1. Not learning how to use EXPLAIN SELECT
I'm really surprised how many developers who use MySQL all the time and who do not know or understand how to use EXPLAIN SELECT. I've seen several times developers proposing serious architectural changes to their code to minimize, partition or cache data in their database when the actual solution was to spend 30 minutes thinking over result of EXPLAIN SELECT and adding or changing couple indexes.
2. Wasting space with redundant indexes
If you have multicolumn index it means you don't need a separate index which is subset of the first index. It is easier to explain with an example:
CREATE TABLE table1 (Index on col1 is redundant as any search on col1 can use primary index. This just wastes disk space and might make some queries which change this table a bit slower.
PRIMARY (col1, col2),
There is one but! See below..
3. Incorrect order of columns in index
Order of columns in multicolumn index is important. From MySQL documentation:
MySQL cannot use an index if the columns do not form a leftmost prefix of the index.Example:
CREATE TABLE table2 (MySQL wont use any indexes for query like
id INT PRIMARY,
KEY (col1, col2)
SELECT * FROM table2 WHERE col2=123EXPLAIN SELECT shows this instantly. If you want to run this query faster either change order of columns in the index or add another one.
4. Not using multicolumn indexes when you need to
MySQL can use only one index per table in a time so if you query by several columns in the table you may need to add multicolumn index. Example:
CREATE TABLE table3 (Query like
id INT PRIMARY,
SELECT * FROM table2 WHERE col1=123 AND col2=456would use the index on col1 to reduce number of rows to check but MySQL can do much better if you add multicolumn index which covers both col1 and col2. The effect of adding such index is very easy to see with EXPLAIN SELECT.