I am changing my db so that certain fields can be longer. Right now they're "varchar(255)" can I just increase that to "varchar(500)" or are there special rules for dealing with this and I should make them "text"?
From the MySQL manual:
"The effective maximum length of a VARCHAR is subject to the maximum row size (65,535 bytes, which is shared among all columns) and the character set used."
So, yes, you can just extend your columns to be a maximum of 500 characters in length (though, you'll want to be sure your application logic doesn't expect the maximum length to be 255 characters).
Edit: see also cballou's note below about character sets.
Modifying to a longer varchar would be fine:
ALTER TABLE my_table MODIFY column_name VARCHAR(500);
Related
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
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.
I would like to know what the max size is for a MySQL VARCHAR type.
I read that the max size is limited by the row size which is about 65k. I tried setting the field to varchar(20000) but it says that that's too large.
I could set it to varchar(10000). What is the exact max I can set it to?
Keep in mind that MySQL has a maximum row size limit
The internal representation of a MySQL table has a maximum row size limit of 65,535 bytes, not counting BLOB and TEXT types. BLOB and TEXT columns only contribute 9 to 12 bytes toward the row size limit because their contents are stored separately from the rest of the row. Read more about Limits on Table Column Count and Row Size.
Maximum size a single column can occupy, is different before and after MySQL 5.0.3
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.
However, note that the limit is lower if you use a multi-byte character set like utf8 or utf8mb4.
Use TEXT types inorder to overcome row size limit.
The four TEXT types are TINYTEXT, TEXT, MEDIUMTEXT, and LONGTEXT. These correspond to the four BLOB types and have the same maximum lengths and storage requirements.
More details on BLOB and TEXT Types
Ref for MySQLv8.0 https://dev.mysql.com/doc/refman/8.0/en/blob.html
Ref for MySQLv5.7 https://dev.mysql.com/doc/refman/5.7/en/blob.html
Ref for MySQLv5.6 https://dev.mysql.com/doc/refman/5.6/en/blob.html
Even more
Checkout more details on Data Type Storage Requirements which deals with storage requirements for all data types.
As per the online docs, there is a 64K row limit and you can work out the row size by using:
row length = 1
+ (sum of column lengths)
+ (number of NULL columns + delete_flag + 7)/8
+ (number of variable-length columns)
You need to keep in mind that the column lengths aren't a one-to-one mapping of their size. For example, CHAR(10) CHARACTER SET utf8 requires three bytes for each of the ten characters since that particular encoding has to account for the three-bytes-per-character property of utf8 (that's MySQL's utf8 encoding rather than "real" UTF-8, which can have up to four bytes).
But, if your row size is approaching 64K, you may want to examine the schema of your database. It's a rare table that needs to be that wide in a properly set up (3NF) database - it's possible, just not very common.
If you want to use more than that, you can use the BLOB or TEXT types. These do not count against the 64K limit of the row (other than a small administrative footprint) but you need to be aware of other problems that come from their use, such as not being able to sort using the entire text block beyond a certain number of characters (though this can be configured upwards), forcing temporary tables to be on disk rather than in memory, or having to configure client and server comms buffers to handle the sizes efficiently.
The sizes allowed are:
TINYTEXT 255 (+1 byte overhead)
TEXT 64K - 1 (+2 bytes overhead)
MEDIUMTEXT 16M - 1 (+3 bytes overhead)
LONGTEXT 4G - 1 (+4 bytes overhead)
You still have the byte/character mismatch (so that a MEDIUMTEXT utf8 column can store "only" about half a million characters, (16M-1)/3 = 5,592,405) but it still greatly expands your range.
Source
The max length of a varchar is subject to the max row size in MySQL,
which is 64KB (not counting BLOBs):
VARCHAR(65535) However, note that the limit is lower if you use a
multi-byte character set:
VARCHAR(21844) CHARACTER SET utf8
From MySQL documentation:
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. For example, utf8 characters
can require up to three bytes per character, so a VARCHAR column that
uses the utf8 character set can be declared to be a maximum of 21,844
characters.
Limits for the VARCHAR varies depending on charset used. Using ASCII would use 1 byte per character. Meaning you could store 65,535 characters. Using utf8 will use 3 bytes per character resulting in character limit of 21,844. BUT if you are using the modern multibyte charset utf8mb4 which you should use! It supports emojis and other special characters. It will be using 4 bytes per character. This will limit the number of characters per table to 16,383. Note that other fields such as INT will also be counted to these limits.
Conclusion:
utf8 maximum of 21,844 characters
utf8mb4 maximum of 16,383 characters
you can also use MEDIUMBLOB/LONGBLOB or MEDIUMTEXT/LONGTEXT
A BLOB type in MySQL can store up to 65,534 bytes, if you try to store more than this much data MySQL will truncate the data. MEDIUMBLOB can store up to 16,777,213 bytes, and LONGBLOB can store up to 4,294,967,292 bytes.
Before Mysql version 5.0.3 Varchar datatype can store 255 character, but from 5.0.3 it can be store 65,535 characters.
BUT it has a limitation of maximum row size of 65,535 bytes. It means including all columns it must not be more than 65,535 bytes.
In your case it may possible that when you are trying to set more than 10000 it is exceeding more than 65,535 and mysql will gives the error.
For more information: https://dev.mysql.com/doc/refman/5.0/en/column-count-limit.html
blog with example: http://sforsuresh.in/mysql_varchar_max_length/
In my case, I tried 20'000 according to #Firze answer (with UTF8 limit) and phpMyAdmin responded with the maximum size; the answer was to decrease or choose BLOB instead.
So, I think, finally, the best is to test yourself according to the version of MySQL you have and the engine used. As MySQL / phpMyAdmin has safeguards.
You can use TEXT type, which is not limited to 64KB.
I have switched from sql server 2005 to mysql which was not really a problem.
I have a slight issue with (n)varchar which exist in sql server. Usually I have used:
mapping.Map(x => x.bla).Length(10000);
to set bla to nvarchar(max). does this work in mysql? I believe there is no nvarchar in mysql and you have to use something like this:
alter table sometable modify bla VARCHAR(21844) CHARACTER SET utf8
to update an existing column to 'nvarchar(max)'. Is this correct because I am getting:
"Row size too large. The maximum row size for the used table type"
If I am using:
alter table sometable modify bla VARCHAR(1000) CHARACTER SET utf8
things work but I am not sure whether this achieves 'nvarchar(max)' in mysql.
As explained in the manual:
Every table (regardless of storage engine) has a maximum row size of 65,535 bytes. Storage engines may place additional constraints on this limit, reducing the effective maximum row size.
The maximum row size constrains the number (and possibly size) of columns because the total length of all columns cannot exceed this size. For example, utf8 characters require up to three bytes per character, so for a CHAR(255) CHARACTER SET utf8 column, the server must allocate 255 × 3 = 765 bytes per value. Consequently, a table cannot contain more than 65,535 / 765 = 85 such columns.
Storage for variable-length columns includes length bytes, which are assessed against the row size. For example, a VARCHAR(255) CHARACTER SET utf8 column takes two bytes to store the length of the value, so each value can take up to 767 bytes.
You must therefore consider what other columns exist in your table and calculate the maximum size available for this VARCHAR.
However, if you require space for long text values, why not use the TEXT data types, which are not constrained by this limit (except for the 9 to 12 bytes they contribute toward it)?
I just noticed in the documentation that in versions greater than 5.0.3 of MySQL you can declare varchar's with larger values than 255. In the past I've switched datatypes for anything larger than 255 but I'm wondering if it's better practice now to define larger string values using varchar(1000) or whatever length is appropriate.
Is this common with other databases now as well, or is it best to stick with 255 as the max value and change datatypes above that?
As the answer #Eric pointed out suggests, VARCHARs are stored in table while TEXTs are stored in a separate file - the only truly important point that you have to keep in mind when designing a table structure is the row size limitation (MySQL limits each row / record to 65 KB).
I suggest you use VARCHARs for "one-liners" - anything that has a text input as its data source.
In my opinion, I would discourage to approach. When you need more than 255 characters, use TEXT are some more suitable.
Update: VARCHAR is now limited to 65535 bytes, but a row in MySQL cannot contain more than 65535 bytes.
You have to know that VARCHAR and fields like that are stored directly into your database when TEXT for example will be stored outisde the row why a pointer inside the row linking to it.
So if you want to use big VARCHAR, make sure they will not be too big and won't interfere with the rest of the data in the row.
For example, having mutltiple VARCHAR fields that can contains up to 65K char would be a bad idea.
The VARCHAR column is limited to 65,535 bytes, which doesn't always mean 65,535 characters depending on which character set you are using.
If your using the latin1 character set which is one byte per character you won't run into any issues because the length of the string in the same as the amount of storage needed.
If you use a character set that stores multi-byte characters you can only set the length to be what the character set will allow. For instance the utf8 character set can have a maximum length of 21,844 characters.