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In-Memory OLTP: Row Visibility in SQL Server’s MVCC

SQL Server's In-memory OLTP is fast, due to its multi-valued concurrency control (MVCC). MVCC avoids the need for locks by arranging for each user connected to the database to see a snapshot of the rows of the tables at a point in time, No changes made by the user will be seen by other users of the database until the changes have been completed and committed. It is conceptually simple but does the user always see the correct version of a row under all circumstances? Shel Burkow explains.

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DELETE Operation in SQL Server HEAPs

You should stick to using tables in SQL Server, rather than heaps that have no clustered index, unless you have well-considered reasons to choose heaps. However, there are uses for heaps in special circumstances, and it is useful to know what these uses are, and when you should avoid heaps. Uwe Ricken explains, and demonstrates why you'd be unwise to use heaps rather than tables when the data is liable to change.

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Question of the Day

Adding new column with DEFAULT

Which number will the COUNT() return after executing the following statements: 

DROP TABLE IF EXISTS #test;
CREATE TABLE #test (id INT)
INSERT INTO #test (id)
SELECT *
  FROM GENERATE_SERIES(1, 3) AS gs
;

ALTER TABLE #test ADD flag BIT CONSTRAINT DF_#test_flag DEFAULT 0;
go
UPDATE #test SET flag = 0 WHERE id = 1
UPDATE #test SET flag = 1 WHERE id = 2

INSERT INTO #test (id) VALUES (4)

SELECT COUNT(*)
  FROM #test AS t
 WHERE flag = 0
 

See possible answers