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.
Related
This question already has answers here:
VARCHAR vs TEXT in MySQL
(3 answers)
Closed 3 years ago.
When we create a table in MySQL with a VARCHAR column, we have to set the length for it. But for TEXT type we don't have to provide the length.
What are the differences between VARCHAR and TEXT?
TL;DR
TEXT
fixed max size of 65535 characters (you cannot limit the max size)
takes 2 + c bytes of disk space, where c is the length of the stored string.
cannot be (fully) part of an index. One would need to specify a prefix length.
VARCHAR(M)
variable max size of M characters
M needs to be between 1 and 65535
takes 1 + c bytes (for M ≤ 255) or 2 + c (for 256 ≤ M ≤ 65535) bytes of disk space where c is the length of the stored string
can be part of an index
More Details
TEXT has a fixed max size of 2¹⁶-1 = 65535 characters.
VARCHAR has a variable max size M up to M = 2¹⁶-1.
So you cannot choose the size of TEXT but you can for a VARCHAR.
The other difference is, that you cannot put an index (except for a fulltext index) on a TEXT column.
So if you want to have an index on the column, you have to use VARCHAR. But notice that the length of an index is also limited, so if your VARCHAR column is too long you have to use only the first few characters of the VARCHAR column in your index (See the documentation for CREATE INDEX).
But you also want to use VARCHAR, if you know that the maximum length of the possible input string is only M, e.g. a phone number or a name or something like this. Then you can use VARCHAR(30) instead of TINYTEXT or TEXT and if someone tries to save the text of all three "Lord of the Ring" books in your phone number column you only store the first 30 characters :)
Edit: If the text you want to store in the database is longer than 65535 characters, you have to choose MEDIUMTEXT or LONGTEXT, but be careful: MEDIUMTEXT stores strings up to 16 MB, LONGTEXT up to 4 GB. If you use LONGTEXT and get the data via PHP (at least if you use mysqli without store_result), you maybe get a memory allocation error, because PHP tries to allocate 4 GB of memory to be sure the whole string can be buffered. This maybe also happens in other languages than PHP.
However, you should always check the input (Is it too long? Does it contain strange code?) before storing it in the database.
Notice: For both types, the required disk space depends only on the length of the stored string and not on the maximum length.
E.g. if you use the charset latin1 and store the text "Test" in VARCHAR(30), VARCHAR(100) and TINYTEXT, it always requires 5 bytes (1 byte to store the length of the string and 1 byte for each character). If you store the same text in a VARCHAR(2000) or a TEXT column, it would also require the same space, but, in this case, it would be 6 bytes (2 bytes to store the string length and 1 byte for each character).
For more information have a look at the documentation.
Finally, I want to add a notice, that both, TEXT and VARCHAR are variable length data types, and so they most likely minimize the space you need to store the data. But this comes with a trade-off for performance. If you need better performance, you have to use a fixed length type like CHAR. You can read more about this here.
There is an important detail that has been omitted in the answer above.
MySQL imposes a limit of 65,535 bytes for the max size of each row.
The size of a VARCHAR column is counted towards the maximum row size, while TEXT columns are assumed to be storing their data by reference so they only need 9-12 bytes. That means even if the "theoretical" max size of your VARCHAR field is 65,535 characters you won't be able to achieve that if you have more than one column in your table.
Also note that the actual number of bytes required by a VARCHAR field is dependent on the encoding of the column (and the content). MySQL counts the maximum possible bytes used toward the max row size, so if you use a multibyte encoding like utf8mb4 (which you almost certainly should) it will use up even more of your maximum row size.
Correction: Regardless of how MySQL computes the max row size, whether or not the VARCHAR/TEXT field data is ACTUALLY stored in the row or stored by reference depends on your underlying storage engine. For InnoDB the row format affects this behavior. (Thanks Bill-Karwin)
Reasons to use TEXT:
If you want to store a paragraph or more of text
If you don't need to index the column
If you have reached the row size limit for your table
Reasons to use VARCHAR:
If you want to store a few words or a sentence
If you want to index the (entire) column
If you want to use the column with foreign-key constraints
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 saw comment "If you have 50 million values between 10 and 15 characters in a varchar(20) column, and the same 50 million values in a varchar(50) column, they will take up exactly the same space. That's the whole point of varchar, as opposed to char.". Can Anybody tell me the reason? See What is a reasonable length limit on person "Name" fields?
MySQL offers a choice of storage engines. The physical storage of data depends on the storage engine.
MyISAM Storage of VARCHAR
In MyISAM, VARCHARs typically occupy just the actual length of the string plus a byte or two of length. This is made practical by the design limitation of MyISAM to table locking as opposed to a row locking capability. Performance consequences include a more compact cache profile, but also more complicated (slower) computation of record offsets.
(In fact, MyISAM gives you a degree of choice between fixed physical row size and variable physical row size table formats depending on column types occuring in the whole table. Occurrence of VARCHAR changes the default method only, but the presence of a TEXT blob forces VARCHARs in the same table to use the variable length method as well.)
The physical storage method is particularly important with indexes, which is a different story than tables. MyISAM uses space compression for both CHAR and VARCHAR columns, meaning that shorter data take up less space in the index in both cases.
InnoDB Storage of VARCHAR
InnoDB, like most other current relational databases, uses a more sophisticated mechanism. VARCHAR columns whose maximum width is less than 768 bytes will be stored inline, with room reserved matching that maximum width. More accurately here:
For each non-NULL variable-length field, the record header contains
the length of the column in one or two bytes. Two bytes will only be
needed if part of the column is stored externally in overflow pages or
the maximum length exceeds 255 bytes and the actual length exceeds 127
bytes. For an externally stored column, the two-byte length indicates
the length of the internally stored part plus the 20-byte pointer to
the externally stored part. The internal part is 768 bytes, so the
length is 768+20. The 20-byte pointer stores the true length of the
column.
InnoDB currently does not do space compression in its indexes, the opposite of MyISAM as described above.
Back to the question
All of the above is however just an implementational detail that may even change between versions. The true difference between CHAR and VARCHAR is semantic, and so is the one between VARCHAR(20) and VARCHAR(50). By ensuring that there is no way to store a 30 character string in a VARCHAR(20), the database makes the life easier and better defined for various processors and applications that it supposedly integrates into a predictably behaving solution. This is the big deal.
Regarding personal names specifically, this question may give you some practical guidance. People with full names over 70 UTF-8 characters are in trouble anyway.
Yes, that is indeed the whole point of VARCHAR. It only takes up as much space as the text is long.
If you had CHAR(50), it would take up 50 bytes (or characters) no matter how short the data really is (it would be padded, usually by spaces).
Can Anybody tell me the reason?
Because people thought it was wasteful to store a lot of useless padding, they invented VARCHAR.
The manual states:
The CHAR and VARCHAR types are declared with a length that indicates the maximum number of characters you want to store. (...)
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.
Notice that VARCHAR(255) is not the same as VARCHAR(256).
This is theory. As habeebperwad suggests, the actual footprint of one row depends on (engine) page size and (hard disk) block size.
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.
How does MySQL store a varchar field? Can I assume that the following pattern represents sensible storage sizes :
1,2,4,8,16,32,64,128,255 (max)
A clarification via example. Lets say I have a varchar field of 20 characters. Does MySQL when creating this field, basically reserve space for 32 bytes(not sure if they are bytes or not) but only allow 20 to be entered?
I guess I am worried about optimising disk space for a massive table.
To answer the question, on disk MySql uses 1 + the size that is used in the field to store the data (so if the column was declared varchar(45), and the field was "FooBar" it would use 7 bytes on disk, unless of course you where using a multibyte character set, where it would be using 14 bytes). So, however you declare your columns, it wont make a difference on the storage end (you stated you are worried about disk optimization for a massive table). However, it does make a difference in queries, as VARCHAR's are converted to CHAR's when MySql makes a temporary table (SORT, ORDER, etc) and the more records you can fit into a single page, the less memory and faster your table scans will be.
MySQL stores a varchar field as a variable length record, with either a one-byte or a two-byte prefix to indicate the record size.
Having a pattern of storage sizes doesn't really make any difference to how MySQL will function when dealing with variable length record storage. The length specified in a varchar(x) declaration will simply determine the maximum length of the data that can be stored. Basically, a varchar(16) is no different disk-wise than a varchar(128).
This manual page has a more detailed explanation.
Edit: With regards to your updated question, the answer is still the same. A varchar field will only use up as much space on disk as the data you store in it (plus a one or two byte overhead). So it doesn't matter if you have a varchar(16) or a varchar(128), if you store a 10-character string in it, you're only going to use 10 bytes (plus 1 or 2) of disk space.