    TransId INT NOT NULL,
    TransXml xml NOT NULL,
CONSTRAINT [PK_XmlData] PRIMARY KEY CLUSTERED (TransId)
) 

--INSERT DATA INTO XML TABLE
INSERT INTO #XmlData
SELECT TransId,
    CAST('<root><rec>' + REPLACE(REPLACE(TransDescription, 'Ticket reopened... Status', 'Status'), '<br />', '</rec><rec>') + '</rec></root>' AS XML) TransXml
FROM #Trans
WHERE TransDate >= '11/1/2015'
    AND (TransDescription LIKE '%Ticket reopened... Status%' OR TransDescription LIKE '%Status%')

--CREATE AN XML INDEX
CREATE PRIMARY XML INDEX PXML_TransXml
ON #XmlData(TransXml)

--APPLY NODES QUERY AGAINST XML INDEX
SELECT TransId,
   TransXml,
   FieldName = T.V.value('span[@class="fieldname"][1]', 'varchar(255)'),
   OldValue = NULLIF(T.V.value('span[@class="oldvalue"][1]', 'varchar(255)'), 'nothing'),
   NewValue = NULLIF(T.V.value('span[@class="newvalue"][1]', 'varchar(255)'), 'nothing')
FROM #XmlData TT
   CROSS APPLY TT.TransXml.nodes('root/rec') T(V);

---------------------------------
--Original Query
;WITH TT AS (
   SELECT TransId,
      CAST('<root><rec>' + REPLACE(REPLACE(TransDescription, 'Ticket reopened... Status', 'Status'), '<br />', '</rec><rec>') + '</rec></root>' AS XML) TransXml
   FROM #Trans--dbo.Trans
   WHERE TransDate >= '11/1/2015'
      AND (TransDescription LIKE '%Ticket reopened... Status%' OR TransDescription LIKE '%Status%'))

SELECT TransId,
   TransXml,
   FieldName = T.V.value('span[@class="fieldname"][1]', 'varchar(255)'),
   OldValue = NULLIF(T.V.value('span[@class="oldvalue"][1]', 'varchar(255)'), 'nothing'),
   NewValue = NULLIF(T.V.value('span[@class="newvalue"][1]', 'varchar(255)'), 'nothing')
--INTO #tmp
FROM TT
   CROSS APPLY TT.TransXml.nodes('root/rec') T(V);

调整查询以解析 SQL Server 2014 上的 XML 数据

Tuning a query to parse XML data on SQL Server 2014

sql-server

xpath

query-performance

query-tuning