in Projects, Programming, Technology

MySQL Explain Command for Query Optimization

EXPLAIN SELECT employee_number,firstname,surname FROM employee WHERE employee_number= '10875';

+----------+------+---------------+------+---------+------+------+------------+
| table    | type | possible_keys | key  | key_len | ref  | rows | Extra      |
+----------+------+---------------+------+---------+------+------+------------+
| employee | ALL  | NULL          | NULL |    NULL | NULL |    2 | where used |
+----------+------+---------------+------+---------+------+------+------------+

So what are all these things?

  • table shows us which table the output is about (for when you join many tables in the query)
  • type is an important one – it tells us which type of join is being used. From best to worst the types are: system, const, eq_ref, ref, range, index, all
  • possible_keys Shows which possible indexes apply to this table
  • key And which one is actually used
  • key_len give us the length of the key used. The shorter the better.
  • ref Tells us which column, or a constant, is used
  • rows Number of rows mysql believes it must examine to get the data
  • extra Extra info – the bad ones to see here are "using temporary" and "using filesort"

Write a Comment

Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.