Sorry, but I'm going to greatly disagree with a number of these 'tips'
8. sp_executeSQL and the KEEPFIXED PLAN options - Both sp_executesql and the KEEPFIXED PLAN option avoid the recompilation of a stored procedure.
Well, maybe. There are still things that will force a recompile. Recompiles aren't always bad, sometimes they are, sometimes running the proc with an outdated and inefficient plan is far, far worse.
11. More WHERE clause hints - Avoid unnecessary conditions in the WHERE Clause. You can easily use the first example instead of the second one. The second example uses one extra OR condition which can be avoided using the first example.
SELECT emp_name FROM table_name WHERE LOWER(emp_name) = 'edu'
SELECT emp_name FROM table_name WHERE emp_name = 'EDU' OR emp_name = 'edu'
Completely the wrong way around. The first, with a function on the column, cannot use an index seek. The best it can run with is a full index scan which won't be cheap on a large table. The second can use an index seek and, in fact, will do two seeks against the table. Unless the table is very small, the second will run way faster than the first.
General rule. Never use a function on a column in the where clause unless there's absolutely no way around. It prevents SQL doing index seeks, forcing full index scans or even table scans.
Also, try to avoid IN. While checking the existence of some values, then use EXISTS instead of IN. Because IN counts the NULL values also, hence slower than EXISTS. Since EXISTS returns Boolean(Yes/No) but IN returns all values hence result set for IN is heavier than EXISTS.
SELECT * FROM employee WHERE emp_no NOT IN (SELECT emp_no from emp_detail)
SELECT * FROM employee WHERE NOT EXISTS (SELECT emp_no FROM emp_detail)
Those two queries are not equivalent, hence they won't be running the same speed.
IN checks for matches, so the first query will return all records in employee where there isn't a match in emp_detail. Exists returns true if there are any rows at all in the subquery, it doesn't care about the values returned in the select. Hence that second query will only return results if there are no rows at all in emp_detail and, if that's the case, it will return all the rows in the employee table.
The equivalent query using exists is this:
SELECT * FROM employee WHERE NOT EXISTS (SELECT 1 FROM emp_detail where employee.emp_no = emp_detail.emp_no)
That will probably run much the same speed as the IN (haven't tested)
EDIT: The queries are only equivalent when the subquery returns no NULLs. If it does, NOT IN and NOT EXISTS return different results.
12. CAST and CONVERT - Try to use CAST instead of CONVERT. CAST is ANSI-92 standard but CONVERT works in MS SQL server only. Also, Convert may be deprecated in future MS SQL releases.
Please provide a reference that states that CONVERT may be deprecated in future versions.
14. Avoid using cursors - Try to use temporary table/table variables with identity column and then iterate all the tables using WHILE loop and a looping counter, which will map with the identity column.
A while loop is no better than a cursor. It's still looping and working on rows one by one. If you're going to remove a cursor, replace it with set-based code. Properly written set-based code will be way faster than cursor or while loop for the vast majority of cases.
16.Subquery vs JOINs - But try to avoid correlated sub queries because it makes the query much slower.
Most of the time correlated subqueries run just as fast as the equivalent query with a join and most of the time have the same exec plan. The exceptions are when the correlation function is an inequality and when there's a TOP (1) ... ORDER BY in the subquery.
18. Try to use table variables instead of Temporary Tables - Temp tables can cause stored procedures to recompile. But table variables were designed specifically to guard against stored procedure recompiles during execution.
In SQL 2005 and higher, with temp table caching, use of temp tables does not always cause a recompile. Adding rows to a temp table may still cause a recompile though.
It's true that table variables don't cause recompiles, but the downside is that the optimiser estimates 1 row in the table variable. If there's a lot more, the plan that the optimise comes up with may be very bad indeed and the query run very, very slow.
Recommendation: Try both see how they behave, use the one that causes the least problems for the particular proc.
19.Index scans are much faster than table scans.
And index seeks are faster still.
But when a table returns smaller rows, then it is better to use a table scan.
I assume you mean 'smaller number of rows' rather than 'smaller rows'. Even so, that's not necessarily true. See this blog post
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