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.

Slow query Mysql 5.7 ordering by varchar column

1 个答案: