Redis HMSET command documentation describes it as:
"Sets the specified fields to their respective values in the hash stored at key. This command overwrites any existing fields in the hash. If key does not exist, a new key holding a hash is created."
What does the word 'hash' mean in this case? Does it mean a hash table? Or, hash code computed for the given the field,value pairs? I would like to think it means the former, i.e., a hash table, but I would still like to clarify as the documentation is not explicit?
Hash refers to the Redis Hash Data-Type:
Redis Hashes are maps between string fields and string values, so they
are the perfect data type to represent objects (e.g. A User with a
number of fields like name, surname, age, and so forth)
Related
I have a table which uses the snowflake hash function to store values in some columns.
Is there any way to reverse the encrytion from the hash function and get the original values from the table?
As per the documentation, the function is not "not a cryptographic hash function", and will always return the same result for the same input expression.
Example :
select hash(1) always returns -4730168494964875235
select hash('a') always returns -947125324004678632
select hash('1234') always returns -4035663806895772878
I was wondering if there is any way to reverse the hashing and get the original input expression from the hashed values.
I think these disclaimers are for preventing potential legal disputes:
Cryptographic hash functions have a few properties which this function
does not, for example:
The cryptographic hashing of a value cannot be inverted to find the
original value.
It's not possible to reserve a hash value in general. If you consider that when you even send a very long text, and it is represented in a 64-bit value, it's obvious that the data is not preserved. On the other hand, if you use a brute force technique, you may find the actual value producing the hash, and it can be counted as reserving the hash value.
For example, if you store all hash values for the numbers between 0 and 5000 in a table, when I came with hash value '-7875472545445966613', you can look up that value in your table, and say it belongs to 1000 (number).
I'm having trouble understanding what the Hash Function does and doesn't do, as well as what exactly a Bucket is.
From my understanding:
A HashTable is a data structure that maps keys to values using a Hash Function.
A HashFunction is meant to map data from an array of arbitrary/unknown size to a data array of fixed size.
There can be duplicate Values in the original data array, but this is irrelevant.
Each Value will have a unique Key. Thus, each Key has exactly 1 Value.
The HashFunction will generate a HashCode for each (Value, Key) pair. However, Collisions can occur in which multiple (Value, Key) pairs map to the same HashCode.
This can be remedied by using either Chaining/Open Addressing methods.
The HashCode is the index value indicating the position of a particular entry from the original data array within the Bucket array.
The Bucket array is the fixed data array constructed that will contain the entries from the original array.
My questions:
How are the Keys generated for each value? Is the HashFunction meant to generate both Key and HashCode values for each entry? Does each Bucket thus contain only one entry (assuming a Chaining implementation to remedy Collision)?
How are the Keys generated for each value?
Key is not generated, it is provided by you and serves as an input to the hash function which in turn converts that key into index of hash table. Simply speaking:
H(key)=index
so the value you are looking for is:
hash_table[index] = value
Is the HashFunction meant to generate HashCode values for each entry?
It all depends on the implementation of hash function and hash table. Some hash functions might generate a hashcode out of provided key and then for example take its modulo(size) where size is the size of hash table, in order to get the index. Others might convert the key directly into index. In either case the ultimate goal of hash function is to find the location of searched data within hash table in constant time.
Does each Bucket thus contain only one entry (assuming a Chaining implementation to remedy Collision)?
Ideally each key should be mapped to a unique index but mostly that's not the case since the number of buckets (i.e. indices) is far smaller than the number of keys so the average length of a chain per bucket (i.e. number of collisions per bucket) is no.of keys/no.of indices
This question is not specific to any programming language, I am more interested in a generic logic.
Generally, associative maps take a key and map it to a value. As far as I know, implementations require the keys to be unique otherwise values get overwritten. Alright.
So let us assume that the above is done by some hash implementation.
What if two DIFFERENT keys get the same hash value? I am thinking of this in the form of an underlying array whose indices are in a result of hash on said keys. It could be possible that more than one unique key gets mapped to the same value yes? If so, how does such an implementation handle this?
How is handling same hash different from handling same key? Since same key results in overwriting and same hash HAS to retain the value.
I understand hashing with collision, so I know chaining and probing. Do implementations iterate over the current values which are hashed to a particular index and determine if the key is the same?
While I was searching for the answer I came across these links:
1. What happens when a duplicate key is put into a HashMap?
2. HashMap with multiple values under the same key
They don't answer my question however. How do we distinguish between same hash vs same key?
By comparing the keys. If you look at object-oriented implementations of hash maps, you'll find that they usually require two methods to be implemented on the key type:
bool equal(Key key1, Key key2);
int hash(Key key);
If only the hash function can be given and no equality function, that restricts the hash map to be based on the language's default equality. This is not always desirable as sometimes keys need to be compared with a different equality function. For example, if the keys are strings, an application may need to do a case-insensitive comparison, and then it would pass a hash function that converts to lowercase before hashing, and an equal function that ignores case.
The hash map stores the key alongside each corresponding value. (Usually, that's a pointer to the key object that was originally stored.) Any lookup into the hash map has to make a key comparison after finding a matching hash, to verify that the key actually matches.
For example, for a very simple hash map that stores a list in each bucket, the list would be a list of (key, value) pairs, and any lookup compares the keys of each list entry until it finds a match. In pseudocode:
Array<List<Pair<Key, Value>>> buckets;
Value lookup(Key k_sought) {
int h = hash(k_sought);
List<Pair<Key, Value>> bucket = buckets[h];
for (kv in bucket) {
Key k_found = kv.0;
Value v_found = kv.1;
if (equal(k_sought, k_found)) {
return v_found;
}
}
throw Not_found;
}
You can not tell what a key is from the index, so no you can not iterate over the values to find any information about the keys. You will either have to guarantee 0 collisions or store the information that was hashed to give the index.
If you only have values stored in your structure, there is no way to tell if they have the same key or just the same hash. You will need to store the key along with the value to know.
I implemented a hashmap based on cuckoo hashing.
My hash functions take values of any length and return keys of type long. To match the keys to my array size n, I do key % n.
I'm thinking about following scenario:
Insert value A with key A.key into location A.key % n
Find value B with key A.key
So for this example I get the entry for value A and it is not recognized that value B hasn't even been inserted. This happens if my hash function returns the same key for two different values. Collisions with different keys but same locations are no problem.
What is the best way to detect those collisions?
Do I have to check every time I insert or search an item if the original values are equal?
As with most hashing schemes, in cuckoo hashing, the hash code tells you where to look in the table for the element in question, but the expectation is that you store both the key and the value in the table so that before returning the stored value, you first check the key stored at that slot against the key you're looking for. That way, if you get the same hash code for two objects, you can determine which object was stored at that slot.
I want to have some objects data in redis and I want to display all objects in a table.
in SQL I would just get the entire row for all object and display it in a view
in redis, I don't want to query each hash separately, since that will be unbearable slow.
Assuming I know the hash keys and the hash names I want to pull, Is there a way to do this effienctly?
I'm not sure why you believe querying each hash would be unbearably slow. If you loop through your hash keys and do an HMGET for each with the field names you should be good, provided you pipeline the requests.
Alternatively, you could do this in a Lua script that accepts (some of) the key names as KEYS and the fields as ARGV, returnint the answer in whatever format you need it.
Store all hash key in a set, let's called it 'hashkeyset'
Use 'sort' command to retrieve all hash values sort hashkeyset get *->field0 get *->field1 ... get *->fieldN
You can find more about 'sort' in this link http://redis.io/commands/sort