In our example above, we defined the url_md5 column as a CHAR(32) character column. The character set determines what characters are allowed to go in the column, and the collation is a set of rules that determines how those characters compare to each other. Storing MD5 hashes in binary columnsĮvery character column in MySQL is defined with a character set and a collation. We can make one more optimization with the current setup before we move on to hashing multiple columns. To perform a strict equality, indexed lookup on this column, we'll create a generated hash in an example table called visits. Since URLs can be incredibly long, you might store them in a TEXT column to adequately accommodate the full possible length. (Use the MD5() or CRC32() function to produce the hash value.)Īn example of a column that might need this is a url column. Rather than testing for equality against a very long text string, you can store a hash of the column value in a separate column, index that column, and test the hashed value in queries. Any B-tree indexes added to BLOB or TEXT columns require a prefix because they are too large to be indexed in their entirety. The first scenario where you might want a generated hash column is when you need to do a strict equality lookup on a value that is too large to add a B-tree index. We're using hashing functions to create very small, deterministic results for speedy lookups, not to protect information. Securely storing sensitive information is an entirely separate topic. It must be noted that, importantly, this technique has nothing to do with securely storing passwords or other sensitive information. Usually, this is an MD5 hash, but it could be a CRC32, SHA256, or any other hash that suits your needs. It's merely a generated column in which the value generated is a hash of something else. Generated hash columns for strict equality lookupsĪ generated hash column is nothing special. Now that we know how generated columns work, let's move on to making generated hash columns. The syntax for creating a generated column is: Performance metrics of VIRTUAL or STORED are highly dependent on the situation, but a good rule of thumb is that if it's expensive to calculate the value, store it. A VIRTUAL generated column is calculated at runtime every time. When using a STORED generated column, the value of the expression is written to disk as if it were a regular column. When creating a generated column, you can make it a STORED or VIRTUAL column. The value in the generated column is always up to date and can never fall out of sync. To create a generated column, you give MySQL an expression, and then the database is in charge of populating the column with the result of that expression. The result of the expression must be scalar and deterministic. The expression can contain literal values, built-in functions, or references to other columns. It is a column whose value results from an expression rather than direct data input. ![]() Generated columns in MySQLĪ generated column can be considered a calculated, computed, or derived column. Instead of creating huge composite indexes, we'll index the compact generated hashes for fast lookups.īefore diving into generated hash columns, let's look at generated columns in general. ![]() We will build on this idea by creating generated hash columns for indexed lookups on large values and enforcing uniqueness across many columns. If this column is short, reasonably unique, and indexed, it might be faster than a “wide” index on many columns. One of the hidden gems in the MySQL documentation is this note in section 8.3.6:Īs an alternative to a composite index, you can introduce a column that is “hashed” based on information from other columns.
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