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I've got this very simple MySQL query:

SELECT target FROM table WHERE goal_id=1 AND year>=2015 AND year<=2020

The table has about 5 millions rows, though. And as a result, it's very slow (about 10 seconds).

What could I do to improve things? Would an index help, and if so on which column?

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  • Show the create table to see the indexes already have and then also a EXPLAIN of your query. But simply said a combined INDEX on goal_id and year shoild move things along.
    – nbk
    Apr 16 '20 at 17:11
  • ...depending on how many distinct goal_id values there are.
    – mustaccio
    Apr 16 '20 at 17:35
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The optimal index for that query is

INDEX(goal_id, year, target)

in that order. And regardless of the cardinality of any of the columns.

  1. Start with the = test(s)
  2. Add on at most one range (the two year tests are effectively one "range")
  3. Add on all the other columns found anywhere in the query; this makes the index "covering" for another boost. Now the entire query is performed in the index's BTree, without also having to bounce over to the data's BTree.

(That is a simplification. For more details and varied cases: http://mysql.rjweb.org/doc.php/index_cookbook_mysql )

But... That query looks like it might return lots and lots of rows. If so, you may be network-bound. If so, then think about whether you can do more processing in SQL without shoveling a million rows to the client.

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I've done the following and it seems to work well so far. Thanks for all the tips!

ALTER TABLE table ADD INDEX index_name (year, goal_id)
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  • 2
    Try to swap fields in the index expression - especially if goal_id have more than 7 distinct values.
    – Akina
    Apr 16 '20 at 18:31

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