MySQL wanted to ensure that its index files could fit within a single page block on older file systems.MySQL was itself trying to be compatible with even older databases (sybase/SAP), and they had a 255 character limit.MySQL being very concerned with speed and memory usage, wanted to store things with the smallest possible data types. 256 is the largest number you can represent with an 8-bit integer.The history is fuzzy as to why MySQL chose a 255 character limit (see the articles linked below), but the most popular theories include: MySQL, the most popular open source database of the early 2000s had a limit of 255 characters in indexed fields. The next stop on our journey is to look at what the default column size was far in the past, and that was 255 characters, e.g.: email_address varchar ( 255 ) NOT NULL But, generically, it seems that an index of any size should work, and while that’s true today, it wasn't always possible. In fact, historically, databases were constructed with limits on how big an index could be to optimize search and how they stored data on disk. For some databases, you aren’t allowed to add a search index to columns of type text because this optimization can't be done, while in others, the index just won’t perform as well. Knowing how long the strings in the index are is one of the best ways speed things up. So, why use varchar? Indexes can be made to perform better when assumptions can be made about the type of data they store. For most applications this is a great tradeoff, since they are "read heavy" and "write lite". In essence, indexes spend computation time (and a little bit of disk space) making writes to the database slower, to speed up reads later. However, if you add a search index, you are telling your database to essentially "pre-compute" popular search patterns with a tree so the next search is much faster. If you are going to search by a column, say email_address, you probably want to add an index to it to speed things up when you do the following: select id from users where email = Īs your table gets bigger, searches get slower because your database has to check every row to find a match. The first question you might ask is why limit the length of the strings you can store in a database at all? All modern popular relational database support (almost) unlimited sized strings with a text or blob-type column, so why not use that? The reason is indexes. 191 is such an odd number - where did it come from? In this post, we’ll look at the historical reasons for the 191 character limit as a default in most relational databases. This means that the column supports strings with a maximum length of 191 characters, and can’t be null. This actually gives the maximum length you can use.Sometimes, when you are looking at a database’s schema, you see that there are text fields defined like this: email_address varchar ( 191 ) NOT NULL Mysql> CREATE TABLE max_len_varchar(fld VARCHAR(16384) CHARSET utf8) ĮRROR 1074 (42000): Column length too big for column 'fld' (max = 16383) use BLOB or TEXT instead (one of the reasons I believe this is that there’s a differentĮrror message when the character set is multi-byte: There is someĪllusion to that on the Forge Internals page……but I have not confirmed Overhead per row, though I have not confirmed that. Why is that? I believe that is because there is also a 1-byte Mysql> CREATE TABLE max_len_varchar(fld VARCHAR(65532) CHARSET latin1) You have to change some columns to TEXT or BLOBs The maximum row size for the used table type, not counting BLOBs, is 65535. Mysql> CREATE TABLE max_len_varchar(fld VARCHAR(65533) CHARSET latin1) ĮRROR 1118 (42000): Row size too large. ![]() Length of a VARCHAR should be 65,533 bytes. Storing the string length is two bytes per row. ![]() Of VARCHAR is greater than 255, the overhead cost of Is used to store the actual length of the string. Length of VARCHAR is less than 255, one byte per row ![]() Only the NDB storage engine has a different maximum value. Row length is the maximum allowed by MySQL, which is 65,535īytes. The maximum length of a VARCHAR is only restrictedīy the maximum row length.
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