Query Tuning in C#

Encode QR Code in C# Query Tuning

4
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TABLE 4-3
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Query Tuning
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Aggregated Duration by Query Pre x
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total_duration
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3928210 89089077 2000 1000
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tsql_code
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SELECT orderid, custid, empid, shipperid, orderdate, filler FROM dbo.Orders WHERE orderdate = '200 SELECT orderid, custid, empid, shipperid, orderdate, filler FROM dbo.Orders WHERE orderdate >= '20 SELECT orderid, custid, empid, shipperid, orderdate, filler FROM dbo.Orders WHERE orderid = 5; SELECT orderid, custid, empid, shipperid, orderdate, filler FROM dbo.Orders WHERE orderid = 7;
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In our case, this pre x length did the trick for some queries, but it wasn t very successful with others. With more realistic trace data, you won t have the privilege of looking at a tiny number of queries and being able to play with the numbers so easily. But the general idea is that you adjust the pre x length by applying trial and error. The following code uses a pre x length of 94 and generates the output shown in Table 4-4:
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SELECT SUBSTRING(tsql_code, 1, 94) AS tsql_code, SUM(duration) AS total_duration FROM dbo.Workload GROUP BY SUBSTRING(tsql_code, 1, 94);
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TABLE 4-4
Aggregated Duration by Query Pre x, Adjusted
total_duration
93017287 93017287
tsql_code
SELECT orderid, custid, empid, shipperid, orderdate, filler FROM dbo.Orders WHERE orderdate SELECT orderid, custid, empid, shipperid, orderdate, filler FROM dbo.Orders WHERE orderdate
Now you end up with overgrouping. In short, nding the right pre x length is a tricky process, and its accuracy and reliability are questionable. A much more accurate approach is to parse the query strings and produce a query signature for each. A query signature is a query template that is the same for queries following the same pattern. After creating these, you can then aggregate the data by query signatures instead of by the query strings themselves. SQL Server 2008 provides you with the sp_get_ query_template stored procedure, which parses an input query string and returns the query template and the de nition of the arguments via output parameters. For example, the following code invokes the stored procedure, providing a sample query string as input:
DECLARE @my_templatetext AS NVARCHAR(MAX); DECLARE @my_parameters AS NVARCHAR(MAX);
Inside Microsoft SQL Server 2008: T-SQL Querying
EXEC sp_get_query_template N'SELECT * FROM dbo.T1 WHERE col1 = 3 AND col2 > 78', @my_templatetext OUTPUT, @my_parameters OUTPUT; SELECT @my_templatetext AS querysig, @my_parameters AS params;
This code generates the following output:
querysig params ------------------------------------------------------- -------------select * from dbo . T1 where col1 = @0 and col2 > @1 @0 int,@1 int
The problem with this stored procedure is that you need to use a cursor to invoke it against every query string from the trace data, and this can take quite a while with large traces. The stored procedure also (by design) returns an error in some cases (see SQL Server Books Online for details), which could compromise its value. It would be much more convenient to have this logic implemented as a function, allowing you to invoke it directly against the table containing the trace data. Fortunately, such a function exists; it was written by Stuart Ozer, who is with the Microsoft SQL Server Customer Advisory Team (SQL CAT). I would like to thank him for allowing me to share the code with the readers of this book. Here s the function s de nition:
IF OBJECT_ID('dbo.SQLSig', 'FN') IS NOT NULL DROP FUNCTION dbo.SQLSig; GO CREATE FUNCTION dbo.SQLSig (@p1 NTEXT, @parselength INT = 4000) RETURNS NVARCHAR(4000) --- This function is provided "AS IS" with no warranties, -- and confers no rights. --Use of included script samples are subject to the terms specified at -- http://www.microsoft.com/info/cpyright.htm --- Strips query strings AS BEGIN DECLARE @pos AS INT; DECLARE @mode AS CHAR(10); DECLARE @maxlength AS INT; DECLARE @p2 AS NCHAR(4000); DECLARE @currchar AS CHAR(1), @nextchar AS CHAR(1); DECLARE @p2len AS INT; SET @maxlength = LEN(RTRIM(SUBSTRING(@p1,1,4000))); SET @maxlength = CASE WHEN @maxlength > @parselength THEN @parselength ELSE @maxlength END; SET @pos = 1; SET @p2 = ''; SET @p2len = 0; SET @currchar = ''; set @nextchar = ''; SET @mode = 'command';
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