How to store hashes in MySQL databases without usi

问题:

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?

回答1:

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.

回答2:

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

回答3:

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.

回答4:

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.

回答5:

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.

回答6:

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.

回答7:

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.

回答8:

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!

回答9:

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...