如何在不使用文本字段的情况下在MySQL数据库中存储哈希值

问题描述：

I'm storing unique user-agents in a MySQL MyISAM table so when I have to look if it exists in the table, I check the md5 hash that is stored next to the TEXT field.

User-Agents 
{
    id         - INT
    user-agent - TEXT
    hash       - VARCHAR(32) // md5
}

There is any way to do the same but using a 32-bit integer and not a text hash? Maybe the md5 in raw format will be faster? That will requiere a binary search.

[EDIT]

MySQL don't handle hash searches for complete case-sensitive strings?

我在MySQL MyISAM表中存储了唯一的用户代理 strong>，所以当我有为了查看表中是否存在，我检查存储在TEXT字段旁边的 md5哈希 strong>。 p>

  User-Agents 
 {  
 id  -  INT 
 user-agent  -  TEXT 
 hash  -  VARCHAR（32）// md5 
} 
  code>  pre> 
 
 有什么方法可以做 相同但使用32位整数而不是文本哈希？ 也许原始格式的md5会更快？ 这将需要二进制搜索。 p> 
 
 
 [编辑]  p> 
 
 
 MySQL不处理完整区分大小写的字符串的哈希搜索？ p>  
  div>

答

Let MySQL do the hard work for you. Use a CHAR column and create an index on that column. You could convert and store the hash as an integer, but there's absolutely no benefit, and it may actually cause problems.

答

Are you REALLY sure the hashes are only 32-bit? MD5 is 128-bit. Cropping the hash to first 4 or 8 bytes would greatly increase risk of collisions.

答

You can't store an MD5 hash in a 32-bit int: it simply won't fit. (It's 32 characters when written in hex, but it's 128-bits of data)

You could look at MySQL's BINARY and VARBINARY types. See http://dev.mysql.com/doc/refman/5.1/en/binary-varbinary.html. These types store binary data. In your case, BINARY(16) or VARBINARY(16), but since MD5 hashes are always 16 bytes, the latter seems a bit pointless.

答

You can store MD5 hash in char(32) which is a bit faster than varchar(32). It's also possible to make two BIGINT fields and keep first half of md5 hash in first field and second part in second field.

答

If your field hash is always an MD5 value generated by PHP, then you can safely set it to CHAR(32). This should not impact the response time to your queries, unless you plan to have millions+ of rows, or even worst! JOIN other tables with this field. The bottom line is that fixed width column is better than variable ones, so if you can optimize do it.

Regarding changing MD5 into int values, see this question; the conclusion to this is that if you really want to change your MD5 into a 128-bit int value, you might as well use a random number instead of an MD5!

答

Have you tried creating a BINARY(16) field, and storing the result of md5($plaintext, true); in it? That might work, make sure you index that field as well.

Because trying to fit a 128-bit value in 32 bits doesn't make any sense...

答

You could do this instead:

User-Agents 
{
    id         - INT
    user-agent - TEXT
    hash       - UNSIGNED INT (CRC32, indexed)
}


$crc32 = sprintf("%u", crc32($user_agent));

SELECT * FROM user_agents WHERE hash=$crc32 AND user_agent='$user_agent';

It's unlikely that you'll get collisions with crc32 for this kind of data.

To guarantee that collisions will not cause problems, add a secondary search parameter. MySQL will be able to use the index to quickly find the record. Then it can do a simple string search to guarantee that match is correct.

PS: The sprintf() is there to work around signed 32-bit integers. Should be unnecessary on 64-bit systems.

答

try MurmurHash. Its a fast hashing algo thats been translated to multiple languages. It takes your input and translates it into a 32/64 bit integer hash.

答

Store the UNHEX(MD5($value)) in a BINARY(16).

如何在不使用文本字段的情况下在MySQL数据库中存储哈希值

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