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)?
Related
In trying to index a VARCHAR(256) fields I get the following error:
MySQL said: Index column size too large. The maximum column size is 767 bytes.
It works using VARCHAR(255) but I'm curious why varchar translates to what seems like 3 reserved bytes for each 'char' in a field, if it's using variable width encoding, does it just assume 'worst case' for ever letter, so three bytes if utf-8?
Additionally, if I'm only using English, would the best way to get larger than 255 size to use ascii encoding on the column instead?
Each CHARACTER SET has a maximum length, in bytes. For latin1 and ascii, it is 1. That each character takes one byte.
For utf8 it may take 3 bytes, hence the factor of 3. 3*255+2 = 767. The "2" is for the length.
utf8mb4 covers all of the currently defined Unicode characters, and takes up to 4 bytes per character.
I say "up to" because English takes only 1 byte per char; European languages take 1 or 2 bytes. Chinese and Emojis take 3 or 4.
The implementation of indexing needs to reserve space for the largest possible number of bytes for the column. And there is (was) a limit of 767. Newer versions raised the limit to 3072.
Meanwhile, do not arbitrarily use VARCHAR(256) or even VARCHAR(255), pick some reasonable limit.
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 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.
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.
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);