I'm developing a database driven website for a Chinese audience in PHP. The content is stored in the database as a longtext field.
I was wondering how can I be sure the data to be stored isn't truncated?
It depends on the characters' sizes and some configuration options.
LONGTEXT [CHARACTER SET charset_name]
[COLLATE collation_name]
A TEXT column with a maximum length of
4,294,967,295 or 4GB (2^32 – 1)
characters. The effective maximum
length is less if the value contains
multi-byte characters. The effective
maximum length of LONGTEXT columns
also depends on the configured maximum
packet size in the client/server
protocol and available memory. Each
LONGTEXT value is stored using a
four-byte length prefix that indicates
the number of bytes in the value.
http://dev.mysql.com/doc/refman/5.0/en/string-type-overview.html
Related
I have a table post:
field | type
---------------------------------
id | int A_I P_K
title | varchar(300)
content | text
When I insert a row with content more than 15k words, my database just save 10k words and I lost 5k words.
How can I fixed this?
I'm using MySQL and PHP framework Laravel 5.1
If you read the MySQL docs on TEXT type—or any other data type—you can find the limitations of each type.
For example, TEXT specifically has a limit of around 65K, however this limit can be decreased depending on the character encoding (e.g. UTF-8 or other multibyte encodings), because it's calculated in bytes and not by character.
A TEXT column with a maximum length of 65,535 (216 − 1) characters. The effective maximum length is less if the value contains multibyte characters. Each TEXT value is stored using a 2-byte length prefix that indicates the number of bytes in the value.
An optional length M can be given for this type. If this is done, MySQL creates the column as the smallest TEXT type large enough to hold values M characters long.
So if your requirements exceed these limits you should pick a type that is equipped to handle larger payloads, like MEDIUMBLOB and LONGBLOB, which can handle up to 16M and 4G repsectively.
Type of field stored in database, gives you something like space in "tiny" workshop, laboratory, "medium" factory, "long" like a a large factory.
but choose wisely, because that "space" sometimes accupies much more resources than it is needed. Use Type of field for it's purpose.
Type | Maximum length
-----------+-------------------------------------
TINYTEXT | 255 (2^8 −1) bytes
TEXT | 65,535 (2^16−1) bytes = 64 KiB
MEDIUMTEXT | 16,777,215 (2^24−1) bytes = 16 MiB
LONGTEXT | 4,294,967,295 (2^32−1) bytes = 4 GiB
Instead of type -> TEXT ( 65535 characters )
use type -> MEDIUMTEXT ( witch can contain 16 Million Characters )
What data type should use in a MySQL database to store 2 text files of code. If I intend to compare similarity later.
It's a MySQL database running on my Windows machine.
Also can you recommend an API that can compare code for me.
As per MySQL documentation
Values in VARCHAR columns are variable-length strings. The length can be specified as a value from 0 to 65,535. 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.
...
Values in CHAR and VARCHAR columns are sorted and compared according to the character set collation assigned to the column.
So, VARCHAR is stored inline with the table, whilst BLOB and TEXT types are stored off the table with the database holding the location of the data. Depending on how long your text is, TEXT might be defined as TINYTEXT, TEXT, MEDIUMTEXT, and LONGTEXT, the only difference is the maximum amount of data it holds.
TINYTEXT 256 bytes
TEXT 65,535 bytes
MEDIUMTEXT 16,777,215 bytes
LONGTEXT 4,294,967,295 bytes
To compare the two strings stored in TEXT (or any other string column) you might want to use STRCMP(expr1,expr2)
STRCMP() returns 0 if the strings are the same, -1 if the first argument is smaller than the second according to the current sort order, and 1 otherwise.
If you specify the desired output of the comparison, I might edit the answer.
EDIT
To compare two strings and calculate the difference percentage, you might want to use similar_text. As the official documentation states:
This calculates the similarity between two strings as described in Programming Classics: Implementing the World's Best Algorithms by Oliver (ISBN 0-131-00413-1). Note that this implementation does not use a stack as in Oliver's pseudo code, but recursive calls which may or may not speed up the whole process. Note also that the complexity of this algorithm is O(N**3) where N is the length of the longest string.
When I first learnt about field types in MySQL I would define anything that was not a number as a VARCHAR and set the length to anything. 500 2000, but am I right in saying the maximum is actually 255?
If this is the case, why does MySQL let me define columns with much larger lengths and what is it actually doing? Will it allow larger lengths? Does it define the column / field as something else?
Any advice welcomed.
This behavior was changed after 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.
From The CHAR and VARCHAR Types documentation reference.
The maximum is 65k
The CHAR and VARCHAR Types
It was 255 before 5.0.3, but now:
Values in VARCHAR columns are variable-length strings. The length can be
specified as a value from 0 to 65,535.
The documentation of 5.0.x shows the transition:
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.
ref: What is the maximum range of varchar in MySQL?
There is a difference between varchar(n) with values of n up to 255 and values larger than 255.
MySQL stores variable length strings by encoding the length of the string in the first one or two bytes. For values less than 256, MySQL uses one byte of overhead for the length. For values of n as 256 or greater, MySQL uses two bytes of overhead for the length.
Note that this is based on the definition of the column, not on the contents. So an empty string ('') could occupy 1 byte as a varchar(255) and 2 bytes as a varchar(256) (not counting space for storing NULL flags).
To refresh your member, one byte can store integers from 0 to 255, and two bytes can store from 0 to 65,535. This is where these limits come from.
MySQL explanation, click on the link for further information.
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).
What's the difference between VARCHAR(255) and TINYTEXT string types in MySQL?
Each of them allows to store strings with a maximum length of 255 characters. Storage requirements are also the same. When should I prefer one over another?
You cannot assign a DEFAULT value to a TINYTEXT and you cannot create an unprefixed index on the latter.
Internally, additional objects are allocated in memory to handle TEXT (incl. TINYTEXT) columns which can cause memory fragmentation on the large recordsets.
Note that this only concerns the column's internal representation in the recordsets, not how they are stored on the disk.
Using VARCHAR you can set the column to NULL or NOT NULL and you can set DEFAULT value, but not with TEXT. Use VARCHAR if you need one or both feature, NULL and DEFAULT.
in varchar you have to set the length of a character whereas in tanytext there is nothing like this it saves the memory of data base for ex:
for address you have to define the varchar(50) than your address may be 50 charecter or less the worse condition is your character more than the 50 character this is the limitation of varchar if character is less than 50 than it occupy the 50 character memory in this case memory is increases
so use tanytext it define the character length depend upon the size of character so memory is saved
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Importance of varchar length in MySQL table
When using VARCHAR (assuming this is the correct data type for a short string) does the size matter? If I set it to 20 characters, will that take up less space or be faster than 255 characters?
Yes, is matter when you indexing multiple columns.
Prefixes can be up to 1000 bytes long (767 bytes for InnoDB tables). Note that prefix limits are measured in bytes, whereas the prefix length in CREATE TABLE statements is interpreted as number of characters. Be sure to take this into account when specifying a prefix length for a column that uses a multi-byte character set.
source : http://dev.mysql.com/doc/refman/5.0/en/column-indexes.html
In a latin1 collation, you can only specify up 3 columns of varchar(255).
While can specify up to 50 columns for varchar(20)
In-directly, without proper index, it will slow-down query speed
In terms of storage, it does not make difference,
as varchar stand for variable-length strings
In general, for a VARCHAR field, the amount of data stored in each field determines its footprint on the disk rather than the maximum size (unlike a CHAR field which always has the same footprint).
There is an upper limit on the total data stored within all fields of an index of 900 bytes (900 byte index size limit in character length).
The larger you make the field, the more likely people will try to use for purposes other than what you intended - and the greater the screen real-estate required to show the value - so its good practice to try to pick the right size, rather than assuming that if you make it as large as possible it will save you having to revisit the design.
The actual differences are:
TINYTEXT and other TEXT fields are stored separately from in-memory row inside MySQL heap, whereas VARCHAR() fields add up to 64k limit (so you can have more than 64k in TINYTEXTs, whereas you won't with VARCHAR).
TINYTEXT and other 'blob-like' fields will force SQL layer (MySQL) to use on-disk temporary tables whenever they are used, whereas VARCHAR will be still sorted 'in memory' (though will be converted to CHAR for the full width).
InnoDB internally doesn't really care whether it is tinytext or varchar. It is very easy to verify, create two tables, one with VARCHAR(255), another with TINYINT, and insert a record to both. They both will take single 16k page - whereas if overflow pages are used, TINYTEXT table should show up as taking at least 32k in 'SHOW TABLE STATUS'.
I usually prefer VARCHAR(255) - they don't cause too much of heap fragmentation for single row, and can be treated as single 64k object in memory inside MySQL. On InnoDB size differences are negligible.
In the documentation of MySQL:
http://dev.mysql.com/doc/refman/5.0/en/char.html
You have a table that indicates the bytes of a VARCHAR(4) (vs a CHAR(4)).
A simple VARCHAR(4) without string, only 1 byte. Then, a simple VARCHAR(255) without string is 1byte. A VARCHAR(4) with 'ab' is 3 bytes, and a VARCHAR(255) with 'ab' is 3 bytes. It's the same, but with the lenght limit :)
This will have no effect on performance. In this case the constraint merely helps ensure data integrity.
If you set it to 20, it will save only the first 20 characters. So yes, it will take up less space than 255 characters :).
The required storage space for VARCHAR is as follows:
VARCHAR(L), VARBINARY(L) — L + 1 bytes if column values require 0 – 255 bytes, L + 2 bytes if values may require more than 255 bytes
So VARCHAR does only require the space for the string plus one or two additional bytes for the length of the string.