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Mysql Interview Questions


  • Current mysql version : 8 ( last was 5.7 ,  5.7 to directly 8 )
  • SHOW FULL PROCESSLIST is used to see all the query executing when refresh the site.
  • 1 second == 1000 mili second, 0-500 ms responce time of any query is ok
How to Read the MySQL Slow Query Log : The MySQL slow query log is where the MySQL database server registers all queries that exceed a given threshold of execution time. This can often be a good starting place to see which queries are slowest and how often they are slow.
MySQL on your server is configured to log all queries taking longer than 0.1 seconds.

/var/log/mysql/mysql-slow.log
Use EXPLAIN or EXPLAIN EXTENDED to explain the query how it is executed.
MySQL describe or ANALYZE command shows the structure of the table.

Best practice in respect of performance :
1 - always use index,
2 - index types , primary index and combined field index like fname & lname in one index not two index,
3 - one index scan is faster than two,
4 - data type matters like in a query we use .... where numeric_value = "66", in this case the CPU takes too much time to execute, because first it will check as a string then check as a number,
so use proper data types.
5 - try to avoid the not indication in query  like IS NOT, IS NOT NULL,NOT IN ,NOT LIKE
6 - use inner join instead of left join
7 - UNION ALL is better than UNION
8 - use latest version of db 

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