In MySQL what is the difference between VARCHAR(1024) and VARCHAR(512)? If my item will never be more than 512 characters, what do I lose by using VARCHAR(1024)?
Don't know where you got that from, but it's not possible to create a table with varchar without specifying the length. It results in a syntax error. So your question is obsolete.
UPDATE:
Nothing. Varchar is as the name implies a datatype of variable length, at least to the maximum length you specified when creating the table. This means, that in a varchar column for each row one additional byte is used to store how long the string in the row actually is.
So the difference between varchar(1024) and varchar(512) is, that your data gets truncated when you try to insert more than 1024 or 512 bytes. Note: bytes, not characters. How much bytes each character uses is dependent on the character set you're using.
There is a actually a difference. And it can have a big performance impact if you manipulate big data. If a temporary table is used, the records on disk will take the full length indicated instead of the variable length. A high value will slow down the request even more in that case. Temporary tables can occur for various reasons (such as memory full, or some combinations of group by /order by).
VARCHAR(1024) 1024 this is lenght.
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.
According to mySQL documentation
In contrast to CHAR, VARCHAR values are stored as a 1-byte or 2-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.
A deeper analysis of the performance impact of larger VARCHARs can be found here.
Related
We know that varchar is a variable length data type then what is the difference between varchar(3) and varchar(300) in mysql ???
In SQL varchar is a string which varies in length. Traditionally, you specify an upper limit for this string. Here are some things to know about varchar:
Strings which are shorter than the specified limit do not take extra space: they only take up the required space.
If the string is longer than the limit, the whole record is rejected, both when you try to INSERT a record, and when you try to UPDATE a record.
Traditionally the upper upper limit was 255 characters. Modern databases no longer have this limit.
Some databases, such as PostgreSQL recommend that you no longer specify the length this way. Instead they recommend an unlimited string (varchar without the length) and limit it using a CHECK CONSTRAINT instead.
Most modern databases handle strings much more efficiently than in the past, so there is less need to be restrictive about the size of the string.
The short answer to your question is that both varchar(3) and varchar(300) are limited-length strings, and both will allow shorter strings without penalty. However clearly one is much shorter than the other.
In addition to #Manngo's answer, varchar(300) has an extra byte of overhead than varchar(3) because its max is over 255. From the MySQL docs...
In contrast to CHAR, VARCHAR values are stored as a 1-byte or 2-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.
Basically, the length of the string must be stored. 1 byte can only hold 0 to 255, so 2 bytes are required to store a length that can go up to 300.
Suppose I want to insert a single character in my column in that scenario what is difference between both ?? Why we use varchar(1) why not varchar(100)??
If the column is only supposed to ever store a single character, use char(1), a single byte, to ensure the integrity of the data (varchar(1) is two bytes). That ensures anyone using the column will only ever get what they expect and don't have to do their own data validation.
Since the size of a varchar is only a max, specifying a smaller one won't make the table use any less disk (except as noted above about >255). Avoid adding arbitrary limits and business rules to your columns. For example, if you're going to store a URL or email address, there's little reason not to allow varchar(255). Limits based on business rules and UX concerns should be handed at a higher layer and not limited by the database schema.
I need to create column in mysql 5.1 that can store user's feedback.
It shouldn't be too long, so I think not more 1000 characters of UTF-8.
The question is how to represent this efficiently in mysql 5.1.
For now I have:
`description` varchar NOT NULL,
But how to constrain varchar to hold at most 1000 characters of UTF-8?
From the documentation:
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.
This means that you can store up to 65,535 bytes in a VARCHAR column. However, from the String Type Overview:
MySQL interprets length specifications in character column definitions in character units. (Before MySQL 4.1, column lengths were interpreted in bytes.) This applies to CHAR, VARCHAR, and the TEXT types.
So, declare your table with a UTF8 collation and set the length of the varchar to 1,000 characters and MySQL will do the work for you behind the scenes.
Since the size is apparently defined in bytes, ...
-correction- Field size is defined in 'character units'. It's a bit unclear what they mean by that, but I guess they mean 'code units'.
Removed the rest of the detailed explanation, since it wasn't (entirely true).
Correction. In MySQL you actually define the number of characters in the field. It is still limited to the 65535 byte boundary though. Above that, MySQL just reserves 3 bytes per character for UTF-8, which means that you cannot have UTF-8 fields of more than 21844 characters, and declaring a field als VARCHAR(21900) will just fail for that reason: " Column length too big for column 'field1' (max = 21845); use BLOB or TEXT instead: ". The number in this message is wrong, by the way. The actual maximum size is 21844. 21845 is 1/3 of 65535, but I guess you need to subtract the two bytes for the field size header as well.
The limit of 3 bytes is weird, though. The unicode definition is designed to be able to expand with extra characters. There are already supplementary characters of 4 bytes, that actually cannot be stored in a UTF-8 varchar(1) field, or any varchar field for that matter, since MySQL just doesn't seem able to read those characters: "Incorrect string value: '\xF0\xA0\x9C\x8E' for column 'field1' at row 1". So I guess you would need an actual binary/blob column to be able to store these characters.
I think the documentation about this subject is pretty poor, but I've tried some things and came to this conclusion. You can see the fiddle here: http://sqlfiddle.com/#!2/4d938
To the question:
So for your specific situation, declaring the field as varchar(1000) will do the trick, presuming you don't want people to use the supplementary characters in their feedback.
Some things to consider though:
I think a 'feedback' field of 1000 characters is pretty small. For many folks this will be enough, but if you have to say more, it is annoying if you can't. So I would make the field bigger.
varchar fields are stored in the record and consume a part of the maximum row size of 65536 bytes. This is an important fact. You cannot have two varchar(20000) fields in a row, because together they would be larger than this maximum row size.
A better alternative for large text fields would be therefor be to make them TEXT or MEDIUMTEXT, which can be even larger and are stored in a different way.
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.
http://dev.mysql.com/doc/refman/5.0/en/char.html
I've begun to get very interested in the memory usage of MySQL. So I'm looking at this here:
http://dev.mysql.com/doc/refman/5.0/en/storage-requirements.html
I get very excited about the prospect of saving memory by (for example) needing only a signed smallint where I was using an unsigned int in many places. Then I read about varchars...
"VARCHAR(M) - Length + 1 bytes if column values require 0 – 255 bytes"
What?! Now it appears to me as though storing a single varchar would use up so much memory, that I may as well not even get excited with my int vs. smallint because it's vastly overshadowed by the varchar field. So I come here asking if this is true, because it simply can't be? Are varchars really that terrible? Or should I really not be getting excited at all for my smallint discovery?
edit: Sorry! I should've been more clear. So, let's say I store a varchar with 7 characters, meaning 8 bytes. That means, then, that it uses the same as a number stored in a BIGINT column? That's what I'm concerned about.
What this is saying is that for a given string length, the amount of storage used is equal to the length of the string in bytes, plus one byte to tell MySQL how long the string is.
So for instance, the word "automobile" is 10 bytes (1 for each character), so if it is stored in a varchar column it will take up 11 bytes. 1 for the number 10 , and 1 each for each of the characters in the string.
From the link you posted:
http://dev.mysql.com/doc/refman/5.0/en/storage-requirements.html
The storage requirements depend on these factors:
-The actual length of the column value
-The column's maximum possible length
-The character set used for the column, because some character sets contain multi-byte characters
For example, a VARCHAR(255) column can hold a string with a maximum length of 255 characters. Assuming that the column uses the latin1 character set (one byte per character), the actual storage required is the length of the string (L), plus one byte to record the length of the string. For the string 'abcd', L is 4 and the storage requirement is five bytes. If the same column is instead declared to use the ucs2 double-byte character set, the storage requirement is 10 bytes: The length of 'abcd' is eight bytes and the column requires two bytes to store lengths because the maximum length is greater than 255 (up to 510 bytes).
While I am no MySQL DBA, it appears there is a very simple answer to this question, and no need to go deeper into storage sizes - because it is NOT configureable.
Per MySQL memory storage documentation,
MEMORY tables use a fixed-length row-storage format. Variable-length types such as VARCHAR are stored using a fixed length.
Thus, you won't have any specific gains by using VARCHAR for a table using the MEMORY storage engine, no matter how VARCHAR is stored on other storage engines such as MyISAM or InnoDB.
Regarding innodb, someone recently told me:
"the varchar content beyond 768 bytes
is stored in supplemental 16K pages"
This is very interesting. If each varchar will be latin1, which I believe stores as 1byte per letter, would a single varchar(500) (<768 bytes) require an extra i/o as a varchar(1000) (>768 bytes) would??
(this question is to find out if all varchars or just big varchars are split into a separate page)
Is the 768 limit per varchar or for all varchars in the row added together? (for example, does this get optimized - varchar(300), varchar(300), varchar(300): [where each individual varchar column is below 768 but together they are above 768 characters]?
I am confused about if the 768 limit relates to each individual varchar or all varchars in the row totaled (as in the question). Any clarification?
EDIT: Removed part about CHARS due to finding out about their limit of 255 max.
The documentation of the current MySQL does not mention this constraint. It actually gives the impression that VARCHAR is stored in the row itself, as the sum of the length of all VARCHARs together in a row cannot be more than 65k bytes.
It also mentions that storage is length+data, which is different from what you describe.
I did find a mention of a splitted storage in MySQL compression internals:
Tables created in previous versions of
InnoDB use the “Antelope” file format,
which supports only
ROW_FORMAT=REDUNDANT and
ROW_FORMAT=COMPACT. In these formats,
InnoDB stores the first 768 bytes of
BLOB, VARCHAR and TEXT columns in the
clustered index record along with the
primary key. The 768-byte prefix is
followed by a 20-byte pointer to the
overflow pages that contain the rest
of the column value.
To me this suggests that each column is considered individually when determining whether that column needs an overflow.
This seems to contrast with newer versions of MySQL which seem to store VARCHAR quite differently. A VARCHAR in a newer version has a max length of 65k bytes, which seems to be stored in the record iself (no overflow), which results in the constraint that all columns together may not be longer than 65k byte.
a CHAR is very different. It's maximum length is 255, and it is stored in the row itself. So no CHAR(500) :)
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
unless the PAD_CHAR_TO_FULL_LENGTH SQL
mode is enabled.
If I have a column in table with field of type VARCHAR(15) and if I try to insert data of length 16, MySQL gives an error stating
Data too long for column 'testname' at row 1
Does anyone know why VARCHAR fields in MySQL take fixed length? Also how many bytes does a VARCHAR field take per record based on the size given?
From the MySQL 5.0 Manual:
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.
I only use VARCHAR when I'm certain that the data the column needs to hold will never exceed a certain length, and even then I'm cautious. If I'm storing a text string I tend to use one of the TEXT types.
Check out the MySQL Storage Requirements for more information on how the bytes are used.
If you set a column to be varchar(15) the maximum bytes allowed is 15. Thus you can't pass it more than 15 characters without modifying the column to support more than 15. If you store a 4 character string it should only use around 4 bytes out of a possible 15, whereas if you used char(15) it would have filled in the other 11 with empty bytes.
http://dev.mysql.com/doc/refman/5.0/en/char.html
( My byte calculation was probably off since it's always -1/+1 or something like that ).
Small extra local note. The number of bytes used will depend on the encoding scheme in use. 1 byte per character in latin1 encoding, but up to 3 in UTF8. See link in mlambie's answer for details.
If you look here it should tell you everything about varchar you want to know:
http://dev.mysql.com/doc/refman/5.0/en/char.html
Basically, depending on the length you chose it will use 1 or two bytes to track the length of the current string in that column, so it will store the number of bytes for the data you put in, plus one or two bytes.
So, if you put in 'abc' then it will be 4 or 5 bytes used for that column in that row.
If you used char(15) then even 'abc' would take up 15 bytes, as the data is the right-padded to use up the full 15 bytes.