Question

I am using the following query on a relatively large table (~20million rows):

SELECT 
    MAX(`col_1`) 
FROM `table` 
WHERE  
    col_2 = X AND
    col_3 = Y AND
    col_4 = Z

I have a combined index on the columns col_2, col_3 and col_4 and a separate one on col_1, but the query is still multiple orders of magnitude slower than the same query without the WHERE part.

How can I use indices to improve the performance on this?

Answer 1

As documented under How MySQL Uses Indexes:

MySQL uses indexes for these operations:

[ deletia ]

• To find the MIN() or MAX() value for a specific indexed column key_col. This is optimized by a preprocessor that checks whether you are using WHERE key_part_N = constant on all key parts that occur before key_col in the index. In this case, MySQL does a single key lookup for each MIN() or MAX() expression and replaces it with a constant. If all expressions are replaced with constants, the query returns at once. For example:

  SELECT MIN(key_part2),MAX(key_part2)
  FROM tbl_name WHERE key_part1=10;

Therefore, MySQL cannot use the simple index that you have defined on col_1 for finding MAX(col_1) when you are applying a filter: it must instead scan all matching rows (albeit it can do this in descending order of col_1 by sorting upon that simple index), as would be shown by the EXPLAIN output for your query.

You should use an index on (col_2, col_3, col_4, col_1).