In relational database business there is offen a need to compare data of two tables with the same structure so see if they are identical or not. If the hash table size M is small compared to the resulting summations, then this hash function should do a good job of distributing strings evenly among the hash table slots, because it gives equal weight to all characters in the string. A hash table is a very fast kind of lookup table, somewhat like an alist (see Association Lists) in that it maps keys to corresponding values.It differs from an alist in these ways: Lookup in a hash table is extremely fast for large tables—in fact, the time required is essentially independent of how many elements are stored in the table. If a new item has a collision in the first hash table, the collided item is moved to the second hash table using the second hash function… A common method to detect if a row has changed is to compare hashes: if the hash of the incoming record is different from the hash found in the destination table, one or more columns have changed. But their hash code can be different if computed with default std::hash<:string>. Value doesn’t vary even if there is a large or small value. XOR/table Fowler–Noll–Vo hash function (FNV Hash) 32, 64, 128, 256, 512, or 1024 bits xor/product or product/XOR Jenkins hash function: 32 or 64 bits XOR/addition Bernstein's hash djb2: 32 or 64 bits shift/add or mult/add or shift/add/xor or mult/xor PJW hash / Elf Hash: 32 or 64 bits add,shift,xor Another use is in cryptography, the science of encoding, and safeguarding data. Hash functions used to accelerate table lookup or data comparison tasks such as finding items in a database, detecting duplicated or similar records in a large file, finding similar stretches in DNA sequences, and other data-driven tasks. Compare Use Case . Hash function is basically performing some calculations in the computer.Data values that are its output is of fixed length. Custom hasher Nemec . Hash table benchmarks usually include theoretical metrics such as the number of collisions or distribution uniformity (see, for example, hash function comparison in the Red Dragon book). The hash function can be described as − h(k) = k mod n. Here, h(k) is the hash value obtained by dividing the key value k by size of hash table n using the remainder. A hash value, from 0 to (HashTableSize - 1), is computed by dividing the key value by the size of the hash table and taking the remainder: Public Function Hash(ByVal Key As Long) As Long Hash = Key Mod HashTableSize End Function Selecting an appropriate HashTableSize is important to the success of this method. 8 Hash Tables. Comparing Tables With these two functions together it’s dead simple to compare the data in two tables. Therefore, we need custom hasher function, so that elements which are equal based on above compare function should have same hash code. This could be a validation, whether two tables from different instancies are in sync or a decision of a JUnit test whether the expected results … Length always varies according to the hash function. This is the easiest method to create a hash function. The following are some of the Hash Functions − Division Method. Comparing Data of Two Oracle Tables Using MD5 Hash Jaromir D.B. For example, according to above compare function “abc” and “def” are equal because their length is same. Cuckoo Hashing  used two hash tables and two different hash functions respectively; each item, is either in the first hash table or in the other one. This is an example of the folding approach to designing a hash function. Obviously, you will have a better distribution with more complex functions, so they are winners in these benchmarks. Different hash functions are given below: Hash Functions. We need to create a checksum for the entire table, this can be done simply by first generating a checksum for each row and then using CHECKSUM_AGG() to give us an aggregated checksum for the table.