Slow query Mysql 5.7 ordering by varchar column

Question

I've a linux VM with Mysql 5.7.19. Logging slow queries and log-queries-not-using-indexes I noticed this query:

# Time: 2017-08-05T11:10:36.881359Z
# User@Host: root[root] @ localhost [127.0.0.1]  Id:  2070
# Query_time: 0.000381  Lock_time: 0.000145 Rows_sent: 1  Rows_examined: 35
SET timestamp=1501931436;
 SELECT * FROM `cliente` order by denominazione;

Because I set long_query_time = 5 , seems the query is logged because it doesn't use indexes. Explaning the query I've:

id  select_type     table   partitions  type    possible_keys   key     key_len     ref     rows    filtered    Extra   
1   SIMPLE  cliente     NULL    ALL     NULL    NULL    NULL    NULL    35  100.00  Using filesort

This is my table ddl:

  CREATE TABLE `cliente` (
  `id` bigint(20) NOT NULL,
  `creatoDa` varchar(255) DEFAULT NULL,
  `dataInserimento` datetime DEFAULT NULL,
  `dataUltimaModifica` datetime DEFAULT NULL,
  `modificatoDa` varchar(255) DEFAULT NULL,
  `ENTITY_UID` varchar(36) NOT NULL,
  `version` int(11) DEFAULT NULL,
  `attivo` tinyint(1) NOT NULL DEFAULT '1',      
  `denominazione` varchar(255) NOT NULL,
  `email` varchar(255) DEFAULT NULL,
  `note` longtext,
  `password` varchar(255) NOT NULL,
  `username` varchar(255) NOT NULL,
  `dataUltimoPing` datetime DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;



 ALTER TABLE `cliente`
  ADD PRIMARY KEY (`id`),
  ADD UNIQUE KEY `UK_75j8vo25r1ws35vccamep965h` (`ENTITY_UID`),
  ADD UNIQUE KEY `UK_sgwkt4eif8lb1e5miakwnu7q9` (`username`),
  ADD KEY `idx_denominazione` (`denominazione`),
  ADD KEY `idx_attivo` (`attivo`)

I'm wondering why Mysql is not using index to sorting results considering that I've a index <768 on denominazione (varchar(255)) field.

Answer 1

MySQL optimizer is cost based and has detected a full table scan is the fastest way to retrieve the records with your limited number of rows/records. Instead of doing random disk i/o to retrieve the records that is much slower for these few existing records. Additional records will automatically cause the optimizer to take advantage of your index when appropriate.