CODEWITHSUNDEEP
mysqlcharvarchar
Lizard
Lizard

Reputation: 45032

Why should I ever choose any other length than 255 for varchar in MySQL?

I know the differnce between CHAR and VARCHAR,

CHAR - Fixed length

VARCHAR - Variable length (size + 1 byte)

But I wanted to know what was the purpse of the having the option for a varchar length e.g. VARCHAR(50), VARCHAR(100), VARCHAR(255)

This seems pointless to me because the actual space used depends on the value stored in the database.

So my questions are:

1) It is fine to set all my varchar's to 255 2) Why would you want to specify any other lenght?

Upvotes: 45

Views: 42484

Answers (9)

musafar006
musafar006

Reputation: 999

Fixed-length (Static) Tables are Faster. When every single column in a table is “fixed-length”, the table is also considered “static” or “fixed-length”. Examples of column types that are NOT fixed-length are: VARCHAR, TEXT, BLOB.

http://net.tutsplus.com/tutorials/other/top-20-mysql-best-practices/

So if your table does not have any other fields that are varchar, text, or blob; you can use char and make your table a static one. That way they are faster.

Upvotes: 2

Aniket Kulkarni
Aniket Kulkarni

Reputation: 12993

CHAR Vs VARCHAR

CHAR is used for Fixed Length Size Variable.
VARCHAR is used for Variable Length Size Variable.

E.g.

create table emp
(f_name CHAR(20),
 l_name VARCHAR(20)
);

insert into emp values('Suraj','Chandak');

select length(f_name), length(l_name) from emp;

Output will be

length(f_name)          Length(l_name)
   20                       7

The best answer for CHAR vs VARCHAR

Edit

  • You can set maximum upper limit for the column.
  • Performance and storage can have effect.

Thanks.

Upvotes: 4

Grygoriy Gonchar
Grygoriy Gonchar

Reputation: 4028

But I wanted to know what was the purpse of the having the option for a varchar length e.g. VARCHAR(50), VARCHAR(100), VARCHAR(255)

This seems pointless to me because the actual space used depends on the value stored in the database.

Specifying e.g. VARCHAR(5) instead of VARCHAR(500) can give you better performance in some cases, e.g. for operations which use in-memory temporary tables.

Another case is to restrict column length to compliment domain requirements (when your value should not be greater then some maximum. Example: full domain name in DNS may not exceed the length of 253 characters)

Upvotes: 1

Alon Kogan
Alon Kogan

Reputation: 3418

The main difference between these two value types comes into place when doing a comparison between strings.

In a CHAR column which its length is predefined you'll have to "run" all the way throughout the column length, while in VARCHAR column you'll need to "run" all the way throughout the value length and not column length, which is way faster in most cases.

Therefore a value length which is smaller than the field length will be compared faster if stored in a VARCHAR field.

Upvotes: 1

Toby Allen
Toby Allen

Reputation: 11211

1) Yes.

2) Historically it was a performance hit.

Look at databases such as sqlite that store everything as text for evidence that it no longer really matters.

Upvotes: 0

Jauzsika
Jauzsika

Reputation: 3251

Excerpt from the MySQL documentation:

The CHAR and VARCHAR types are similar, but differ in the way they are stored and retrieved. As of MySQL 5.0.3, they also differ in maximum length and in whether trailing spaces are retained.

The CHAR and VARCHAR types are declared with a length that indicates the maximum number of characters you want to store. For example, CHAR(30) can hold up to 30 characters.

The length of a CHAR column is fixed to the length that you declare when you create the table. The length can be any value from 0 to 255. When CHAR values are stored, they are right-padded with spaces to the specified length. When CHAR values are retrieved, trailing spaces are removed.

Values in VARCHAR columns are variable-length strings. The length can be specified as a value from 0 to 255 before MySQL 5.0.3, and 0 to 65,535 in 5.0.3 and later versions. The effective maximum length of a VARCHAR in MySQL 5.0.3 and later is subject to the maximum row size (65,535 bytes, which is shared among all columns) and the character set used.

In contrast to CHAR, VARCHAR values are stored as a one-byte or two-byte length prefix plus data. The length prefix indicates the number of bytes in the value. A column uses one length byte if values require no more than 255 bytes, two length bytes if values may require more than 255 bytes.

Upvotes: 19

Greg Gauthier
Greg Gauthier

Reputation: 1436

I have read elsewhere that varchar comes with a performance hit relative to char, when you run selects against columns defined with them. So, maybe you want to choose char, if you know for sure the field will always be a certain length, and you have performance issues...

Upvotes: 1

user174624
user174624

Reputation:

1) Technically it is fine, because the fields are created with only 1 or 2 bytes in length in the beginning. Afterwards, they'll grow as necessary.

2) Having said that though, good design principles suggest that you set field lengths appropriately so a) If someone goes through the table scheme and tries to work out how much data is stored in particular fields, they can see that certain fields will hold less data than others and b) you can prevent small amounts of extra work done by the database engine because it has to truncate less space from a VARCHAR(10) field than a VARCHAR(255) during an insert.

You can view extra details about it here:

http://dev.mysql.com/doc/refman/5.0/en/char.html

Upvotes: 1

MarkD
MarkD

Reputation: 4954

1) If you dont want to limit the maximum size of a stored varchar, then yes it is fine. That being said...

2) In many cases you want to set an upper limit for the size of a varchar. Lets say you are storing a mailing list, and have a limited amount of space for an address line. By setting an upper limit for your address field, you now allow the database to enforce a maximum address line length for you.

Upvotes: 25

Related Questions