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MySQL: Identify The Worst Performing Indexes

16 Apr 2010

This script shows the Top 10 worst performing indexes (in terms of selectivity %) on the whole MySQL server instance. Selectivity is the percentage of distinct values in an indexed field compared to the number of records in the table. When constructing indexes, you want to create indexes on columns that have a good chance of "high selectivity". This requires some understanding of the data in the column, which you may or may not have depending on your knowledge of the domain and the availability of sample data. Keep in mind that selectivity is {" Index Selectivity = Number of Distinct Values / Total Number of Rows "}

Lets consider a table "People" with three columns;  name, surname, gender, and age.

For example, creating an index on a column such as gender (where gender is constrained to (NULL), M or F) would not provide much benefit during a query (especially if the query already results in a table scan for other reasons). In any scenario, the selectivity of this index would be extremely low. Depending on the database, using this index may actually be worse that a full table scan.

However, creating a composite index on (name , surname) would provide benefits when executing queries against those columns. The selectivity of this index (for most populations) would be pretty good.

An index with selectivity of 1 is the ideal, however, the only way to achieve a selectivity of 1 is to have a unique index on a non-nullable column.

In general we say, that indexes help us to search the rows faster, however

  1. If the index column is not used for searches there is no point in defining it.
  2. If the values in that column keep changing very frequently it will be extra work for database server (for re-indexing)
  3. If there are too many inserts and deletes from table it will be extra work for server

Query to Identify The Worst Performing Indexes

Note that this query can take some time to complete on servers with lots of databases or lots of tables.

Example output on a MySQL server with a few wordpress databases.

x2q@x2q:~$ mysql --table -u x2q -p < mysql-worst-indexes.sql
+-----+------------------+---------------------------+------------------+--------------+--------+------+----------+-------+
| db  | table            | index name                | field name       | seq in index | # cols | card | est rows | sel % |
+-----+------------------+---------------------------+------------------+--------------+--------+------+----------+-------+
| aa  | wp_commentmeta   | meta_key                  | meta_key         |            1 |      1 |    0 |     1360 |  0.00 |
| aa1 | wp_commentmeta   | meta_key                  | meta_key         |            1 |      1 |    6 |    14023 |  0.04 |
| aa  | wp_comments      | comment_approved_date_gmt | comment_approved |            1 |      2 |    1 |     2213 |  0.05 |
| aa  | wp_comments      | comment_parent            | comment_parent   |            1 |      1 |    1 |     2213 |  0.05 |
| aa  | wp_postmeta      | meta_value                | meta_value       |            1 |      1 |    1 |     1693 |  0.06 |
| aa  | wp_term_taxonomy | taxonomy                  | taxonomy         |            1 |      1 |    2 |     3548 |  0.06 |
| aa  | wp_posts         | post_related              | post_name        |            1 |      2 |    1 |      808 |  0.12 |
| aa  | wp_posts         | yarpp_title               | post_title       |            1 |      1 |    1 |      808 |  0.12 |
| aa  | wp_posts         | post_author               | post_author      |            1 |      1 |    1 |      808 |  0.12 |
| aa  | wp_posts         | yarpp_content             | post_content     |            1 |      1 |    1 |      808 |  0.12 |
+-----+------------------+---------------------------+------------------+--------------+--------+------+----------+-------+

MySQL and Performance

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