When we creating database for our application, We limited lengths of database columns.
example -
String (200)
int (5)
etc
Is there any effect on Speed or some effect?
First of all, one does not limit the length of a "database". Instead, one does limit the size of columns of tables in a database.
Why do we do this, you ask?
We don't want to waste any space for data that's never going to use it (that's what the varchar, varbinary and the like are for).
It's a best practice because it forces you to think of your data structure BEFORE you actually use it.
The less data there is the faster the processing of the application (that's a tautology).
It makes it easier to validate your data if you know exactely how much space it is allowed to take.
Full text indexes gain greatly when limited in size
One reason I can think of is, When you didn't specify the length of a column data type, the MYsql engine would assume a default length value that may be a lot larger of the length of the actual data that would be stored in that column. So it is always a best practice never to ignore the length property of a column.
Limiting the length of database fields ensures validation of data, you won't get any unexpected data of a length other than what has been specified. Also certain fields cannot be indexed such as LONG so choose appropriately and wisely. With regard to performance the effect is negligible. You need to also think about the data itself, for example storing data in a unicode encoding such as UTF-8 may increase the storage requirements.
Related
I have a MariaDB InnoDB table with several million rows, but with short, fixed-width rows consisting of numbers and timestamps only.
We usually search, filter and sort the rows using any of the existing columns.
We want to add a column to store an associated "url" to each row. Ideally every row will have it's url.
We know for a fact that we won't be sorting, searching and filtering by the url column.
We don't mind truncating the URL to it's first 255 bytes, so we are going to give it the VARCHAR type.
But of course that column's width would be variable. The whole record will become variable-width and the width of the original record will double in many cases.
We were considering the alternative of using a different, secondary table for storing the varchar.
We could join them when querying the data, or even more efficiently -probably- just fetch the url's for the page we are showing.
Would this approach be advisable?
Is there a better alternative that would also allow us to preserve performance?
Update: As user Bill Karwin noted in one comment below, InnoDB does not benefit from fixed width as much as MyISAM does, so the real issue here is about the size of the row and not so much about the fixed versus variable width discussion.
Assuming you have control over how the URL is generated, you may want to change it to a fixed-length state. Youtube videos' URIs, for instance, are always 11 characters long and base-64. This fixes the variable length problem and avoids joining tables.
If changing URI generation is not an option, you have a few alternatives to make it fixed-length:
You could fill in the blanks with a special character to force every url to be 255 within the database, and removing it just before returning it. This is not a clean solution but makes DQL operations faster than joining.
You could fetch the url as you have stated, but beware that two http requests may be more time consuming than any other option with just one request.
You could join with another table only when the user requires it, as opposed to it being the default.
Consider that having variable length may not be as big a problem, depending on your needs. The only issue might be if you're grossly oversizing fields, but it doesn't seem to be your case.
I am new to web application developing. I am using php+mysql, so just wanted to check that what is best and safe way to create a database schema.
I have a table with 15 columns, the first column is unique id and remaining all are type of "text" and I am planning to put approx 1-20kb data in each column.
So is it good or bad or it should not be like that? So could anyone please help me on the same?
I am using engine type MYISAM.
The best idea depends on what you're using it for. VARCHAR is faster when the size is reasonable and also that is dependent on hardware and data that you are using. You should almost aways try to store text format messages as TEXT whereas string attributes as VARCHAR.
TEXT columns will not be stored in memory and will have to be read from the disk, which is much slower. Check this reference
Also check Blob Storage in Innodb
Dedicated data types exist for a reason. Some random things you can't do reliably (or can't do at all) with large blobs of text:
Maths
Date calculations
Proper indexes
Ensure a bare minimum of data integrity
Numeric sort
A database is much more than a dumb storage system.
Sometimes its must to use Text type. But try to use varchar instead od text/mediumtext/longtext etc. as often as its possible.
Usung text will have impact of performance.
Queries on text types are slower.
Data with text types will not be cached in memory.
I'm looking for some advice on the best way to store long strings of data from the mySQL experts.
I have a general purpose table which is used to store any kind of data, by which I mean it should be able to hold alphanumeric and numeric data.
Currently, the table structure is simple with an ID and the actual data stored in a single column as follows:
id INT(11)
data VARCHAR(128)
I now have a requirement to store a larger amount of data (up to 500 characters) and am wondering whether the best way would be to simply increase the varchar column size, or whether I should add a new column (a TEXT type column?) for the times I need to store longer strings.
If any experts out there has any advice I'm all ears!
My preferred method would be to simply increase the varchar column, but that's because I'm lazy.
The mySQL version I'm running is 5.0.77.
I should mention the new 500 character requirement will only be for the odd record; most records in the table will be not longer than 50 characters.
I thought I'd be future-proofing by making the column 128. Shows how much I knew!
Generally speaking, this is not a question that has a "correct" answer. There is no "infinite length" text storage type in MySQL. You could use LONGTEXT, but that still has an (absurdly high) upper limit. Yet if you do, you're kicking your DBMS in the teeth for having to deal with that absurd blob of a column for your 50-character text. Not to mention the fact that you hardly do anything with it.
So, most futureproofness(TM) is probably offered by LONGTEXT. But it's also a very bad method of resolving the issue. Honestly, I'd revisit the application requirements. Storing strings that have no "domain" (as in, being well-defined in their application) and arbitrary length is not one of the strengths of RDBMS.
If I'd want to solve this on the "application design" level, I'd use NoSQL key-value store for this (and I'm as anti-NoSQL-hype as they get, so you know it's serious), even though I recognize it's a rather expensive change for such a minor change. But if this is an indication of what your DBMS is eventually going to hold, it might be more prudent to switch now to avoid this same problem hundred times in the future. Data domain is very important in RDBMS, whereas it's explicitly sidelined in non-relational solutions, which seems to be what you're trying to solve here.
Stuck with MySQL? Just increase it to VARCHAR(1000). If you have no requirements for your data, it's irrelevant what you do anyway.
Careful if using text. TEXT data is not stored in the database server’s memory, therefore, whenever you query TEXT data, MySQL has to read from it from the disk, which is much slower in comparison with CHAR and VARCHAR as it cannot make use of indexes.The better way to store long string will be nosql databases
We can use varchar(<maximum_limit>). The maximum limit that we can pass is 65535 bytes.
Note: This maximum length of a VARCHAR is shared among all columns except TEXT/BLOB columns and the character set used.
When I add user info to MySQL through a PHP registration form, there are with limits on the data fields (e.g. name is 20 max chars, email 18 chars, additional info 200, pass 12 chars, etc.)
Should I create exact same fields in the MySQL table, or I should define longer fields?
Is there any benefits of doing so rather than just creating all string fields e.g. 500 characters long?
When storing age as an integer, should I use a small int (i.e. with max 256) or not?
In general, it doesn't really matter. The important part is how you validate the information on the server side.
Make sure the entered data does not exceed the size of the column. If you don't, you can run into issues where mysql will auto-truncate the data.
Don't limit the password size. If someone wants to enter a 200 character password, let them. You should be storing it in a storing hash and not in plain text, so the exact length shouldn't make a difference.
Always store your data types properly. If you expect an integer age, store it in an integer column. There's no real reason to store it in a string column type.
As far as the rest of your limits, it's really application dependent more than anything. If you expect 200 character info limit, then store it in a VARCHAR(200). But if you're just assuming, store it in a TEXT type so that the user can enter as much as they'd like. But that's more application and use-case dependent than anything else...
Suggest you be liberal with your database column lengths (for your varchar), but strict on your application in enforcing size/lengths.
The business logic may change over time. Your application tier will be the keeper and enforcer of those rules.
Your database shouldn't have to adjust often to the changing business rules regarding length. Defining a column of type varchar(100) doesn't cost you anything today. The length is variable up to 100, so your performance and storage won't suffer at all.
Application and database changes/maintenance are expensive; database storage is cheap.
Some other detailed suggestions, if you will:
don't store age. Derive it from a date (birthdate) by using math (Today-Birthdate).
passwords shouldn't be stored or have a max length!
all your string fields -- define them as varchar(256) or 1024 and be done with them. Let your application enforce the business rules of the day.
If you're using the MyISAM table type it will be a bit more efficient for queries if you keep the record length static. So if you can use char fields of a fixed size instead of varchar it's better (as long as all the fields in a record are static). However, the whole number of characters you specify will be blocked out in memory so you need to decide if memory usage is more important.
Unless your application is huge and your DB is going to be massive, this should matter very little in terms of performance. I would say that you should give yourself a bit of extra room in the MySQL fields as it is a lot easier to change the registration form max lengths than the MySQL max lengths later. Using smallint for age is fine. Or not. Generally you shouldn't allow for fields to take up more space than they are going to need, but I would give myself some padding just in case. Again, though, it shouldn't make much of a difference.
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Are there disadvantages to using a generic varchar(255) for all text-based fields?
In MYSQL you can choose a length for the VARCHAR field type. Possible values are 1-255.
But what are its advantages if you use VARCHAR(255) that is the maximum instead of VARCHAR(20)? As far as I know, the size of the entries depends only on the real length of the inserted string.
size (bytes) = length+1
So if you have the word "Example" in a VARCHAR(255) field, it would have 8 bytes. If you have it in a VARCHAR(20) field, it would have 8 bytes, too. What is the difference?
I hope you can help me. Thanks in advance!
Check out: Reference for Varchar
In short there isn't much difference unless you go over the size of 255 in your VARCHAR which will require another byte for the length prefix.
The length indicates more of a constraint on the data stored in the column than anything else. This inherently constrains the MAXIMUM storage size for the column as well. IMHO, the length should make sense with respect to the data. If your storing a Social Security # it makes no sense to set the length to 128 even though it doesn't cost you anything in storage if all you actually store is an SSN.
There are many valid reasons for choosing a value smaller than the maximum that are not related to performance. Setting a size helps indicate the type of data you are storing and also can also act as a last-gasp form of validation.
For instance, if you are storing a UK postcode then you only need 8 characters. Setting this limit helps make clear the type of data you are storing. If you chose 255 characters it would just confuse matters.
I don't know about mySQL but in SQL Server it will let you define fields such that the total number of bytes used is greater than the total number of bytes that can actually be stored in a record. This is a bad thing. Sooner or later you will get a row where the limit is reached and you cannot insert the data.
It is far better to design your database structure to consider row size limits.
Additionally yes, you do not want people to put 200 characters in a field where the maximum value should be 10. If they do, it is almost always bad data.
You say, well I can limit that at the application level. But data does not get into the database just from one application. Sometimes multiple applications use it, sometimes data is imported and sometimes it is fixed manually from the query window (update all the records to add 10% to the price for instance). If any of these other sources of data don't know about the rules you put in your application, you will have bad, useless data in your database. Data integrity must be enforced at the database level (which doesn't stop you from also checking before you try to enter data) or you have no integrity. Plus it has been my experience that people who are too lazy to design their database are often also too lazy to actually put the limits into the application and there is no data integrity check at all.
They have a word for databases with no data integrity - useless.
There is a semantical difference (and I believe that's the only difference): if you try to fill 30 non-space characters into varchar(20), it will produce an error, whereas it will succeed for varchar(255). So it is primarily an additional constraint.
Well, if you want to allow for a larger entry, or limit the entry size perhaps.
For example, you may have first_name as a VARCHAR 20, but perhaps street_address as a VARCHAR 50 since 20 may not be enough space. At the same time, you may want to control how large that value can get.
In other words, you have set a ceiling of how large a particular value can be, in theory to prevent the table (and potentially the index/index entries) from getting too large.
You could just use CHAR which is a fixed width as well, but unlike VARCHAR which can be smaller, CHAR pads the values (although this makes for quicker SQL access.
From a database perspective performance wise I do not believe there is going to be a difference.
However, I think a lot of the decision on the length to use comes down to what you are trying to accomplish and documenting the system to accept just the data that it needs.