Need an explanation about a slow query with MariaDB

September 30, 2022

I have a "simple" query how takes 0.7678 seconds or more to be executed with MariaDB.

Here is the query:

select `referenceNumber` from `invoice` where `groupId` = 3550 and `referenceNumber` >= 301 order by `referenceNumber` desc limit 1;

These columns have an index: "referenceNumber", "groupId"

Here is the result of an EXPLAIN:

I found a solution by creating a subquery like that:

select `referenceNumber` from (select id from `invoice` where `groupId` = 3550 and `referenceNumber` >= 301) as subquery JOIN invoice as invoice on invoice.id = subquery.id order by `referenceNumber` desc limit 1;

This query takes like 0.0011 seconds.

Here is the result of an EXPLAIN:

Do you have an explanation about the poor performance of the first query?

Two surprising findings:

The query without the where `groupId` = 3550 takes only 0.0005 seconds like that:

select `referenceNumber` from `invoice` where `referenceNumber` >= 301 order by `referenceNumber` desc limit 1;

The query without the order by `referenceNumber` desc takes only 0.0011 seconds like that:

select `referenceNumber` from `invoice` where `groupId` = 3550 and `referenceNumber` >= 301 limit 1;

Here is the schema of this table:

CREATE TABLE `invoice` (
  `id` int(10) UNSIGNED NOT NULL,
  `groupId` int(11) NOT NULL,
  `referenceNumber` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;


ALTER TABLE `invoice`
  ADD PRIMARY KEY (`id`),
  ADD KEY `invoice_groupid_index` (`groupId`),
  ADD KEY `invoice_referencenumber_index` (`referenceNumber`);

ALTER TABLE `invoice`
  MODIFY `id` int(10) UNSIGNED NOT NULL AUTO_INCREMENT;
COMMIT;

Thank you really much for your help!

>Solution :

When it comes to indexes, having an index on column A and column B will not help if there’s a query involving both of them.

Adding an index on A creates a look-up table to records with various A values, and can provide support for ORDER, BETWEEN and other operations involving ordered values. Importantly it does not account for the order of anything B related.

Likewise, an index on B does much the same thing, ignoring the order of A.

In general, if you want to query WHERE A=? ORDER BY B then you need an index on A,B. This creates an index with data sorted on A, then sub-sorted (for equal values of A) on B. This makes comparisons very quick, they can often happen entirely within the index.