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Storing large strings for future equality checks

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  Storing large strings for future equality checks Abu Yahya <abu_yahya@invalid.com> - 2011-06-08 22:05 +0530
    Re: Storing large strings for future equality checks markspace <-@.> - 2011-06-08 09:49 -0700
      Re: Storing large strings for future equality checks Willem <willem@toad.stack.nl> - 2011-06-08 17:28 +0000
        Re: Storing large strings for future equality checks Abu Yahya <abu_yahya@invalid.com> - 2011-06-08 23:45 +0530
      Re: Storing large strings for future equality checks Abu Yahya <abu_yahya@invalid.com> - 2011-06-08 23:45 +0530
    Re: Storing large strings for future equality checks David Kerber <dkerber@WarrenRogersAssociates.invalid> - 2011-06-08 12:58 -0400
      Re: Storing large strings for future equality checks Abu Yahya <abu_yahya@invalid.com> - 2011-06-08 23:49 +0530
      Re: Storing large strings for future equality checks Lothar Kimmeringer <news200709@kimmeringer.de> - 2011-06-08 20:31 +0200
    Re: Storing large strings for future equality checks Gene Wirchenko <genew@ocis.net> - 2011-06-08 11:07 -0700
      Re: Storing large strings for future equality checks Abu Yahya <abu_yahya@invalid.com> - 2011-06-08 23:58 +0530
      Re: Storing large strings for future equality checks Hallvard B Furuseth <h.b.furuseth@usit.uio.no> - 2011-06-09 12:38 +0200
      Re: Storing large strings for future equality checks Michael Wojcik <mwojcik@newsguy.com> - 2011-06-09 17:32 -0400
        Re: Storing large strings for future equality checks bugbear <bugbear@trim_papermule.co.uk_trim> - 2011-06-10 10:51 +0100
    Re: Storing large strings for future equality checks Lothar Kimmeringer <news200709@kimmeringer.de> - 2011-06-08 20:28 +0200
    Re: Storing large strings for future equality checks rossum <rossum48@coldmail.com> - 2011-06-08 21:38 +0100
    Re: Storing large strings for future equality checks Robert Klemme <shortcutter@googlemail.com> - 2011-06-08 23:20 +0200
    Re: Storing large strings for future equality checks Tom Anderson <twic@urchin.earth.li> - 2011-06-08 23:02 +0100
    Re: Storing large strings for future equality checks Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-09 15:01 -0700

#458 — Storing large strings for future equality checks

A small application that I'm making requires me to store very long 
strings (>1000 characters) in a database.

I will need to use these strings later to compare for equality to 
incoming strings from another application. I will also want to add some 
of the incoming strings to the storage, if they meet certain criteria.

For my application, I get a feeling that storing these strings in my 
table will be a waste of space, and will impact performance due to 
retrieval and storage times, as well as comparison times.

I considered using an SHA-512 hash of these strings and storing them in 
the database. However, while these will save on storage space, it will 
take time to do the hashing before comparing an incoming string. So I'm 
still wasting time. (Collisions due to hashing will not be a problem, 
since an occasional false positive will not be fatal for my application).

What would be the best approach?

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#459

On 6/8/2011 9:35 AM, Abu Yahya wrote:

> I considered using an SHA-512 hash of these strings and storing them in
> the database. However, while these will save on storage space, it will
> take time to do the hashing before comparing an incoming string. So I'm
> still wasting time. (Collisions due to hashing will not be a problem,
> since an occasional false positive will not be fatal for my application).


You have to store the whole string.  Even if the SHA-512 hash codes are 
equal, it could be that the strings are different.  You'll have to 
eventually compare the raw string, even if the SHA is used as a 
quick-out case.

No one can really tell what is "faster" or "wasting time" until you can 
better characterize the usage patterns.  How big can these strings get? 
  How often will you get an actual duplicate?  What's the penalty when 
you need to add a new string?  You'll need to implement a few 
algorithms, profile them and then make a decision based on actual data.

For Java, I'd store the strings in a WeakHashMap or similar to allow 
them to be cached, but tossed away when more storage is needed.  Also 
you should look into getting some DB caching library, much easier than 
implementing this yourself (sorry I can't personally recommend any).

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#461

markspace wrote:
) On 6/8/2011 9:35 AM, Abu Yahya wrote:
)> I considered using an SHA-512 hash of these strings and storing them in
)> the database. However, while these will save on storage space, it will
)> take time to do the hashing before comparing an incoming string. So I'm
)> still wasting time. (Collisions due to hashing will not be a problem,
)> since an occasional false positive will not be fatal for my application).
)
) You have to store the whole string.  Even if the SHA-512 hash codes are 
) equal, it could be that the strings are different.  You'll have to 
) eventually compare the raw string, even if the SHA is used as a 
) quick-out case.

No he doesn't.  Read again.  Especially the last bit between parentheses.


SaSW, Willem
-- 
Disclaimer: I am in no way responsible for any of the statements
            made in the above text. For all I know I might be
            drugged or something..
            No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT

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#464

On 6/8/2011 10:58 PM, Willem wrote:
> markspace wrote:
> ) On 6/8/2011 9:35 AM, Abu Yahya wrote:
> )>  I considered using an SHA-512 hash of these strings and storing them in
> )>  the database. However, while these will save on storage space, it will
> )>  take time to do the hashing before comparing an incoming string. So I'm
> )>  still wasting time. (Collisions due to hashing will not be a problem,
> )>  since an occasional false positive will not be fatal for my application).
> )
> ) You have to store the whole string.  Even if the SHA-512 hash codes are
> ) equal, it could be that the strings are different.  You'll have to
> ) eventually compare the raw string, even if the SHA is used as a
> ) quick-out case.
>
> No he doesn't.  Read again.  Especially the last bit between parentheses.
>

Yeah, it seems he missed that last part or read it incorrectly.

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#463

On 6/8/2011 10:19 PM, markspace wrote:
> On 6/8/2011 9:35 AM, Abu Yahya wrote:
>
>> I considered using an SHA-512 hash of these strings and storing them in
>> the database. However, while these will save on storage space, it will
>> take time to do the hashing before comparing an incoming string. So I'm
>> still wasting time. (Collisions due to hashing will not be a problem,
>> since an occasional false positive will not be fatal for my application).
>
>
> You have to store the whole string. Even if the SHA-512 hash codes are
> equal, it could be that the strings are different. You'll have to
> eventually compare the raw string, even if the SHA is used as a
> quick-out case.

You're right about comparing the whole strings anyway, but, for this 
application I'm creating, I wouldn't mind very very rare incorrect results.

>
> No one can really tell what is "faster" or "wasting time" until you can
> better characterize the usage patterns. How big can these strings get?
> How often will you get an actual duplicate? What's the penalty when you
> need to add a new string? You'll need to implement a few algorithms,
> profile them and then make a decision based on actual data.

That makes sense. I'd need to analyze my input data and then run some 
empirical tests.

>
> For Java, I'd store the strings in a WeakHashMap or similar to allow
> them to be cached, but tossed away when more storage is needed. Also you
> should look into getting some DB caching library, much easier than
> implementing this yourself (sorry I can't personally recommend any).

The WeakHashMap idea looks useful. As for a DB caching library, would 
you recommend this as a replacement for the WeakHashMap, or as a complement?

Thanks for the useful pointers.

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#460

[This followup was posted to comp.lang.java.databases and a copy was 
sent to the cited author.]

In article <iso8cm$a80$1@speranza.aioe.org>, abu_yahya@invalid.com 
says...
> 
> A small application that I'm making requires me to store very long 
> strings (>1000 characters) in a database.

Unless you're storing millions of these strings or using Access, I'd say 
just store and use the string.  I think you'll find that the speed 
penalty is nearly unnoticeable.

D

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#465

On 6/8/2011 10:28 PM, David Kerber wrote:
> [This followup was posted to comp.lang.java.databases and a copy was
> sent to the cited author.]
>
> In article<iso8cm$a80$1@speranza.aioe.org>, abu_yahya@invalid.com
> says...
>>
>> A small application that I'm making requires me to store very long
>> strings (>1000 characters) in a database.
>
> Unless you're storing millions of these strings or using Access, I'd say
> just store and use the string.  I think you'll find that the speed
> penalty is nearly unnoticeable.

Thanks - simply storing them the way they are does seem to be the best 
way forward.

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#468

David Kerber wrote:

> In article <iso8cm$a80$1@speranza.aioe.org>, abu_yahya@invalid.com 
> says...
>> 
>> A small application that I'm making requires me to store very long 
>> strings (>1000 characters) in a database.
> 
> Unless you're storing millions of these strings or using Access, I'd say 
> just store and use the string.  I think you'll find that the speed 
> penalty is nearly unnoticeable.

Depends on the database. Some of them force you to use CLOBS for
text-columns with more than 255 characters. CLOBS are a PITA in
terms of indexing.


Regards, Lothar
-- 
Lothar Kimmeringer                E-Mail: spamfang@kimmeringer.de
               PGP-encrypted mails preferred (Key-ID: 0x8BC3CD81)

Always remember: The answer is forty-two, there can only be wrong
                 questions!

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#462

On Wed, 08 Jun 2011 22:05:30 +0530, Abu Yahya <abu_yahya@invalid.com>
wrote:

>A small application that I'm making requires me to store very long 
>strings (>1000 characters) in a database.
>
>I will need to use these strings later to compare for equality to 
>incoming strings from another application. I will also want to add some 
>of the incoming strings to the storage, if they meet certain criteria.
>
>For my application, I get a feeling that storing these strings in my 
>table will be a waste of space, and will impact performance due to 
>retrieval and storage times, as well as comparison times.

     Your feeling is irrelevant.  You should benchmark.  If you do
not, you may end up jumping through hoops for something that is
unneeded (though you may never find out it that it is unneeded).

>I considered using an SHA-512 hash of these strings and storing them in 
>the database. However, while these will save on storage space, it will 
>take time to do the hashing before comparing an incoming string. So I'm 
>still wasting time. (Collisions due to hashing will not be a problem, 
>since an occasional false positive will not be fatal for my application).

     What does "occasional" mean?

>What would be the best approach?

     If it is a small app, why are you worrying about it so much?
Create a straightforward design, code it, and see if it is fast
enough.  If it is, keep it.  If it is not, then and only then, concern
yourself with how to speed it up.

     Quit wasting time posting here, and benchmark something.  <BEG>

Sincerely,

Gene Wirchenko

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#467

On 6/8/2011 11:37 PM, Gene Wirchenko wrote:
> On Wed, 08 Jun 2011 22:05:30 +0530, Abu Yahya<abu_yahya@invalid.com>
> wrote:
>
>> I considered using an SHA-512 hash of these strings and storing them in
>> the database. However, while these will save on storage space, it will
>> take time to do the hashing before comparing an incoming string. So I'm
>> still wasting time. (Collisions due to hashing will not be a problem,
>> since an occasional false positive will not be fatal for my application).
>
>       What does "occasional" mean?
>

The application will be doing some probabilistic evaluation using the 
data it is storing, and needs to be fairly close to the actual /most/ of 
the time. Because it would anyway be impossible to be correct 100% of 
the time, using a wrong value for a particular piece of input data will 
not matter, provided it does not happen too often. (In my text above, 
"rare" would have been a better choice of word than "occasional")

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#472

Gene Wirchenko writes:
>On Wed, 08 Jun 2011 22:05:30 +0530, Abu Yahya <abu_yahya@invalid.com>
>wrote:
>>For my application, I get a feeling that storing these strings in my 
>>table will be a waste of space, and will impact performance due to 
>>retrieval and storage times, as well as comparison times.
>
>      Your feeling is irrelevant.  You should benchmark.  If you do
> not, you may end up jumping through hoops for something that is
> unneeded (though you may never find out it that it is unneeded).

Indeed.  No point in using a lot of time to solve a non-problem.  But
after the benchmark, the decision can depend on who the application is
for.  If it scales poorly, that can bite other users with different
input data.

OTOH if he'll be the only user and he finds that full strings and SHA
are both too slow: Another approach would be to use a faster hash, count
hash collisions, and don't bother with more if the count is acceptable.

Or try tries, as Tom Anderson suggested.

-- 
Hallvard

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#473

Gene Wirchenko wrote:
> On Wed, 08 Jun 2011 22:05:30 +0530, Abu Yahya <abu_yahya@invalid.com>
> wrote:
> 
>> I considered using an SHA-512 hash of these strings and storing them in 
>> the database. However, while these will save on storage space, it will 
>> take time to do the hashing before comparing an incoming string. So I'm 
>> still wasting time. (Collisions due to hashing will not be a problem, 
>> since an occasional false positive will not be fatal for my application).
> 
>      What does "occasional" mean?

Assuming the application doesn't accidentally run afoul of a
hitherto-unknown flaw in SHA-512 - a tremendously unlikely event - it
means "once in every N/2**256 uses", where N is the current number of
hashes in the database. (2**256 because of the Birthday Paradox; we're
interested in the probability of two arbitrary colliding pre-images.)

Or, in other words, "probably not before the heat death of the
universe". (Or false vacuum decay, zombie apocalypse, Rapture, etc.)

Worrying about the time to hash the string is silly. It's linear in
the length of the string, so it's roughly equivalent to the time taken
to do a few comparisons (where "a few" depends on how long, on
average, the matching prefix of the new string and the candidates is).

However, as various others have pointed out, this is premature
optimization. There's no reason to use any design other than the
obvious until a problem is demonstrated.

-- 
Michael Wojcik
Micro Focus
Rhetoric & Writing, Michigan State University

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#475

Michael Wojcik wrote:
> Gene Wirchenko wrote:
>> On Wed, 08 Jun 2011 22:05:30 +0530, Abu Yahya<abu_yahya@invalid.com>
>> wrote:
>>
>>> I considered using an SHA-512 hash of these strings and storing them in
>>> the database. However, while these will save on storage space, it will
>>> take time to do the hashing before comparing an incoming string. So I'm
>>> still wasting time. (Collisions due to hashing will not be a problem,
>>> since an occasional false positive will not be fatal for my application).
>>
>>       What does "occasional" mean?
>
> Assuming the application doesn't accidentally run afoul of a
> hitherto-unknown flaw in SHA-512 - a tremendously unlikely event - it
> means "once in every N/2**256 uses", where N is the current number of
> hashes in the database. (2**256 because of the Birthday Paradox; we're
> interested in the probability of two arbitrary colliding pre-images.)
>
> Or, in other words, "probably not before the heat death of the
> universe". (Or false vacuum decay, zombie apocalypse, Rapture, etc.)
>
> Worrying about the time to hash the string is silly. It's linear in
> the length of the string, so it's roughly equivalent to the time taken
> to do a few comparisons (where "a few" depends on how long, on
> average, the matching prefix of the new string and the candidates is).
>
> However, as various others have pointed out, this is premature
> optimization. There's no reason to use any design other than the
> obvious until a problem is demonstrated.

I any case, it's plausible that the DB is using "something"
to index the strings, in other words the string search in the DB
already has some-kind-of optimisation.

   BugBear

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#466

F'up to cljp

Abu Yahya wrote:

> I considered using an SHA-512 hash of these strings and storing them in 
> the database. However, while these will save on storage space, it will 
> take time to do the hashing before comparing an incoming string. So I'm 
> still wasting time. (Collisions due to hashing will not be a problem, 
> since an occasional false positive will not be fatal for my application).

If you write seldom and read often, why not using two columns:

string_hashcode
sha1_hashcode

If the first is equal, you can calculate the sha1-hash for the string
to be checked and if that is equal as well, you can consider the
string as equal. That both hashes collide I expect to be very
very unlikely (which is why I changed the other alg to sha-1, that
should be considerably more performant than sha512).

So calculation of the more complex algorithm is only done while
storing to the database and when checking a string that is already
in the database. If you have that case very often you still might
get a better performance with String.hashcode and SHA1 than with
just SHA512.

> What would be the best approach?

There is no single best approach, only an optimal one. Which
one it is dependend on what defines one way to be better than
the other (in terms of performance, storage-space, collision-
rates, etc).


Regards, Lothar
-- 
Lothar Kimmeringer                E-Mail: spamfang@kimmeringer.de
               PGP-encrypted mails preferred (Key-ID: 0x8BC3CD81)

Always remember: The answer is forty-two, there can only be wrong
                 questions!

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#469

On Wed, 08 Jun 2011 22:05:30 +0530, Abu Yahya <abu_yahya@invalid.com>
wrote:

>A small application that I'm making requires me to store very long 
>strings (>1000 characters) in a database.
>
>I will need to use these strings later to compare for equality to 
>incoming strings from another application. I will also want to add some 
>of the incoming strings to the storage, if they meet certain criteria.
>
>For my application, I get a feeling that storing these strings in my 
>table will be a waste of space, and will impact performance due to 
>retrieval and storage times, as well as comparison times.
>
>I considered using an SHA-512 hash of these strings and storing them in 
>the database. However, while these will save on storage space, it will 
>take time to do the hashing before comparing an incoming string. So I'm 
>still wasting time. (Collisions due to hashing will not be a problem, 
>since an occasional false positive will not be fatal for my application).
>
>What would be the best approach?
As others have said, write the simple obvious approach and see if that
is good enough.  Tune where required after measuring.

Lothar's suggestion of using SHA-1 is good.  You could even drop back
to MD-4 if you are sure that nobody is going to be deliberately trying
to create false collisions.  MD-4 is much too badly broken for any
cryptographic purposes, but is even faster than SHA-1.

If the amount of storage needed is a problem then you might want to
zip the strings before storing them.

If you can be sure that the zipped versions are identical (not always
possible with unicode combining characters) then you could hash the
zipped version rather than the originals for more time saving.

rossum

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#470

On 08.06.2011 18:35, Abu Yahya wrote:
> A small application that I'm making requires me to store very long
> strings (>1000 characters) in a database.
>
> I will need to use these strings later to compare for equality to
> incoming strings from another application. I will also want to add some
> of the incoming strings to the storage, if they meet certain criteria.
>
> For my application, I get a feeling that storing these strings in my
> table will be a waste of space, and will impact performance due to
> retrieval and storage times, as well as comparison times.
>
> I considered using an SHA-512 hash of these strings and storing them in
> the database. However, while these will save on storage space, it will
> take time to do the hashing before comparing an incoming string. So I'm
> still wasting time. (Collisions due to hashing will not be a problem,
> since an occasional false positive will not be fatal for my application).
>
> What would be the best approach?

Just out of curiosity: what do those strings represent?  What do you do 
with them?

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

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#471

On Wed, 8 Jun 2011, Abu Yahya wrote:

> A small application that I'm making requires me to store very long 
> strings (>1000 characters) in a database.

So >1000, but by any chance <2000? <4000?

> I will need to use these strings later to compare for equality to 
> incoming strings from another application. I will also want to add some 
> of the incoming strings to the storage, if they meet certain criteria.
>
> For my application, I get a feeling that storing these strings in my 
> table will be a waste of space, and will impact performance due to 
> retrieval and storage times, as well as comparison times.

I assume by 'table' you mean an in-memory hashtable. I wouldn't be so 
quick to discount this - how many strings do you have? If you had 25 000 
2000-character strings, that would be 100 MB; that's not an exorbitant 
amount. Access would be pretty quick.

> I considered using an SHA-512 hash of these strings and storing them in 
> the database. However, while these will save on storage space, it will 
> take time to do the hashing before comparing an incoming string. So I'm 
> still wasting time. (Collisions due to hashing will not be a problem, 
> since an occasional false positive will not be fatal for my 
> application).

If you're using a database, don't bother with a hash, just put the whole 
string in, index the column, and then do equality queries on it. A B-tree 
index will deal with this kind of query pretty efficiently.

If you wanted to pursue in-memory approaches, i'd suggest constructing a 
trie of some sort. Tries are very fast at retrieval, don't need to access 
the whole key to identify a miss, and only need to access the whole key 
once to identify a hit - you always walk through the key from beginning to 
end (perhaps skipping some characters in some kinds of tree), stopping as 
soon as the key doesn't match, and only reaching the end if it does match. 
I haven't found a good overview of the current state of the art in tries, 
but look up Patricia tries, Judy tries, burst tries, fusion tries, and HAT 
tries. You could consider only storing a prefix of the strings - the first 
100 characters, say - in the trie, to save memory, with the leaves having 
pointers to where the complete strings are stored on disk.

tom

-- 
The sun just came out, I can't believe it

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#474

On Jun 8, 9:35 am, Abu Yahya <abu_ya...@invalid.com> wrote:
> A small application that I'm making requires me to store very long
> strings (>1000 characters) in a database.
>
> I will need to use these strings later to compare for equality to
> incoming strings from another application. I will also want to add some
> of the incoming strings to the storage, if they meet certain criteria.
>
> For my application, I get a feeling that storing these strings in my
> table will be a waste of space, and will impact performance due to
> retrieval and storage times, as well as comparison times.
>
> I considered using an SHA-512 hash of these strings and storing them in
> the database. However, while these will save on storage space, it will
> take time to do the hashing before comparing an incoming string. So I'm
> still wasting time. (Collisions due to hashing will not be a problem,
> since an occasional false positive will not be fatal for my application).
>
> What would be the best approach?

If it's that relevant that you're asking, measure first to see if it's
a problem. If you're that concerned that it will be, then code a
number of reasonable alternatives and measure.

Presumably you need to do a Map lookup on the incoming strings. I
thought about some itern scheme, but that won't work if you're
receiving a lot of incoming new strings. Storing hashs could work. Do
you need to store the strings in a database? If you can store them
locally, maybe a trie?
http://en.wikipedia.org/wiki/Trie
I somewhat doubt (maybe?) that you're going to get much better lookup
performance than a trie (but of course I would measure too).

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