Should Higher Cardinality Columns Come First in Multi-Column Indexes with Range Queries?

Indexing with Higher Cardinality Columns First When Involving a Range

Consider the following table:

CREATE TABLE `files` (
  `did` int(10) unsigned NOT NULL DEFAULT '0',
  `filename` varbinary(200) NOT NULL,
  `ext` varbinary(5) DEFAULT NULL,
  `fsize` double DEFAULT NULL,
  `filetime` datetime DEFAULT NULL,
  PRIMARY KEY (`did`,`filename`),
  KEY `fe` (`filetime`,`ext`),          -- This?
  KEY `ef` (`ext`,`filetime`)           -- or This?
) ENGINE=InnoDB DEFAULT CHARSET=utf8 ;

With a million rows and a high cardinality for filetime and a lower cardinality for ext, the question arises as to which index is more advantageous: fe or ef.

Analysis with Force Index and EXPLAIN

Using FORCE INDEX to test both indexes reveals a clear difference in performance:

-- Forcing the range on filetime first
mysql> EXPLAIN SELECT COUNT(*), AVG(fsize)
    FROM files FORCE INDEX(fe)
    WHERE ext = 'gif' AND filetime >= '2015-01-01'
                      AND filetime <pre class="brush:php;toolbar:false">-- Forcing the low-cardinality ext first
mysql> EXPLAIN SELECT COUNT(*), AVG(fsize)
    FROM files FORCE INDEX(ef)
    WHERE ext = 'gif' AND filetime >= '2015-01-01'
                      AND filetime <p>The EXPLAIN output indicates that ef is significantly faster, using less rows to retrieve the results.</p><p><strong>Analysis with the Optimizer Trace</strong></p><p>The Optimizer trace confirms the superiority of ef:</p><pre class="brush:php;toolbar:false">"potential_range_indices": [
    ...
    {
        "index": "fe",
        "usable": true,
        ...
    },
    {
        "index": "ef",
        "usable": true,
        ...
    }
],
"analyzing_range_alternatives": {
    "range_scan_alternatives": [
        {
            "index": "fe",
            "ranges": [
                "2015-01-01 00:00:00 <p>The trace reveals that ef can use both columns of the index, resulting in a more efficient search. Additionally, it highlights that the Optimizer will only examine the first 'range' column, making the cardinality of ext irrelevant.</p><p><strong>Conclusions</strong></p><p>Based on the analysis, it is clear that when dealing with a range query involving multiple indexed columns, the order of the columns should be:</p>

• Put columns involved in equality tests first, regardless of cardinality.
• Other columns involved in the range should be placed after the equality columns.

This approach ensures that the index is used most effectively, resulting in optimal query performance.

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