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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.

383
votes
4answers
714k views

How do you mysqldump specific table(s)?

How can I dump a specific table or set of tables without including the rest of the db tables?
279
votes
15answers
144k views

Can MySQL reasonably perform queries on billions of rows?

I am planning on storing scans from a mass spectrometer in a MySQL database and would like to know whether storing and analyzing this amount of data is remotely feasible. I know performance varies ...
236
votes
10answers
232k views

What are the main differences between InnoDB and MyISAM?

What are the main differences between InnoDB and MyISAM?
195
votes
11answers
154k views

How can I monitor the progress of an import of a large .sql file?

I am importing a 7 GB foobar.sql to restore a table in a local database. $ mysql -h localhost -u root 'my_data' < foobar.sql $ mysql --version /usr/local/mysql/bin/mysql Ver 14.12 Distrib 5.0....
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 ...
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 ...
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, ...
125
votes
2answers
237k views

Create a MySQL database with charset UTF-8

I'm new to MySQL and I would like to know: How can I create a database with charset utf-8 like I did in navicat? create mydatabase; ...seems to be using some kind of default charset.
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/...
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: ...
103
votes
3answers
58k views

Advantages and Disadvantages to using ENUM vs Integer types?

Lets say in some random table, you have a column named status. It's real-world values would be either enabled or disabled. Is it better for this column's data type to be an int/bool (1 or zero) or to ...
99
votes
3answers
360k views

ALTER TABLE - Rename a column

This is driving me nuts! I want to rename a column from read-more to read_more in my blog table I tried all this: ALTER TABLE blog RENAME COLUMN read-more to read_more; ALTER TABLE blog CHANGE ...
98
votes
5answers
162k views

Good explanation of cascade (ON DELETE/UPDATE) behavior

I don't design schemas everyday, but when I do, I try to setup cascade updates/deletes correctly to make administration easier. I understand how cascades work, but I can never remember which table is ...
96
votes
5answers
24k views

Storing vs calculating aggregate values

Are there any guidelines or rules of thumb to determine when to store aggregate values and when to calculate them on the fly? For example, suppose I have widgets which users can rate (see schema ...
85
votes
3answers
118k views

Is it safe to delete mysql-bin files?

I have MM Replication in mysql, and I want to squeeze some free space in the box be deleting unnecessary files, I came across these mysql-bin files inside /var/db/mysql/ There are hundreds of those ...
81
votes
9answers
220k views

mysql: Show GRANTs for all users

MySQL's SHOW GRANTS shows the permissions of the current user. Is there a way to log in as root and show the permissions of all users?
74
votes
6answers
126k views

Safest way to perform mysqldump on a live system with active reads and writes?

I'm not sure if this is true but I remember reading if you run the following command in linux mysqldump -u username -p database_name > backup_db.sql while reads and writes are being made to a ...
67
votes
5answers
10k views

How can a group track database schema changes?

What version control methodologies help teams of people track database schema changes?
64
votes
7answers
108k views

How to easily convert utf8 tables to utf8mb4 in MySQL 5.5

I have a database which now needs to support 4 byte characters (Chinese). Luckily I already have MySQL 5.5 in production. So I would just like to make all collations which are utf8_bin to utf8mb4_bin....
62
votes
7answers
120k views

How do I completely disable MySQL replication

I am running dual master MySQL replication and now want to move to a single database with no replication. How do I completely disable replication on both databases?
62
votes
4answers
83k views

What is the default order of records for a SELECT statement in MySQL?

Suppose you have the following table and data: create table t ( k int, v int, index k(k) ) engine=memory; insert into t (k, v) values (10, 1), (10, 2), (10, 3); When ...
62
votes
1answer
105k views

What is the best way to reduce the size of ibdata in mysql?

I have some Production servers whose ibdata files increase in size day by day. It has already consumed 290GB of space. The tables in the servers are mostly InnoDB and there are high read and write ...
62
votes
2answers
92k views

What is faster, one big query or many small queries?

I have been working for different companies, and I have noticed that some of them prefer to have views that will join a table with all its "relatives". But then on the application some times, we only ...
59
votes
4answers
81k views

MySQL any way to import a huge (32 GB) sql dump faster?

I have this huge 32 GB SQL dump that I need to import into MySQL. I haven't had to import such a huge SQL dump before. I did the usual: mysql -uroot dbname < dbname.sql It is taking too long. ...
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 ...
57
votes
3answers
78k views

MySQL: Create index If not exists

Is there a way to create an index in MySQL if it does not exist? MySQL does not support the obvious format: CREATE INDEX IF NOT EXISTS index_name ON table(column) ERROR 1064 (42000): You have an ...
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 ...
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?
52
votes
5answers
84k views

Is it safe to use innodb_flush_log_at_trx_commit = 2

I turned innodb_flush_log_at_trx_commit = 2 and get a very fast write speed. But is it safe be used in production web site?
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 ...
51
votes
3answers
73k views

When to use views in MySQL?

When creating tables from multiple joins for use in analysis, when is it preferred to use views versus creating a new table? One reason that I would prefer to use views is that the database schema ...
50
votes
1answer
20k views

How do I swap tables in MySQL?

Suppose, I have a table foo, which contains some statistics that are computed every now and then. It is heavily used by other queries. That's why I want to compute more recent statistics in foo_new ...
47
votes
12answers
27k views

Why aren't primary key / foreign key matches used for joins?

As far as I could find out many DBMSs (e.g. mysql, postgres, mssql) use fk and pk combinations only to constrain changes to data, but they are rarely natively used to automatically select columns to ...
47
votes
7answers
36k views

Why should I create an ID column when I can use others as key fields? [duplicate]

Possible Duplicate: Why use an int as a lookup table's primary key? So far, I'm accustomed to creating an ID column for every table and it is practical in a way that it makes me not think ...
46
votes
3answers
23k views

Is it possible to have passwords configured per database or per host in .my.cnf

I have the following in my ~/.my.cnf [client] password="somepass" but this is not the password I use for every user@host/database I connect to. Is there some way to specify in the config different ...
45
votes
6answers
27k views

What problems will I get creating a database per customer?

I remember from the stackoverflow podcasts that Fog Creek use a database per customer for Fogbugz. I assume that means the Fogbugz On Demand servers have 10s of thousands of databases. We are just ...
44
votes
5answers
61k views

Why is DROP DATABASE taking so long? (MySQL)

New CentOS installation. I was running an import of a large DB (2GB sql file) and had a problem. The SSH client seemed to lose the connection and the import seemed to freeze. I used another window to ...
43
votes
9answers
25k views

Is there a tool like Microsoft's “SQL Server Profiler” for MySQL? [closed]

While developing on MySQL I really miss being able to fire up a profiler. I find SQLyog is a good enough replacement for Query Analyzer but have not found a tool that works like SQL profiler. For ...
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 ...
42
votes
1answer
33k views

Scalability limitations of PostgreSQL and MySQL

I've heard that performance of non-sharded relational database such as MySQL or PostgreSQL "breaks" beyond 10 TB. I suspect that limits as such do exist as one would not come up with Netezza, ...
41
votes
2answers
82k views

InnoDB: Error: Table “mysql”.“innodb_table_stats” not found after upgrade to mysql 5.6

I upgraded to mysql 5.6 from 5.5, and now my logs are littered with such messages on startup I found a possible solution here, but it does not seem official. http://forums.mysql.com/read.php?22,...
41
votes
6answers
72k views

SSD vs HDD for databases

I am trying to purchase a new Server to run MySQL Server on. This new server will be a slave of my main machine. However, this server will be dedicated for reporting only "Lots of reads and complex ...
41
votes
3answers
124k views

How to make MySQL table name case insensitive in Ubuntu?

I am using Ubuntu 13.10 and MySQL 5.6 and I know database name and table name are case sensitive in Ubuntu (and some other *nix environments) by default. Now, I want to make MySQL work as case ...
41
votes
3answers
189k views

How to grant super privilege to the user?

I have created a user and given privileges to the user1. `grant all privileges on db1.* to user1@'%' with grant option; Am using mysql workbench to import dumps to my database. While importing dumps ...
40
votes
3answers
93k views

Difference between On Delete Cascade & On Update Cascade in mysql

I have two tables in MySQL database- parent, child. I'm trying to add foreign key references to my child table based on the parent table. Is there any significant difference between ON UPDATE CASCADE ...
40
votes
2answers
70k views

MySQL: How to check foreign keys related to a table

How to see foreign keys related to a table in MySql? Background : I wanted to drop a table in MySql which has a foreign key constraint. When I do it I get this: Error Code: 1217. Cannot delete or ...
40
votes
1answer
56k views

intermittent MySQL crashes with error “Fatal error: cannot allocate memory for the buffer pool”

Added on edit, 2013-05-29: Because this is a long question and discussion, here's a short summary of the question and the solution. I had problems running MySQL and Apache on a small Linux server (1 ...
40
votes
2answers
18k views

NULL or NOT NULL by default?

In MySQL, is it better to always allow nulls unless you know a field is required, or always use Not Null unless you know a field will contain nulls? Or doesn't it matter? I know in some DBMSs they ...
39
votes
14answers
159k views

Trying to use MySQL Workbench with TCP/IP over SSH - failed to connect

I can't connect using TCP/IP over SSH connection in MySQL Workbench from a PC. What's going on? I created a MySQL 5.1 database on an Ubuntu server mysql.myhost.com. I can access it locally. MySQL ...
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 ...