hashing strings to integers for sqlite3 keys
Tim Chase
python.list at tim.thechases.com
Thu May 22 09:09:59 EDT 2014
On 2014-05-22 12:47, Adam Funk wrote:
> I'm using Python 3.3 and the sqlite3 module in the standard library.
> I'm processing a lot of strings from input files (among other
> things, values of headers in e-mail & news messages) and suppressing
> duplicates using a table of seen strings in the database.
>
> It seems to me --- from past experience with other things, where
> testing integers for equality is faster than testing strings, as
> well as from reading the SQLite3 documentation about INTEGER
> PRIMARY KEY --- that the SELECT tests should be faster if I am
> looking up an INTEGER PRIMARY KEY value rather than TEXT PRIMARY
> KEY. Is that right?
If sqlite can handle the absurd length of a Python long, you *can* do
it as ints:
>>> from hashlib import sha1
>>> s = "Hello world"
>>> h = sha1(s)
>>> h.hexdigest()
'7b502c3a1f48c8609ae212cdfb639dee39673f5e'
>>> int(h.hexdigest(), 16)
703993777145756967576188115661016000849227759454L
That's a pretty honkin' huge int for a DB key, but you can use it.
And it's pretty capped on length regardless of the underlying
string's length.
> If so, what sort of hashing function should I use? The "maxint" for
> SQLite3 is a lot smaller than the size of even MD5 hashes. The only
> thing I've thought of so far is to use MD5 or SHA-something modulo
> the maxint value. (Security isn't an issue --- i.e., I'm not
> worried about someone trying to create a hash collision.)
You could truncate that to something like
>>> int(h.hexdigest()[-8:], 16)
which should give you something that would result in a 32-bit number
that should fit in sqlite's int.
-tkc
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