If I know a value being stored in MySQL is always going to be exactly 32 characters, is it better performance to make the column type CHAR instead of VARCHAR? What exactly is the performance difference between using VARCHAR and CHAR?
Thanks.
I would use CHAR.
If you are likely to search on the column, CHAR presents a small performance upgrade over VARCHAR.
Since your data size is going to be fixed, there is no downside to using CHAR, as an VARCHAR equivalent will store anywhere from one to two bytes as a prefix.
Reference: MySQL CHAR vs VARCHAR
The answer to your first question, "is it better performance to make the column type CHAR instead of VARCHAR?"... is yes. Definitely.
If you ALWAYS know the length is going to be 32, then you should definitely use CHAR.
Several good answers were also given here:
Why would I ever pick CHAR over VARCHAR in SQL?
VARCHAR columns use a one or two byte prefix to store the length:
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.
So, a VARCHAR column will be very slightly larger. If you know it's always going to be exactly 32 characters, then CHAR seems reasonable. In pretty much any case where you're not completely sure of the length, I'd go with VARCHAR.
If the string will always be 32 chars (probably you're having an md5 hash) then go for Char(32)
You can always run this script against any table and see what mysql has to say
SELECT * FROM table PROCEDURE ANALYSE();
This will analyze your entire rows and give you some suggestions. Works well only if you have a decent number or rows in your table.
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 have heard that in MsSQL/Access databases that if you declare a varchar of length 100, it declares those 100 chars every row, even if there is only one char in that column.
I have two questions about this.
First: is this true? And if yes, does this also work like this in MySQL?
Why i'm asking this:
I'm working a lot with mysql, and i came across a table database with 128 longtext-columns. The reasoning behind this was "We cannot be certain how much data gets stored in these columns. sometimes it's 1 char, sometimes thousands." I was wondering if this was the right way storage-wise, or that he has to do some changes.
No, VARCHAR is meant for variable length text, while CHAR is fixed length. The number parameter is the character limit for the text but VARCHAR only uses up as much space as the actual characters you enter in that row (+ some bytes to store the length used).
MySQL, Microsoft SQL Server and pretty much all relational databases work the same way with VARCHAR. Every column takes up some minimum amount of space in a row but with VARCHAR it would be the bytes to store the text + bytes to store the length of the text. No text entered would mean just a 1 or 2 bytes used to save '0' as the length.
If you don't know how much text data will be entered, then use LONGTEXT in MySQL or NVARCHAR(MAX) in MS-SQL. This datatype allows you to store an unlimited amount of text efficiently (up to the row size limitations of the database itself). It's just a bigger, unlimited length version of standard VARCAHR.
For SQL Server the answer is no. From the documentation on MSDN:
varchar [ ( n | max ) ]
Variable-length, non-Unicode string data. n
defines the string length and can be a value from 1 through 8,000. max
indicates that the maximum storage size is 2^31-1 bytes (2 GB). The
storage size is the actual length of the data entered + 2 bytes. The
ISO synonyms for varchar are char varying or character varying.
It is possible someone was confusing VARCHAR and CHAR. The CHAR data type requires a fixed amount of storage, based on the maximum allowed size.
EDIT
Rereading your question I'm not entirely sure I've followed your meaning. If you were not referring to the required storage space then please disregard.
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.
CHAR is stored as a fixed length string, VARCHAR is stored as a variable length string. I can use VARCHAR to store a fixed length string, but why people still want to use CHAR to store a fixed length string? Is there any benefit of using CHAR over VARCHAR? If none of benefit, why mySQL database doesn't remove the option of CHAR?
VARCHAR
varchar stores variable-length character string. it can require less storage than fixed-length types because it uses only as much space as it needs.
varchar also uses 1 or 2 extra bytes to record the value's length. for example varchar(10) will use up to 11 bytes of storage space. varchar helps performance because it saves space. however because the rows are variable length, they can grow when you update them, which can cause extra work. if a row grows and no longer fits in its original location, the behavior is storage engine-dependent...
CHAR
char is fixed-length , mysql always allocates enough space for the specified number of characters. When storing a CHAR value, MySQL removes any trailing spaces. Values are padded with spaces as needed for comparisons.
char is useful if you want to store very short strings, or if all the values are nearly
the same length. For example, CHAR is a good choice for MD5 values for user passwords,
which are always the same length.
char is also better than VARCHAR for data that’s changed frequently, because a fixed-length row is not prone to fragmentation.
Char's are great because they are fixed size and thus make for a faster table. They are however limited to 255 characters. I want to hold 500 characters but a blob is variable length and that's not what I want.
Is there some way to have a fixed length field of 500 characters in MySQL or am I going to have to use 2 char fields?
I would suggest using a varchar(500). Even though varchar isn't a fixed length, the database should reserve the correct amount of space. You shouldn't notice any performance difference using varchar(500) over 2xchar(255).
You're also probably going to cause extra overhead by joining two char fields together.
I would suggest using a varchar(500)
... if you have MySQL 5.0.3 or higher. In previous versions, VARCHAR was restricted to 255 characters.
Also, CHAR and VARCHAR do not work the same regarding trailing spaces. Be sure to read 10.4.1. The CHAR and VARCHAR Types (this is for MySQL 5.0).
You're worrying too much about internal implementation details. Don't pre-optimize.
Go with VARCHAR(500)