vb net 2d barcode generator Temporary Tables and Table Variables in .NET

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7 Temporary Tables and Table Variables
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FiguRe 7-1 Estimated execution plans for queries against a table variable
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The code creates a table variable called @T with two columns . The values in col1 are unique, and each value in col2 appears 10 times . The code creates two unique indexes underneath the covers: one on col1, and one on (col2, col1) . The first important thing to notice in the estimated plans is the number of rows the optimizer estimates to be returned from each operator one in all five cases, even when looking for a non-unique value or ranges . You realize that unless you filter a unique column, the optimizer simply cannot estimate the selectivity of queries for lack of statistics . So it assumes one row . This hard-coded assumption is based on the fact that SQL Server assumes that you use table variables only with small sets of data . As for the efficiency of the plans, the first two queries get a good plan (seek, followed by a partial scan in the second query) . But that s because you have a clustered index on the filtered column, and the optimizer doesn t need statistics to figure out what the optimal plan is in this case . However, with the third and fourth queries you get a table scan (an unordered clustered index scan) even though both queries are very selective and would benefit from using the index on (col2, col1), followed by a small number of lookups . The fifth query would benefit from a table scan because it has low selectivity . Fortunately, it got an adequate plan, but that s by chance . To analyze I/O costs, run the code after turning on the SET STATISTICS IO option . The amount of I/O involved with each of the last three queries is 2,713 reads, which is equivalent to the number of pages consumed by the table .
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Inside Microsoft SQL Server 2008: T-SQL Programming
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Next, go through the same analysis process with the following code, which uses a temporary table instead of a table variable:
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SELECT n AS col1, (n - 1) % 10000 + 1 AS col2, CAST('a' AS CHAR(200)) AS filler INTO #T FROM dbo.Nums WHERE n <= 100000; ALTER TABLE #T ADD PRIMARY KEY(col1); CREATE UNIQUE INDEX idx_col2_col1 ON #T(col2, col1); GO SELECT * FROM #T WHERE col1 = 1; SELECT * FROM #T WHERE col1 <= 50000; SELECT * FROM #T WHERE col2 = 1; SELECT * FROM #T WHERE col2 <= 2; SELECT * FROM #T WHERE col2 <= 5000;
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The estimated execution plans generated for these queries are shown in Figures 7-2 and 7-3 .
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FiguRe 7-2 Estimated execution plans for queries 1, 2, and 3 against a temporary table
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7 Temporary Tables and Table Variables
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FiguRe 7-3 Estimated execution plans for queries 4 and 5 against a temporary table
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As an aside, in case you re curious about the Missing Index messages, SSMS 2008 reports this information in the graphical execution plan . Both SQL Server 2005 and SQL Server 2008 may enter a phase in optimization where they report missing index info . In both versions this information is available in the XML form of the execution plan . The new feature in SSMS 2008 is that it exposes this info graphically with the green-colored messages, whereas SSMS 2005 didn t . Now that statistics are available, the optimizer can make educated estimations . You can see that the estimated number of rows returned from each operator is more reasonable . You can also see that high-selectivity queries 3 and 4 use the index on (col2, col1), and the low-selectivity query 5 does a table scan, as it should . STATISTICS IO reports dramatically reduced I/O costs for queries 3 and 4 . These are 32 and 62 reads, respectively, against the temporary table versus 2,713 for each of these queries against the table variable . When you re done, drop #T for cleanup:
DROP TABLE #T;
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