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Questions tagged [mysql]

All versions of MySQL (not Microsoft SQL Server). Please also add a version-specific tag like mysql-5.7 if that is relevant to the question.

122
votes
5answers
180k views

Possible to make MySQL use more than one core?

I've been presented with some dedicated MySQL servers that never use more than a single core. I'm more developer than DBA for MySQL so need some help Setup The servers are quite hefty with an OLAP/...
236
votes
10answers
232k views

What are the main differences between InnoDB and MyISAM?

What are the main differences between InnoDB and MyISAM?
37
votes
2answers
57k views

Multi cores and MySQL Performance

Importance of RAM is an established fact but far less material is available about the importance of cores and multithreading when it comes to the usage of CPU by MySQL. I am talking about the ...
104
votes
5answers
165k views

How to safely change MySQL innodb variable 'innodb_log_file_size'?

So I'm fairly new to tuning InnoDB. I'm slowly changing tables (where necessary) from MyIsam to InnoDB. I've got about 100MB in innodb, so I increased the innodb_buffer_pool_size variable to 128MB: ...
169
votes
7answers
196k views

How can I optimize a mysqldump of a large database?

I have a symfony application with an InnoDB database that is ~2GB with 57 tables. The majority of the size of the database resides in a single table (~1.2GB). I am currently using mysqldump to ...
57
votes
3answers
64k views

Find highest level of a hierarchical field: with vs without CTEs

note: this question has been updated to reflect that we are currently using MySQL, having done so, I would like to see a how much easier it would be if we switched to a CTE-supporting database. I ...
16
votes
3answers
5k views

Problem with MySQL subquery

Why does this query DELETE FROM test WHERE id = ( SELECT id FROM (SELECT * FROM test) temp ORDER BY RAND() LIMIT 1 ); sometimes delete 1 row, ...
51
votes
3answers
65k views

Why does InnoDB store all databases in one file?

It was convenient that MyISAM used to store each table in a corresponding file. InnoDB has made advancements in many aspects, but I wonder why InnoDB stores all databases in one file (ibdata1 by ...
28
votes
4answers
14k views

MySQL InnoDB crash post-mortem

MySQL crashed on me this morning. With the exception of the standard MySQL included databases everything I use is InnoDB. I attempted to restart the MySQL daemon but it failed twice. I then ...
39
votes
5answers
66k views

How do you tune MySQL for a heavy InnoDB workload?

Assuming a production OLTP system with predominantly InnoDB tables What are the common symptoms of a mistuned/misconfigured system? What configuration parameters do you most commonly change from ...
10
votes
1answer
8k views

Why mysql 5.5 slower than 5.1 (linux,using mysqlslap)

my.cnf (5.5 and 5.1 is the same) : back_log=200 max_connections=512 max_connect_errors=999999 key_buffer=512M max_allowed_packet=8M table_cache=512 sort_buffer=8M read_buffer_size=8M thread_cache=8 ...
163
votes
4answers
262k views

How large should be mysql innodb_buffer_pool_size?

I have a busy database with solely InnoDB tables which is about 5GB in size. The database runs on a Debian server using SSD disks and I've set max connections = 800 which sometimes saturate and grind ...
12
votes
2answers
6k views

Will these two queries result in a deadlock if executed in sequence?

This is almost certainly the cause of my other question, but I thought it was worth separating the two as I have a hypothesis based on the following log that I would love to have falsified or verified....
55
votes
3answers
61k views

What is the performance impact of using CHAR vs VARCHAR on a fixed-size field?

I have an indexed column that stores an MD5 hash. Thus, the column will always store a 32-character value. For whatever reason, this was created as a varchar rather than a char. Is it worth the ...
16
votes
2answers
6k views

Trouble deciphering a deadlock in an innodb status log

We are accessing MySQL from the Microsoft ADO.NET connector. Occasionally we are seeing the following deadlock in our innodb status and haven't been able to identify the cause of the problem. It ...
27
votes
2answers
9k views

How do I properly perform a MySQL bake-off?

I want to performance test (aka bake-off) MySQL server rpm against some other forks such as Percona server, MariaDB, and possibly some others. I'm hoping that by asking this question I can better ...
19
votes
3answers
17k views

Is innodb_file_per_table advisable?

We have an application where just one of the table will be growing into million of lines but the rest will just below a million. So what is the advice should we go with innodb_file_per_table or leave ...
17
votes
3answers
16k views

Best of MyISAM and InnoDB

Is it possible to make InnoDB to use indexes same as MyISAM instead of clustered index due to limitation of RAM while getting benefit of its concurrency performance?
34
votes
1answer
7k views

MySQL : Why are there “test” entries in mysql.db?

Recently, I posted an answer to a question about mysql.db. Then, I got to thinking I should ask everyone this question: I have noticed for years that upon installation of MySQL 5.0+, mysql.db is ...
53
votes
4answers
110k views

Which is faster, InnoDB or MyISAM?

How can MyISAM be "faster" than InnoDB if MyISAM needs to do disk reads for the data? InnoDB uses the buffer pool for indexes and data, and MyISAM just for the index?
31
votes
3answers
27k views

For absolute performance, is SUM faster or COUNT?

This relates to counting the number of records that match a certain condition, e.g. invoice amount > $100. I tend to prefer COUNT(CASE WHEN invoice_amount > 100 THEN 1 END) However, this is ...
16
votes
2answers
8k views

Reasons for occasionally slow queries?

We are running MySQL 5.1 on Windows Server 2008 R2. We have been doing some diagnostics on our database of late and have found some disturbing artifacts which we can't explain. We added some code to ...
33
votes
9answers
215k views

restore table from .frm and .ibd file?

I have previously saved a copy of /var/lib/mysql/ddms directory ("ddms" is the schema name). Now I installed a new MySQL on a freshly installed Ubuntu 10.04.3 LTS by running apt-get install mysql-...
22
votes
3answers
8k views

How is LIKE implemented?

Can anyone explain how the LIKE operator is implemented in current database systems (e.g. MySQL or Postgres)? or point me to some references that explain it? The naive approach would be to inspect ...
14
votes
1answer
17k views

How is INFORMATION_SCHEMA implemented in MySQL?

The INFORMATION_SCHEMA is, in theory, a set of views specified in the SQL standard that allow the user to inspect the system's metadata. How is it this implemented in MySQL? When I connect to a fresh ...
15
votes
4answers
16k views

How costly is opening and closing of a DB connection?

How CPU intensive is opening and closing of a DB connection (for a web app) in MySQL ... when the DB software is on localhost? ... when the DB software is on another machine?
10
votes
2answers
4k views

How to model an entity type that can have different sets of attributes?

I’m having some trouble in recreating a database with a one-to-many (1:M) relationship between Users and Items. This is pretty straightforward, yes; however, each Item belongs to a certain Category (...
8
votes
1answer
10k views

MySQL high availability, failover and replication with Latency

We are in the process of implementing a new CMS (Drupal 6.x) that runs on MySQL. We have two data centers -- primary and secondary -- with known latency between them. We're unsure which version of ...
8
votes
1answer
6k views

Are InnoDB Deadlocks exclusive to INSERT/UPDATE/DELETE?

I am Working around MySQL error "Deadlock found when trying to get lock; try restarting transaction". I am going to have to update a program to allow deadlocks. Is it possible that SELECT statement ...
33
votes
3answers
22k views

Is the CPU performance relevant for a database server?

This is a purely theoretical question. Let's say I have an application deployed on multiple servers. A load balancer, Multiple/scalable applications servers A (single) database server (for the ...
22
votes
4answers
165k views

MySQL error: Access denied for user 'a'@'localhost' (using password: YES)

I use the root account created the account 'a'@'%'. But I can't use the account to connect to MySQL server when I specify the host parameter. I can successfully connect without the -h parameter. ...
20
votes
1answer
12k views

Clarification on MySQL innodb_flush_method variable

Let me begin by admitting that I'm very ignorant of the inner workings of hard disks. So when I read over the manual for the variable innodb_flush_method, it confused me. Can I get an explanation in ...
6
votes
2answers
9k views

Query runs a long time in some newer MySQL versions

I have created a database on MySQL 5.0.15. I have a query and when I run this query on this MySQL version, I get 0.9 s run time. When I import this database to another MySQL server with same hardware ...
14
votes
3answers
5k views

Implementing versioning system with MySQL

I know this has been asked here and here, but I have the same idea with a different possible implementation and I need some help. Initially I had my blogstories table with this structure: | Column ...
21
votes
3answers
9k views

Is the overhead of frequent query cache invalidation ever worth it?

I'm currently working on a MySQL database where we are seeing a large number of invalidations from the query cache, primarily because of the high number of INSERT, DELETE and UPDATE statements that ...
10
votes
2answers
20k views

Using multiple cores for single MySQL queries on Debian

I am running a MySQL server for tests on a VM (VMWare) with Debian as guest OS. The guest has four emulated CPU cores, so I set thread_concurrency to four. I am doing expensive joins on large tables, ...
23
votes
3answers
30k views

check constraint does not work?

I have the following table. create table test ( id smallint unsigned AUTO_INCREMENT, age tinyint not null, primary key(id), check (age<20) ); The problem is that the CHECK constraint ...
6
votes
1answer
3k views

How can Innodb ibdata1 file grows by 5X even with innodb_file_per_table set?

I have innodb_file_per_table set and just today my ibdata1 file jumped from 59M to 323M after I made several changes to an 800M table to reduce it to about 600M. That particular table's .ibd file was ...
4
votes
3answers
2k views

mysql replication master 5.5 slave 5.1 error on create database

I have a replication system where my master has a mysql 5.5 version while the slave is mysql 5.1. I know that this is not supported because the slave version should be greater than the master, but we ...
14
votes
1answer
48k views

Recover MySQL database from data folder without ibdata1 from ibd files

My WAMP directory accidentally get deleted by another user. Only data folder in MySQLis available. And, in that only database-folders (folders in "\bin\mysql\mysql5.6.12\data\" with name of databases) ...
13
votes
3answers
27k views

How do I output MySQL logs to syslog?

I want to use syslog for logging MySQL(5.1.41) on Ubuntu(10.04 LTS). I found information that output error log to syslog. [mysqld_safe] syslog But I want to use syslog for logging general logs and ...
14
votes
3answers
9k views

From where does the MySQL Query Optimizer read index statistics?

I'm trying to determine from where the MySQL optimizer obtains the list of indexes that are available for a table when it estimates the cost of (prepares) a query from.
130
votes
13answers
238k views

How can I move a database from one server to another?

How can I move MySQL tables from one physical server to another? Such as this exact scenario: I have a MySQL server that uses innodb table and is about 20GB size. I want to move it to a new server, ...
27
votes
3answers
34k views

Why query_cache_type is disabled by default start from MySQL 5.6?

We've upgraded to MySQL 5.6 and start seeing the loading of db server increased significantly, and finally found out the query_cache_type is defaulted to off start from 5.6. We enabled it again and ...
13
votes
2answers
62k views

How to Recover an InnoDB table whose files were moved around

So I have a test db server that was setup on a replication stream. Over the name an optimize came through that quickly filled up the space on the slaves datadir. Mysql dutifully was just waiting for ...
7
votes
2answers
5k views

Trigger in MySQL to prevent insertion

I want to create the trigger which prevents insertion in the reservation table of flights (numflights) which does not exist. Tables ---------------------------------------------- flights(numflight, ...
42
votes
6answers
23k views

Performance implications of MySQL VARCHAR sizes

Is there a performance difference in MySQL between varchar sizes? For example, varchar(25) and varchar(64000). If not, is there a reason not to declare all varchars with the max size just to ensure ...
25
votes
3answers
38k views

innodb_file_format Barracuda

I have a couple questions for those more familiar. Most of my instances have been running Antelope despite having support for Barracuda. I was looking to play around with some compresses innodb ...
8
votes
3answers
3k views

MySQL single table static and dynamic pivot

I have a table that looks like this: +----------------------------------------+ |Name | kode | jum | +----------------------------------------+ | aman |kode1 |...
8
votes
4answers
38k views

Cannot GRANT privileges as root

Because of having some problems, I decided to re-create all users except for root@localhost. This works fine, but the newly created user has no right to do anything. What I want is to simply give all ...