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Convert Implicit and the related performance issues with SQL Server

I was a running a routine query using an equal operator on the only column of the primary key for a table and I noticed that the performance was terrible. These queries should have been flying because all I was doing was retrieving one row of data which should have been doing an index seek. When I looked at the query plan it was doing a scan instead. This tip shows you what I found and how to resolve the problem.

2009-04-27

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Question of the Day

Adding new column with DEFAULT II

Which number did the two COUNT(*) return: 

DROP TABLE IF EXISTS #tmp
CREATE TABLE #tmp (id INT NOT NULL)

INSERT INTO #tmp (id) 
SELECT gs.value
  FROM GENERATE_SERIES(1, 5) AS gs

ALTER TABLE #tmp ADD my_value INT NOT NULL CONSTRAINT df_tmp_my_value DEFAULT 1

SELECT COUNT(*)  FROM #tmp AS t WHERE my_value = 1

ALTER TABLE #tmp DROP CONSTRAINT df_tmp_my_value

ALTER TABLE #tmp ADD CONSTRAINT df_tmp_my_value DEFAULT 2 FOR my_value

SELECT COUNT(*)  FROM #tmp AS t WHERE my_value = 1

See possible answers