One of the things that always worries me in MySQL is that my string fields will not be large enough for the data that need to be stored. The PHP project I'm currently working on will need to store strings, the lengths of which may vary wildly.
Not being familiar with how MySQL stores string data, I'm wondering if it would be overkill to use a larger data type like TEXT for strings that will probably often be less than 100 characters. What does MySQL do with highly variable data like this?
See this: http://dev.mysql.com/doc/refman/5.1/en/storage-requirements.html
VARCHAR(M), VARBINARY(M) L + 1 bytes
if column values require 0 – 255
bytes, L + 2 bytes if values may
require more than 255 bytes
BLOB, TEXT L + 2 bytes, where L < 2^16
So in the worst case, you're using 1 byte per table cell more when using TEXT.
As for indexing: you can create a normal index on a TEXT column, but you must give a prefix length - e.g.
CREATE INDEX part_of_name ON customer (name(10));
and moreover, TEXT columns allow you to create and query fulltext indexes if using the MyISAM engine.
On the other hand, TEXT columns are not stored together with the table, so performance could, theoretically, become an issue in some cases (benchmark to see about your specific case).
In recent versions of MySQL, VARCHAR fields can be quite long - up to 65,535 characters depending on character set and the other columns in the table. It is very efficient when you have varying length strings. See:
http://dev.mysql.com/doc/refman/5.1/en/char.html
If you need longer strings than that, you'll probably just have to suck it up and use TEXT.
Related
I'd like to set out my websites url path like so:
http://example.com/categories/a-specific-theme-or-subject/1
And use PHP to select all pages like this:
SELECT * FROM table WHERE category = 'a-specific-theme-or-subject'
But I was wondering, is this good practice, or does this have a negative effect on the speed at which the data is pulled from the database - In comparison to using a simpler single word:
http://example.com/categories/subject/1
SELECT * FROM table WHERE category = 'subject'
A VARCHAR column is allowed 255 characters, but I would assume that the lower the number actually used is better, but how many is too many?
I would use no more than 26 characters for any complete category name (a-specific-theme-or-subject).
Short answer: Don't worry.
Long answer:
The hard limit on how long an indexed column can be is 191 or 255 or 767 or 3072 bytes or some intermediate values, depending on MySQL version and CHARACTER SET.
The practical limit on how long is, again, "don't worry".
Indexing shorter column is slightly better than a long column, but the overhead of switching to a longer column is probably worse than simply using what your application needs.
A category VARCHAR(26) is fine; don't worry. And you could make that ascii or latin1 or utf8 or utf8mb4. (The 26 characters would have a max of 26, 26, 78, 104 _bytes, respectively.)
The most important thing in designing a good index is tailoring it to the query. WHERE category = 'subject' calls for exactly INDEX(category).
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.
I plan to be storing strings that have a maximum size of 4,500 VarChar, but MOST of the entries will be under 200 characters. Is MySql smart enough to optimize?
My current solution is to use 5 tables, data_small, data_medium, data_large, etc and insert based on the length of the string. The other solution would be to save files to disk, which would mean a second hit to the database, but result in a smaller return.
MySQL would do fine as would most every RDBMS for that matter. When you specify a field as type CHAR() the number of characters is always used regardless of how many characters are in your string. For instance: If you have Char(64) field and you insert 'ABCD' then the field is still 64 bytes (assuming non-unicode).
When using VARCHAR(), however, the cell only uses as many bytes as are in the string, plus the number of bytes necessary to store the size of the string. So: If you have VARCHAR(64) and insert 'ABCD' you will only use 5 bytes. 4 for the characters 'ABCD' and one for the number of characters '4'.
Your extremely varying string lengths are exactly the reason we have VARCHAR(), so feel free to use VARCHAR(4500) and rest assured you will only be using as much space as necessary to store the characters in the string, and a little bit extra for the length.
Somewhat related: This is why it's not a great idea to use VARCHAR() for fields that don't have varying length strings being inserted. You are wasting space storing the size of the string when it's already known. For instance, telephone numbers of the form x-xxx-xxx-xxxx should just use Char(14) since it will always take up 14 characters and only 14 bytes are necessary. If you used VARCHAR(14) instead you would actually end up using 15 bytes.
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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.
Can we declare a variable with fixed length in PHP?
I'm not asking about trimming or by putting condition do substring.
Can we declare variable just like database char(10).
The reason I'm asking am doing an export process, PHP export data to DB.
In DB I have a field with size 100, and I'm passing a field with length 25, using PHP.
When I look in DB, it's showing some extra space for that field.
Maybe it's your database that is the problem.
The CHAR datatype will always fill up the remaining unused characters when storing data. If you have CHAR(3) and pass 'hi', it will store it as 'hi '. This is true for a lot of relational database engines (MySQL, Postgres, SQLite, etc.).
This is why some database engines also have the VARCHAR datatype (which is variable, like the name says). This one doesn't pad the content with spaces if the data stored in isn't long enough.
In most cases, you are looking for the VARCHAR datatype. CHAR is mostly useful when you store codes, etc. that always have the same length (e.g.: a CHAR(3) field for storing codes like ADD, DEL, CHG, FIX, etc.).
No, a string in PHP is always variable length. You could trim the string to see if extra space is still passed to your DB.
Nope. PHP has no provision to limit string size.
You could simulate something in an object using setter and getter variables, though, throwing an error (or cutting off the data) if the incoming value is larger than allowed.
No, but I really don't think you're having a problem with php. I think you should check your DB2 configuration, perhaps it automatically completes strings with spaces... How much spaces are added? Are they added before? After?
As others have said: No.
I don't understand how it would help anyway. I'm not familiar with DB2 but it sounds like if you have extra spaces, they are either coming in the variable (and thus it should be trimmed) or DB2 does space padding to make the value have 100 characters. If your input is only 25 characters long then if it is doing space padding, it seems it would do it anyway.
If you want to store variable length strings in DB2 then go with VARCHAR, if you always want the same length for each string in the column, define the exact length using CHAR (for postal codes, for instance).
Details on character strings is available here: http://publib.boulder.ibm.com/infocenter/db2luw/v9r7/topic/com.ibm.db2.luw.sql.ref.doc/doc/r0008470.html with a good summary:
Fixed-length character string (CHAR)
All values in a fixed-length string column have the same length, which is determined by the length attribute of the column. The length attribute must be between 1 and 254, inclusive.
Varying-length character strings
There are two types of varying-length character strings:
A VARCHAR value can be up to 32,672 bytes long.
A CLOB (character large object) value can be up to 2 gigabytes minus 1 byte (2,147,483,647 bytes) long.
Of course it then gets more detailed, depending on what sort of encoding you're using, etc... ( like UTF-16 or UTF-32 )