SQL performance

1. Choose Appropriate Data Type

   Choose appropriate SQL Data Type to store your data since it also helps in to improve the query performance. Example: To store strings use varchar in place of text data type since varchar performs better than text. Use text data type, whenever you required storing of large text data (more than 8000 characters). Up to 8000 characters data you can store in varchar.

2. Avoid nchar and nvarchar

   Practice to avoid nchar and nvarchar data type since both the data types takes just double memory as char and varchar. Use nchar and nvarchar when you required to store Unicode (16-bit characters) data like as Hindi, Chinese characters etc.

3. Avoid NULL in fixed-length field

   Practice to avoid the insertion of NULL values in the fixed-length (char) field. Since, NULL takes the same space as desired input value for that field. In case of requirement of NULL, use variable-length (varchar) field that takes less space for NULL.

4. Avoid * in SELECT statement

   Practice to avoid * in Select statement since SQL Server converts the * to columns name before query execution. One more thing, instead of querying all columns by using * in select statement, give the name of columns which you required.

   
    -- Avoid
   SELECT * FROM tblName
   --Best practice 
   SELECT col1,col2,col3 FROM tblName 
   
   

5. Use EXISTS instead of IN

   Practice to use EXISTS to check existence instead of IN since EXISTS is faster than IN.

   
    -- Avoid 
   SELECT Name,Price FROM tblProduct 
   where ProductID IN (Select distinct ProductID from tblOrder)
   --Best practice 
   SELECT Name,Price FROM tblProduct 
   where ProductID EXISTS (Select distinct ProductID from tblOrder)
   
   

6. Avoid Having Clause

   Practice to avoid Having Clause since it acts as filter over selected rows. Having clause is required if you further wish to filter the result of an aggregations. Don't use HAVING clause for any other purpose.

7. Create Clustered and Non-Clustered Indexes

   Practice to create clustered and non clustered index since indexes helps in to access data fastly. But be careful, more indexes on a tables will slow the INSERT,UPDATE,DELETE operations. Hence try to keep small no of indexes on a table.

8. Keep clustered index small

   Practice to keep clustered index as much as possible since the fields used in clustered index may also used in nonclustered index and data in the database is also stored in the order of clustered index. Hence a large clustered index on a table with a large number of rows increase the size significantly. Please refer the article Effective Clustered Indexes

9. Avoid Cursors

   Practice to avoid cursor since cursor are very slow in performance. Always try to use SQL Server cursor alternative. Please refer the article Cursor Alternative.

10. Use Table variable inplace of Temp table

    Practice to use Table varible in place of Temp table since Temp table resides in the TempDb database. Hence use of Temp tables required interaction with TempDb database that is a little bit time taking task.

11. Use UNION ALL inplace of UNION

    Practice to use UNION ALL in place of UNION since it is faster than UNION as it doesn't sort the result set for distinguished values.

12. Use Schema name before SQL objects name

    Practice to use schema name before SQL object name followed by "." since it helps the SQL Server for finding that object in a specific schema. As a result performance is best.

    
     --Here dbo is schema name
    SELECT col1,col2 from dbo.tblName
    -- Avoid
    SELECT col1,col2 from tblName 
    
    

13. Keep Transaction small

    Practice to keep transaction as small as possible since transaction lock the processing tables data during its life. Some times long transaction may results into deadlocks. Please refer the article SQL Server Transactions Management

14. SET NOCOUNT ON

    Practice to set NOCOUNT ON since SQL Server returns number of rows effected by SELECT,INSERT,UPDATE and DELETE statement. We can stop this by setting NOCOUNT ON like as:

    
     CREATE PROCEDURE dbo.MyTestProc
    AS
    SET NOCOUNT ON
    BEGIN
    .
    .
    END 
    
    

15. Use TRY-Catch

    Practice to use TRY-CATCH for handling errors in T-SQL statements. Sometimes an error in a running transaction may cause deadlock if you have no handle error by using TRY-CATCH. Please refer the article Exception Handling by TRY…CATCH

16. Use Stored Procedure for frequently used data and more complex queries

    Practice to create stored procedure for quaery that is required to access data frequently. We also created stored procedure for resolving more complex task.

17. Avoid prefix "sp_" with user defined stored procedure name

    Practice to avoid prefix "sp_" with user defined stored procedure name since system defined stored procedure name starts with prefix "sp_". Hence SQL server first search the user defined procedure in the master database and after that in the current session database. This is time consuming and may give unexcepted result if system defined stored procedure have the same name as your defined procedure.