So I was creating a table for comments, and I was wondering. What would be a good type for comment details? I put longtext. Well then, why would people need varchar if longtext can handle it? Also, which type would I want for usernames?
What is the purpose of "primary" for index? What is the purpose of index?
Update:
Let's say a comment was actually a review.
It is true that TEXT can handle any input you'd place in VARCHAR or CHAR field. In fact TEXT could handle and data you might want to put in DECIMAL, INT, or almost any other type as well. Following this logic we might as well make every column a TEXT type.
But this would be a mistake. Why? Because using the appropriate column type for the expected input allows the database to better optimize queries, uses less disk space and makes the data model easier to understand and maintain.
In regards to the questions: a username column should use VARCHAR(20), since you would want and expect that most usernames are going to short, usually no more than 10 - 20 characters long. For a review column (like a movie review or book review) a TEXT type would be appropriate as reviews can span a single paragraph to several pages.
In regards to indexes, try this link:
http://20bits.com/articles/interview-questions-database-indexes/
That depends on what a "comment" is in your system. Typically VARCHAR is pretty standard for both comments and usernames. This limits you to about 255 characters, which is generally pretty acceptable. If you need more characters in your comments, you can bump it up to a text, which gives you a little over 65k chars.
For more information, see the String Types Reference.
TEXT NOT NULL. That gives sufficient room, has a 2 byte overhead, and generally presents no problems.
Regarding TEXT
On comparisons, TEXT is space extended
to fit the compared object, exactly
like CHAR and VARCHAR.
For indexes on BLOB and TEXT columns,
you must specify an index prefix
length. For CHAR and VARCHAR, a prefix
length is optional. See Section 7.4.2,
“Column Indexes”.
BLOB and TEXT columns cannot have
DEFAULT values.
If you use the BINARY attribute with a
TEXT data type, the column is assigned
the binary collation of the column
character set.
Regarding VARCHAR:
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.
More at: http://dev.mysql.com/doc/refman/5.0/en/blob.html
Have a look at this web page, it lists all the MySQL field types and describes what they are and how they're different from each other.
Related
I am building a table that will receive different values in a field, think of a log table that the "value" field can be a number, a tiny string or a big text etc.
So I was wonderig if I should create that "value" field as Text or create two fields, one for small inputs, like date, numbers and some string and another one only for the Texts inputs.
so, my question is this:
Should this "value" field be a Varchar along with some other "value2" as Text or create one field Text that the mysql will manage this corretcly?
I am afraid that creating only one Text field can be a bad thing for performance.
EDIT: the number, datetime etc are going to be cast as string before insertion, thats not the point
Thanks,
Joe
Do you know how large the largest input will be? If you impose a limit, or know how large the maximum input will be, then you could use varchar (which caps at 255 characters in versions < 5.0.3, and 65,535 in versions >= 5.0.3). Otherwise, you're probably better off with Text, since it holds significantly more (65,535*2^16-1).
As an alternative, if users are creating things that already have tables (such as adding events to a calendar), you could just put an "is_approved" column on the table and only display approved ones, or search through everything to check for duplicates.
http://dev.mysql.com/doc/refman/5.0/en/string-type-overview.html
If there is a limit to the length of that the data stored use varchar() (as MySQL 5.0.3 varchar max length can be up to 65,535)
If there is no concrete limit then use a 'text' field type.
The varchar field can handle the different input's that you are mentioning but as a string, not as an integer or datetime.
I can't help but believe this topic has been written about over and over again but I'm having trouble finding any good, solid information.
What data type should I use to store 200 to 400 words of text? What about longer articles that could approach two or three thousand words?
What options should affect my decision? I don't plan to search this data but I can't completely rule out the possibility that I may want to do that later.
Unfortunately my background is MS Access where the only option for this was a memo field. It doesn't appear to be quite so simple with MySQL.
If you're using MySQL 5.0.3 or later, go VARCHAR. It can hold 65k bytes. As long as you have only 1 long VARCHAR per row, you should be fine.
Otherwise go with text.
From the mysql manual:
BLOB and TEXT differ from VARBINARY
and VARCHAR in the following ways:
There is no trailing-space removal for
BLOB and TEXT columns when values are
stored or retrieved. Before MySQL
5.0.3, this differs from VARBINARY and VARCHAR, for which trailing spaces are
removed when values are stored.
On comparisons, TEXT is space extended
to fit the compared object, exactly
like CHAR and VARCHAR.
For indexes on BLOB and TEXT columns,
you must specify an index prefix
length. For CHAR and VARCHAR, a prefix
length is optional. See Section 7.5.1,
“Column Indexes”.
BLOB and TEXT columns cannot have
DEFAULT values.
Also nice to know (from the manual):
Instances of BLOB or TEXT columns in
the result of a query that is
processed using a temporary table
causes the server to use a table on
disk rather than in memory because the
MEMORY storage engine does not support
those data types
which you really should take into account when formulating queries which use TEXT.
A TEXT field should be big enough to store most articles. Seems to be about equivalent to Access's Memo type. It can hold up to 65535 chars, which would be somewhere around...i dunno...10-12,000 words, on average?
The TEXT data type is a safe bet for your situation, VARCHARs are usually used when they need to be indexed or there is a well-defined value to be stored (IP address, zip code, etc).
We have a very large historical table that contains a column with at most 500 UTF8 characters, and the disk space grows really fast!
We're having at least 2 million rows a day... and we were wondering which would do a better job (mostly in storage but in performance as well)? TEXT or VARCHAR(512)?
VARCHAR is probably preferable in your case from both the storage and performance perspective. View this oft-reposted article.
This is useful information; I think in general, the answer is the varchar is usually the better bet.
From the MySQL manual:
In most respects, you can regard a
BLOB column as a VARBINARY column that
can be as large as you like.
Similarly, you can regard a TEXT
column as a VARCHAR column. BLOB and
TEXT differ from VARBINARY and VARCHAR
in the following ways:
There is no trailing-space removal for BLOB and TEXT columns when values
are stored or retrieved. Before MySQL
5.0.3, this differs from VARBINARY and VARCHAR, for which trailing spaces are
removed when values are stored.
On comparisons, TEXT is space extended to fit the compared object,
exactly like CHAR and VARCHAR.
For indexes on BLOB and TEXT columns, you must specify an index
prefix length. For CHAR and VARCHAR, a
prefix length is optional. See Section
7.5.1, “Column Indexes”.
BLOB and TEXT columns cannot have DEFAULT values.
http://dev.mysql.com/doc/refman/5.0/en/blob.html
I noticed that in MySQL, both VARCHAR and TEXT offer variable-sized data. Well, VARCHAR is a bit more efficient in data storage, but still, TEXT MEDIUMTEXT and LONGTEXT offer a lot more potential. So, what are the real uses of VARCHAR?
First of all, you should read the 10.4. String Types section of the MySQL's manual : it'll give you all the informations you are looking for :
10.4.1. The CHAR and VARCHAR Types
10.4.3. The BLOB and TEXT Types
A couple of important differences :
Difference in the amount of text those can contain :
varchar have a quite small size limit ; with the newest versions of MySQL, it's 64 KB, for the total of all varchar columns of a row -- which is not that much.
TEXT have virtually no limit, as they can contain something like 2^32 bytes.
There are differences in indexing and sorting, if I'm not mistake ; quoting the page about TEXT :
About sorting : "Only the first max_sort_length bytes of the column are used when sorting."
And, about performances : "Instances of BLOB or TEXT columns in the result of a query that is processed using a temporary table causes the server to use a table on disk rather than in memory"
Considering these informations, if you are sure that your strings will not be too long, and that you'll always be able to store them in a varchar, I would use a varchar.
This question is based on two things performance and size
Which DATATYPE is better to use TEXT or VARCHAR? Based on performance which will affect and which will impove?
It depends on what you're using it for. I hate to give such a generic answer, but it's true. Generally, try to get the data type as specific as you can. If your strings will never exceed some upper limit of characters, then go with VARCHAR because it will be a little more efficient. If you need more space, go with TEXT. If you aren't sure how much space your text will take up, you should probably go with TEXT; the performance difference isn't very large, and it's better to be future-proof than risk having to change it later when your requirements change. Just my two cents.
In the comments, Pitarou points out that, if MySQL creates a temporary table for your query (see this), TEXT columns will not be stored in memory and will have to be read from the disk, which is much slower. (Source, bottom of the page.) This shouldn't matter for most queries, though.
In case anyone was wondering how PostgreSQL compares, I found this benchmark that shows that CHAR, VARCHAR, and TEXT all perform equally well. So if you're using Postgres, it doesn't matter what type you use.
From V 5.0.3 onwards, Limit of VARCHAR is increased from 0-256 to 0-65,535 (subject to the maximum row size (65,535 bytes, which is shared among all columns) and the character set used.)
Ref. http://dev.mysql.com/doc/refman/5.0/en/char.html
If you are using TEXT that is fixed 64k length, even if you required lesser limit
So Better to go with VARCHAR with higher limit than TEXT.
If requirement is more than 64K go with MEDIUMTEXT or LONGTEXT accordingly.
Queries against the TEXT table were always 3 times slower than those against the VARCHAR table (averages: 0.10 seconds for the VARCHAR table, 0.29 seconds for the TEXT table). The difference is 100% repeatable.
Benchmark from http://forums.mysql.com/read.php?24,105964,105964
VARCHAR you can set a limit for how many chars it will accept per record, text is (virtually) unlimited... not exactly sure about performance, but i would assume a more specific datatype (varchar) would be faster.
VARCHAR should have a better performance since it has a limited size. In fact, in all of my experiences with MySQL, the search operation was always faster with VARCHAR than TEXT. Anyway, it's based on my experience. You should check the documentation to find out more about it.
It really depends on your data type.
If your field is fixed-length (e.g. a 32-character hash value), then use CHAR. This has better performance because every entry takes up the same space per row.
The standard limit for VARCHAR was 255 characters but I think it's been increased now. TEXT is pretty damn long and is generally only used for big content like a whole blog post, and comments if you don't want a limit.
With regard to size there is no (or very little) difference between VARCHAR and TEXT since they just store what they need to. CHAR fields will always take up their allotted length.
Performance-wise, VARCHAR is usually faster. VARCHARs can be indexed too which leads to faster searching.
MySQL will internally convert TEXT to varchar while creating temporary tables. So it is better to use VARCHAR if possible. There are a few minor bugs related to TEXT column such as...
http://bugs.mysql.com/bug.php?id=36676
As per my Opinion VARCHAR is best option when u know the length of characters. It will also reduce garbage Memory Allocations and space issue. TEXT will consume 255 where as VARCHAR will consume as u give the values to it.
As per performance, VARCHAR is also faster then TEXT.
There is a subtle difference in text and varchar. I have a table as shown:
CREATE TABLE `test`.`tbl`(
`kee` BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
`txt` TEXT(100),
`vrchr` VARCHAR(100),
PRIMARY KEY (`kee`)
);
I insert a row:
INSERT INTO `tbl`
(`txt`,
`vrchr`)
VALUES ('1
2
3',
'1
2
3');
The column txt has value:
1
2
3
and column vrchr has value:
1