I am updating a row in MySQL which currently has 15 varchar(2500) columns plus a few other small sized columns. I need to allow the user to enter more data in these columns, would it be better to convert them to BLOBS? Is there a maximum recommended number of BLOBS per row?
What is best practice with regards to not exceeding the maximum number of bytes per row whilst still allowing the user flexibility with regards to how much they enter in each column?
Many thanks.
No, there's no maximum recommended number of BLOBS on a row. You are really only limited by the maximum size of the row. There is some performance overhead for accessing each BLOB (the row contains a pointer to the BLOB data which is stored elsewhere, vs. accessing a column value actually stored within the row.)
The best practice is to include the most frequently accessed columns within the row, and limit the size of variable length columns to practical limits. Consider TEXT and/or BLOB columns when the size of the column exceeds the maximum allowed by other datatypes, or when the length of the row exceeds the maximum allowed. Alternatively, consider separate child table(s) for less frequently accessed "long-ish" columns.
Related
What is the upper limit of records for MySQL database table. I'm wondering about autoincrement field. What would happen if I add milions of records? How to handle this kind of situations?
Thx!
The greatest value of an integer has little to do with the maximum number of rows you can store in a table.
It's true that if you use an int or bigint as your primary key, you can only have as many rows as the number of unique values in the data type of your primary key, but you don't have to make your primary key an integer, you could make it a CHAR(100). You could also declare the primary key over more than one column.
There are other constraints on table size besides number of rows. For instance you could use an operating system that has a file size limitation. Or you could have a 300GB hard drive that can store only 300 million rows if each row is 1KB in size.
The limits of database size is really high:
http://dev.mysql.com/doc/refman/5.1/en/source-configuration-options.html
The MyISAM storage engine supports 232 rows per table, but you can build MySQL with the --with-big-tables option to make it support up to 264 rows per table.
http://dev.mysql.com/doc/refman/5.1/en/innodb-restrictions.html
The InnoDB storage engine has an internal 6-byte row ID per table, so there are a maximum number of rows equal to 248 or 281,474,976,710,656.
An InnoDB tablespace also has a limit on table size of 64 terabytes. How many rows fits into this depends on the size of each row.
The 64TB limit assumes the default page size of 16KB. You can increase the page size, and therefore increase the tablespace up to 256TB. But I think you'd find other performance factors make this inadvisable long before you grow a table to that size.
mysql int types can do quite a few rows: http://dev.mysql.com/doc/refman/5.0/en/numeric-types.html
unsigned int largest value is 4,294,967,295
unsigned bigint largest value is 18,446,744,073,709,551,615
I suggest, never delete data. Don't say if the tables is longer than 1000 truncate the end of the table. There needs to be real business logic in your plan like how long has this user been inactive. For example, if it is longer than 1 year then put them in a different table. You would have this happen weekly or monthly in a maintenance script in the middle of a slow time.
When you run into to many rows in your table then you should start sharding the tables or partitioning and put old data in old tables by year such as users_2011_jan, users_2011_feb or use numbers for the month. Then change your programming to work with this model. Maybe make a new table with less information to summarize the data in less columns and then only refer to the bigger partitioned tables when you need more information such as when the user is viewing their profile. All of this should be considered very carefully so in the future it isn't too expensive to re-factor. You could also put only the users which comes to your site all the time in one table and the users that never come in an archived set of tables.
In InnoDB, with a limit on table size of 64 terabytes and a MySQL row-size limit of 65,535 there can be 1,073,741,824 rows. That would be minimum number of records utilizing maximum row-size limit. However, more records can be added if the row size is smaller .
According to Scalability and Limits section in http://dev.mysql.com/doc/refman/5.6/en/features.html,
MySQL support for large databases. They use MySQL Server with databases that contain 50 million records. Some users use MySQL Server with 200,000 tables and about 5,000,000,000 rows.
Row Size Limits
The maximum row size for a given table is determined by several factors:
The internal representation of a MySQL table has a maximum row size
limit of 65,535 bytes, even if the storage engine is capable of
supporting larger rows. BLOB and TEXT columns only contribute 9 to 12
bytes toward the row size limit because their contents are stored
separately from the rest of the row.
The maximum row size for an InnoDB table, which applies to data
stored locally within a database page, is slightly less than half a page. For example, the maximum row size is slightly less than 8KB for the default 16KB InnoDB page size, which is defined by the innodb_page_size configuration option. “Limits on InnoDB Tables”.
If a row containing variable-length columns exceeds the InnoDB maximum row size, InnoDB selects variable-length columns for external off-page storage until the row fits within the InnoDB row size limit.
The amount of data stored locally for variable-length columns that are
stored off-page differs by row format. For more information, see
“InnoDB Row Storage and Row Formats”.
Different storage formats use different amounts of page header and trailer data, which affects the amount of storage available for rows.
Link http://dev.mysql.com/doc/refman/5.7/en/column-count-limit.html
Row Size Limits
The maximum row size for a given table is determined by several factors:
The internal representation of a MySQL table has a maximum row size limit of 65,535 bytes, even if the storage engine is capable of supporting larger rows. BLOB and TEXT columns only contribute 9 to 12 bytes toward the row size limit because their contents are stored separately from the rest of the row.
The maximum row size for an InnoDB table, which applies to data stored locally within a database page, is slightly less than half a page for 4KB, 8KB, 16KB, and 32KB innodb_page_size settings. For example, the maximum row size is slightly less than 8KB for the default 16KB InnoDB page size. For 64KB pages, the maximum row size is slightly less than 16KB. See Section 15.8.8, “Limits on InnoDB Tables”.
If a row containing variable-length columns exceeds the InnoDB maximum row size, InnoDB selects variable-length columns for external off-page storage until the row fits within the InnoDB row size limit. The amount of data stored locally for variable-length columns that are stored off-page differs by row format. For more information, see Section 15.11, “InnoDB Row Storage and Row Formats”.
Different storage formats use different amounts of page header and trailer data, which affects the amount of storage available for rows.
For information about InnoDB row formats, see Section 15.11, “InnoDB Row Storage and Row Formats”, and Section 15.8.3, “Physical Row Structure of InnoDB Tables”.
For information about MyISAM storage formats, see Section 16.2.3, “MyISAM Table Storage Formats”.
http://dev.mysql.com/doc/refman/5.7/en/innodb-restrictions.html
There is no limit. It only depends on your free memory and system maximum file size. But that doesn't mean you shouldn't take precautionary measure in tackling memory usage in your database. Always create a script that can delete rows that are out of use or that will keep total no of rows within a particular figure, say a thousand.
What are problems occur when mysql server table contains more than 120 columns ?
from a technical point of view without any consideration on the reasons for which you need 120 columns in a table mysql documentation 5.7 says:
Column Count Limits
MySQL has hard limit of 4096 columns per table, but the effective
maximum may be less for a given table.
https://dev.mysql.com/doc/refman/5.7/en/column-count-limit.html
This is bad practice to have 120 columns in a table better to split into multiple tables.
Since MYSQL is famous for relation database, So make relation based tables structure.
List of issues comes once your application become bigger.
Application gets slow (Since data fetching from table is slowly).
If your internet is slow, then you may not load the application page.
If huge amount of data is loaded at once due to the numbers of columns, then your server require more bandwidth.
Might you may not able you open in mobile, Since mobile better work with small amount of data.
https://dba.stackexchange.com/questions/3972/too-many-columns-in-mysql
There is a hard limit of 4096 columns per table, but the effective maximum may be less for a given table. The exact limit depends on several interacting factors.
Every table (regardless of storage engine) has a maximum row size of 65,535 bytes. Storage engines may place additional constraints on this limit, reducing the effective maximum row size.
The maximum row size constrains the number (and possibly size) of columns because the total length of all columns cannot exceed this size.
...
Individual storage engines might impose additional restrictions that limit table column count. Examples:
InnoDB permits up to 1000 columns.
I usually use maximum chars possible for varchar fields, so in most cases I set 255 but only using 16 chars in columns...
does this decreases performance for my database?
When it comes to storage, a VARCHAR(255) column will take up 1 byte to store the length of the actual value plus the bytes required to store the actual value.
For a latin1 VARCHAR(255) column, that's at most 256 bytes. For a UTF8 column, where each character can take up to 3 bytes (though rarely), the maximum size is 766 bytes. As we know the maximum index length for a single column in bytes in InnoDB is 767 bytes, hence perhaps the reason some declare 255 as the maximum supported column length.
So, again, when storing the value, it only takes up as much room as is actually needed.
However, if the column is indexed, the index automatically allocates the maximum possible size so that each node in the index has enough room to store any possible value. When searching through an index, MySQL loads the nodes in specific byte size chunks at a time. Large nodes means less nodes per read, which means it takes longer to search the index.
MySQL will also use the maximum size when storing the values in a temp table for sorting.
So, even if you aren't using indexes, but are ever performing a query that can't utilize an index for sorting, you will get a performance hit.
Therefore, if performance is your goal, setting any VARCHAR column to 255 characters should not be a rule of thumb. Instead, you should use the minimum required.
There may be edge cases where you'd rather suffer the performance every day so that you never have to lock a table completely to increase the size of a column, but I don't think that's the norm.
One possible exception is if you are joining on a VARCHAR column between two tables. MySQL says:
MySQL can use indexes on columns more efficiently if they are declared
as the same type and size.
In that case, you might use the max size between the two.
Whenever you're talking about "performance" you can only find out one way: Benchmarking.
In theoretical terms there's no difference between VARCHAR(20) and VARCHAR(255) if they're both populated with the same data. Keep in mind if you get your length wrong you will have massive truncation problems and MySQL does not warn you before it starts chopping data to fit.
I try to avoid setting limits on VARCHAR columns unless the data would be completely invalid if it was longer. For instance, two-character ISO country codes can be stored in VARCHAR(2) because longer strings are meaningless. For other things, especially names or phone numbers, limiting the length is potentially and probably harmful.
Still, you will want to test any schema you create to be sure it meets your performance requirements. I expect you'd have a hard time detecting any difference at all between VARCHAR(25) and VARCHAR(16).
There are two ways in which this will decrease performance.
if you're loading those columns many many times, performing a join on the column, or other such thing that means they need to be accessed a large number of times. The number of times depends on your machine, but think on the order of millions.
if you're always filling the field (using 20 chars in a varchar(20), then the length checks are adding a little overhead whenever you perform an insert.
The best way to determine this though is to benchmark your database though.
What is the MySQL VARCHAR max size?
I have searched on stackoverflow about varchar vs text and studied the documentation.
Iam not sure if i understood it the right way but let me ask you this. I have got Mysql version 5.5.34 and yes the maximum varchar size is 65535 bytes and the text content on my website cannot exceed more than 600 characters, I would prefer using varchar(600) rather than Text.
Since varchar is stored inline with table and faster and when I read all the answers of the question.
rajukoyilandy answer
varchar has 64K row limit
paxdiablo answer
if your row size is approaching 64K, you may want to examine the schema of your database. It's a rare table that needs to be that wide in a properly set up (3NF) database - it's possible, just not very common.
If you want to use more than that, you can use the BLOB or TEXT types. These do not count against the 64K limit of the row
So I have decided to use Text
http://dev.mysql.com/doc/refman/5.0/en/column-count-limit.html
When I have seen this documentation on the very top it says
Every table (regardless of storage engine) has maximum row size of 65,535 bytes.
Storage engines may place additional constraints on this limit,
reducing the effective maximum row size
My question is, So no matter what you use Text or Varchar, the row limit is 65kb? or if you use Text there is no row limit? Any help is greatly appreciated.
http://dev.mysql.com/doc/refman/5.0/en/column-count-limit.html
BLOB and TEXT columns count from one to four plus eight bytes each
toward the row-size limit because their contents are stored separately
from the rest of the row.
So yes no matter what you use the row limit is 65KB, but since BLOB and TEXT types are stored separately the length of the contents do not count towards the row size, just the initial one to four plus eight bytes do.
This is why you can use LONGBLOB LONGTEXT to store huge files and still be able to have more than just a single LONGBLOB/LONGTEXT column in a table.
As for which is better VARCHAR(600) vs TEXT, with VARCHAR you would be taking up 1802 bytes per column so you still have lots of room to work with when it comes to the row limit. But for performance you would have to do benchmarks as this comment points out inline storage is faster if it frequently used in your queries.
The row size limit applies to the amount of space required to store the row itself, which is not the same as the amount of space required to store the data in the row. From the page you linked to:
BLOB and TEXT columns count from one to four plus eight bytes each toward the row-size limit because their contents are stored separately from the rest of the row.
This is why you can store huge amounts of data in a BLOB or TEXT field without exceeding the row-size limit.
What is the upper limit of records for MySQL database table. I'm wondering about autoincrement field. What would happen if I add milions of records? How to handle this kind of situations?
Thx!
The greatest value of an integer has little to do with the maximum number of rows you can store in a table.
It's true that if you use an int or bigint as your primary key, you can only have as many rows as the number of unique values in the data type of your primary key, but you don't have to make your primary key an integer, you could make it a CHAR(100). You could also declare the primary key over more than one column.
There are other constraints on table size besides number of rows. For instance you could use an operating system that has a file size limitation. Or you could have a 300GB hard drive that can store only 300 million rows if each row is 1KB in size.
The limits of database size is really high:
http://dev.mysql.com/doc/refman/5.1/en/source-configuration-options.html
The MyISAM storage engine supports 232 rows per table, but you can build MySQL with the --with-big-tables option to make it support up to 264 rows per table.
http://dev.mysql.com/doc/refman/5.1/en/innodb-restrictions.html
The InnoDB storage engine has an internal 6-byte row ID per table, so there are a maximum number of rows equal to 248 or 281,474,976,710,656.
An InnoDB tablespace also has a limit on table size of 64 terabytes. How many rows fits into this depends on the size of each row.
The 64TB limit assumes the default page size of 16KB. You can increase the page size, and therefore increase the tablespace up to 256TB. But I think you'd find other performance factors make this inadvisable long before you grow a table to that size.
mysql int types can do quite a few rows: http://dev.mysql.com/doc/refman/5.0/en/numeric-types.html
unsigned int largest value is 4,294,967,295
unsigned bigint largest value is 18,446,744,073,709,551,615
I suggest, never delete data. Don't say if the tables is longer than 1000 truncate the end of the table. There needs to be real business logic in your plan like how long has this user been inactive. For example, if it is longer than 1 year then put them in a different table. You would have this happen weekly or monthly in a maintenance script in the middle of a slow time.
When you run into to many rows in your table then you should start sharding the tables or partitioning and put old data in old tables by year such as users_2011_jan, users_2011_feb or use numbers for the month. Then change your programming to work with this model. Maybe make a new table with less information to summarize the data in less columns and then only refer to the bigger partitioned tables when you need more information such as when the user is viewing their profile. All of this should be considered very carefully so in the future it isn't too expensive to re-factor. You could also put only the users which comes to your site all the time in one table and the users that never come in an archived set of tables.
In InnoDB, with a limit on table size of 64 terabytes and a MySQL row-size limit of 65,535 there can be 1,073,741,824 rows. That would be minimum number of records utilizing maximum row-size limit. However, more records can be added if the row size is smaller .
According to Scalability and Limits section in http://dev.mysql.com/doc/refman/5.6/en/features.html,
MySQL support for large databases. They use MySQL Server with databases that contain 50 million records. Some users use MySQL Server with 200,000 tables and about 5,000,000,000 rows.
Row Size Limits
The maximum row size for a given table is determined by several factors:
The internal representation of a MySQL table has a maximum row size
limit of 65,535 bytes, even if the storage engine is capable of
supporting larger rows. BLOB and TEXT columns only contribute 9 to 12
bytes toward the row size limit because their contents are stored
separately from the rest of the row.
The maximum row size for an InnoDB table, which applies to data
stored locally within a database page, is slightly less than half a page. For example, the maximum row size is slightly less than 8KB for the default 16KB InnoDB page size, which is defined by the innodb_page_size configuration option. “Limits on InnoDB Tables”.
If a row containing variable-length columns exceeds the InnoDB maximum row size, InnoDB selects variable-length columns for external off-page storage until the row fits within the InnoDB row size limit.
The amount of data stored locally for variable-length columns that are
stored off-page differs by row format. For more information, see
“InnoDB Row Storage and Row Formats”.
Different storage formats use different amounts of page header and trailer data, which affects the amount of storage available for rows.
Link http://dev.mysql.com/doc/refman/5.7/en/column-count-limit.html
Row Size Limits
The maximum row size for a given table is determined by several factors:
The internal representation of a MySQL table has a maximum row size limit of 65,535 bytes, even if the storage engine is capable of supporting larger rows. BLOB and TEXT columns only contribute 9 to 12 bytes toward the row size limit because their contents are stored separately from the rest of the row.
The maximum row size for an InnoDB table, which applies to data stored locally within a database page, is slightly less than half a page for 4KB, 8KB, 16KB, and 32KB innodb_page_size settings. For example, the maximum row size is slightly less than 8KB for the default 16KB InnoDB page size. For 64KB pages, the maximum row size is slightly less than 16KB. See Section 15.8.8, “Limits on InnoDB Tables”.
If a row containing variable-length columns exceeds the InnoDB maximum row size, InnoDB selects variable-length columns for external off-page storage until the row fits within the InnoDB row size limit. The amount of data stored locally for variable-length columns that are stored off-page differs by row format. For more information, see Section 15.11, “InnoDB Row Storage and Row Formats”.
Different storage formats use different amounts of page header and trailer data, which affects the amount of storage available for rows.
For information about InnoDB row formats, see Section 15.11, “InnoDB Row Storage and Row Formats”, and Section 15.8.3, “Physical Row Structure of InnoDB Tables”.
For information about MyISAM storage formats, see Section 16.2.3, “MyISAM Table Storage Formats”.
http://dev.mysql.com/doc/refman/5.7/en/innodb-restrictions.html
There is no limit. It only depends on your free memory and system maximum file size. But that doesn't mean you shouldn't take precautionary measure in tackling memory usage in your database. Always create a script that can delete rows that are out of use or that will keep total no of rows within a particular figure, say a thousand.