I was wondering if it is possible to increase the maximum row length of MySQL with InnoDB engine. The current is 8KB.
I would also like to know what enforces this limitation. I do not remember having such a limitation with Oracle 10 or MSSQL 2005.
Thank you!
You have to change the inno_db_page_size, check http://www.mysqlperformanceblog.com/2006/06/04/innodb-page-size/ It can be 8K, 16K, 32K or 64K, 16K is the default.
MS SQL Server has this limitation as well. To get around this, you may use LOBs (like CLOB, BLOB, TEXT, etc) since they are not part of the record itself.
Alternatively you may split your table into two tables linked 1:1. Maybe you could give some background what you try to achieve and why your table is that big.
This is a restriction on InnoDB tables and cannot be changed. According to the docs:
The maximum row length, except for
variable-length columns (VARBINARY,
VARCHAR, BLOB and TEXT), is slightly
less than half of a database page.
That is, the maximum row length is
about 8000 bytes.
You will have to think about redesigning your table.
Related
In mysql there is a hard limit of row size of 65,535 bytes which I believe is based of page size. Is there a similar restriction with Sqlite3?
I tried reading through https://www.sqlite.org/limits.html on the Sqlite site but found no specific indication there is. But then I am not a DB expert so maybe I missed something.
The limits page you link to says this:
During part of SQLite's INSERT and SELECT processing, the complete content of each row in the database is encoded as a single BLOB. So the SQLITE_MAX_LENGTH parameter also determines the maximum number of bytes in a row.
(emphasis mine)
So SQLITE_MAX_LENGTH is the maximum length of a row, including any BLOB values.
The default is 1000000000 bytes.
For what it's worth, the row length limit in MySQL of 65535 bytes does not apply to BLOB/TEXT columns. And it has nothing to do with page size.
I understand the difference between char and varchar but I'm not sure of exact meaning of varchar maximum length.
When we store string whose length is less than 10, is there any difference between varchar(10) and varchar(100)?
I think both cases use same space and have same performance. If so, why do we need varchar max limit?
Is it enough to use just "varchar" instead of "varchar(xxx)"?
(added) I'm using MySQL 5.0.67
This depends entirely on the DBMS engine being used. SQL itself does not mandate how things are stored physically, just how they're seen logically.
For example, your DBMS may allocate space in the row for the maximum size, plus some extra bytes to store the length. In that case, there would be a big difference between varchar(10) and varchar(1000) since you would waste quite a bit of space per row.
Alternatively, it may use a buffer pool for the varchar data and store only the length and the buffer pool "starting address" in the row. In that case, every single row would store identically-sized information for a varchar column regardless of its size, but there would be an added step to extract the actual data in that column (following the link to the buffer pool).
The reason you use a varchar is exactly why it's named varchar. It allows you to store variable-sized data elements. Typically, char(10) gives you ten characters, no matter what, padding it with spaces if you insert something shorter. You can trim trailing spaces off as you extract it but that won't work so well if the data you want to store is actually "hello ", with a trailing space you want preserved.
A decent DBMS engine may decide to make a trade-off depending on the maximum size of the varchar column. For short ones, it could just store it inline in the row and consume the extra bytes for the size.
Longer varchar columns could be "outsourced" to a separate buffer pool to ensure row-reading is kept efficient (at least until you need the large varchar column, anyway).
What you need to do is re-ask the question for your specific DBMS so as to get a more targeted answer.
Or, in all honesty, engineer your database to only store the maximum size. If you know it's 10, then varchar(1000) is a waste. If, in the future, you need to enlarge the column, that is the time to do it, rather than now (see YAGNI).
For MySQL, you'll want to look at Chapter 14 Storage Engines of the online documentation.
It covers the various storage engines (such as InnoDB and MyISAM) that MySQL uses and, looking deep enough, you can see how the information is physically stored.
For example, in MyISAM, the presence of variable length data in a table (varchar included) usually means dynamic tables. This follows a scheme roughly analogous to the buffer pool concept I mentioned above, with the advantage that less space is wasted for variable sized columns, and the disadvantage that rows may become fragmented.
The other storage format (discounting compressed format since it's only really used for read-only tables) is the static one, where data is stored in a single physical row.
Information on the InnoDB physical structures can be found here. Depending on whether you use the Antelope or Barracuda file format, you end up with the "all information is a physical row" or "buffer pool" situation, similar to the MyISAM distinction between dynamic and static.
In SQL Server, the limit does not affect how the data is stored on disk. What it does provide, though, is one constraint for free. If you, as the database designer, only want up to 10 characters stored, you've prevented someone from storing a novel instead.
Is it enough to use just "varchar"
Again, for SQL Server, almost certainly not what you want. In most circumstances, if you don't specify a limit, you get a varchar(1) (surely the most pointless data type ever conceived). Occasionally, it's a varchar(30).
In Oracle, the Varchar stretches in size depending on its use, just up to the point where you set the limit. This means that indeed, a varchar(10) and a varchar(100) containing 2 characters use the same space (which is different for a char, which always uses the full allocated space).
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.
I have a table with a field that needs to store 512 characters. My question is this: can you limit the length of a TEXT field (VARCHAR stops at 255 - I can't use it) to 512 characters? If I do that, will MySQL run through the table quicker due to the fixed data length, or does it act a bit like VARCHAR in that the length can vary?
I don't have a table (at all) yet, but I'm very conscious about speed and size for the future. I found this article: link text incredibly useful - maybe you will to!
Thanks a lot,
James
Update your mysql server. Since 5.0.3 you can store up to 65535 bytes in VARCHAR.
You don't really say how you are using the table, so I'll just throw this in. One option is to separate out the text field into another table and just have a reference to it in your original table.
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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.