💻IT: Blockchain (Definitions)

"A block chain is a perfect place to store value, identities, agreements, property rights, credentials, etc. Once you put something like a Bit coin into it, it will stay there forever. It is decentralized, disinter mediated, cheap, and censorship-resistant." (Kirti R Bhatele et al, "The Role of Artificial Intelligence in Cyber Security", 2019)

"A system made-up of blocks that are used to record transactions in a peer-to-peer cryptocurrency network such as bitcoins." (Murad Al Shibli, "Hybrid Artificially Intelligent Multi-Layer Blockchain and Bitcoin Cryptology", 2020)

"A chain of blocks containing data that is bundled together. This database is shared across a network of computers (so-called distributed ledger network). Each data block links to the previous block in the blockchain through a cryptographic hash of the previous block, a timestamp, and transaction data. The blockchain only allows data to be written, and once that data has been accepted by the network, it cannot be changed." (Jurij Urbančič et al, "Expansion of Technology Utilization Through Tourism 4.0 in Slovenia", 2020)

"A system in which a record of transactions made in Bitcoin or another cryptocurrency is maintained across several computers that are linked in a peer-to-peer network. Amany M Alshawi, "Decentralized Cryptocurrency Security and Financial Implications: The Bitcoin Paradigm", 2020)

"An encrypted ledger that protects transaction data from modification." (David T A Wesley, "Regulating the Internet, Encyclopedia of Criminal Activities and the Deep Web", 2020)

"Blockchain is a decentralized, immutable, secure data repository or digital ledger where the data is chronologically recorded. The initial block named as Genesis. It is a chain of immutable data blocks what has anonymous individuals as nodes who can transact securely using cryptology. Blockchain technology is subset of distributed ledger technology." (Umit Cali & Claudio Lima, "Energy Informatics Using the Distributed Ledger Technology and Advanced Data Analytics", 2020)

"Blockchain is a meta-technology interconnected with other technologies and consists of several architectural layers: a database, a software application, a number of computers connected to each other, peoples’ access to the system and a software ecosystem that enables development. The blockchain runs on the existing stack of Internet protocols, adding an entire new tier to the Internet to ensure economic transactions, both instant digital currency payments and complicated financial contracts." (Aslı Taşbaşı et al, "An Analysis of Risk Transfer and Trust Nexus in International Trade With Reference to Turkish Data", 2020) 

"Is a growing list of records, called blocks, which are linked using cryptography. Each block contains a cryptographic hash of the previous block a timestamp, and transaction data. (Vardan Mkrttchian, "Perspective Tools to Improve Machine Learning Applications for Cyber Security", 2020)

"This is viewed as a mechanism to provide further protection and enhance the security of data by using its properties of immutability, auditability and encryption whilst providing transparency amongst parties who may not know each other, so operating in a trustless environment." (Hamid Jahankhani & Ionuț O Popescu, "Millennials vs. Cyborgs and Blockchain Role in Trust and Privacy", 2020)

"A blockchain is a data structure that represents the record of each accounting move. Each account transaction is signed digitally to protect its authenticity, and no one can intervene in this transaction." (Ebru E Saygili & Tuncay Ercan, "An Overview of International Fintech Instruments Using Innovation Diffusion Theory Adoption Strategies", 2021)

"A system in which a record of transactions made in bitcoin or another cryptocurrency are maintained across several computers that are linked in a peer-to-peer network." (Silvije Orsag et al, "Finance in the World of Artificial Intelligence and Digitalization", 2021)

"It is a decentralized computation and information sharing platform that enables multiple authoritative domains, who don’t trust each other, to cooperate, coordinate and collaborate in a rational decision-making process." (Vinod Kumar & Gotam Singh Lalotra, "Blockchain-Enabled Secure Internet of Things", 2021)

"A concept consisting of the methods, technologies, and tool sets to support a distributed, tamper-evident, and reliable way to ensure transaction integrity, irrefutability, and non-repudiation. Blockchains are write-once, append-only data stores that include validation, consensus, storage, replication, and security for transactions or other records." (Forrester)

[hybrid blockchain:] "A network with a combination of characteristics of public and private blockchains where a blockchain may incorporate select privacy, security and auditability elements required by the implementation." (AICPA)

[private blockchain:] "A restricted access network controlled by an entity or group which is similar to a traditional centralized network." (AICPA)

"A technology that records a list of records, referred to as blocks, that are linked using cryptography. Each block contains a cryptographic hash of the previous block, a timestamp and transaction data." (AICPA)

[public blockchain:] "An open network where participants can view, read and write data, and no one participant has control (e.g., Bitcoin, Ethereum)." (AICPA)

🔬Data Science: Hash Function (Definition)

"A function that maps a set of keys onto a set of addresses." (S. Sumathi & S. Esakkirajan, "Fundamentals of Relational Database Management Systems", 2007)

"A function that maps a string of arbitrary length to a fixed size value in a deterministic manner. Such a function may or may not have cryptographic applications." (Mark S Merkow & Lakshmikanth Raghavan, "Secure and Resilient Software Development", 2010)

[cryptographic hash function:] "A function that takes an input string of arbitrary length and produces a fixed-size output for which it is unfeasible to find two inputs that map to the same output, and it is unfeasible to learn anything about the input from the output." (Mark S Merkow & Lakshmikanth Raghavan, "Secure and Resilient Software Development", 2010)

[one-way hash function:] "A hash function for which it is computationally unfeasible to determine anything about the input from the output." (Mark S Merkow & Lakshmikanth Raghavan, "Secure and Resilient Software Development", 2010)

"A function that operates on an arbitrary-length input value and returns a fixed-length hash value." (Oracle, "Database SQL Tuning Guide Glossary", 2013)

[one-way hash:] "A one-way hash is an algorithm that transforms one string into another string (a fixed-length sequence of seemingly random characters) in such a way that the original string cannot be calculated by operations on the one-way hash value (i.e., the calculation is one way only). One-way hash values can be calculated for any string, including a person’s name, a document, or an image. For any input string, the resultant one-way hash will always be the same. If a single byte of the input string is modified, the resulting one-way hash will be changed and will have a totally different sequence than the one-way hash sequence calculated for the unmodified string. One-way hash values can be made sufficiently long (e.g., 256 bits) that a hash string collision (i.e., the occurrence of two different input strings with the same one-way hash output value) is negligible." (Jules H Berman, "Principles of Big Data: Preparing, Sharing, and Analyzing Complex Information", 2013)

"A hash function is an algorithm that maps from an input, for example, a string of characters, to an output string. The size of the input can vary, but the size of the output is always the same." (Dan Sullivan, "NoSQL for Mere Mortals®", 2015)

[one-way hash:] "Cryptographic process that takes an arbitrary amount of data and generates a fixed-length value. Used for integrity protection." (Adam Gordon, "Official (ISC)2 Guide to the CISSP CBK" 4th Ed., 2015)

"A function that takes as input the key of an element and produces an integer as output" (Nell Dale et al, "Object-Oriented Data Structures Using Java" 4th Ed., 2016)

"encryption methods that use no keys." (Manish Agrawal, "Information Security and IT Risk Management", 2014)

"A function that operates on an arbitrary-length input value and returns a fixed-length hash value." (Oracle, "Oracle Database Concepts")

⛏️Data Management: Hashing (Definitions)

"A technique for providing fast access to data based on a key value by determining the physical storage location of that data." (Jan L Harrington, "Relational Database Dessign: Clearly Explained" 2nd Ed., 2002)

"A mathematical technique for assigning a unique number to each record in a file." (S. Sumathi & S. Esakkirajan, "Fundamentals of Relational Database Management Systems", 2007)

"A technique that transforms a key value via an algorithm to a physical storage location to enable quick direct access to data. The algorithm is typically referred to as a randomizer, because the goal of the hashing routine is to spread the key values evenly throughout the physical storage." (Craig S Mullins, "Database Administration", 2012)

"A mathematical technique in which an infinite set of input values is mapped to a finite set of output values, called hash values. Hashing is useful for rapid lookups of data in a hash table." (Oracle, "Database SQL Tuning Guide Glossary", 2013)

"An algorithm converts data values into an address" (Daniel Linstedt & W H Inmon, "Data Architecture: A Primer for the Data Scientist", 2014)

"The technique used for ordering and accessing elements in a collection in a relatively constant amount of time by manipulating the element’s key to identify the element’s location in the collection" (Nell Dale et al, "Object-Oriented Data Structures Using Java" 4th Ed., 2016)

"The application of an algorithm to a search key to derive a physical storage location." (George Tillmann, "Usage-Driven Database Design: From Logical Data Modeling through Physical Schmea Definition", 2017)

"Hashing is the process of mapping data values to fixed-size hash values (hashes). Common hashing algorithms are Message Digest 5 (MD5) and Secure Hashing Algorithm (SHA). It’s impossible to turn a hash value back into the original data value." (Piethein Strengholt, "Data Management at Scale", 2020)

"A mathematical technique in which an infinite set of input values is mapped to a finite set of output values, called hash values. Hashing is useful for rapid lookups of data in a hash table." (Oracle, "Oracle Database Concepts")

"A process used to convert data into a string of numbers and letters." (AICPA)

"A technique for arranging a set of items, in which a hash function is applied to the key of each item to determine its hash value. The hash value identifies each item's primary position in a hash table, and if this position is already occupied, the item is inserted either in an overflow table or in another available position in the table." (IEEE 610.5-1990)

⛏️Data Management: Hash (Definitions)

"A number (often a 32-bit integer) that is derived from column values using a lossy compression algorithm. DBMSs occasionally use hashing to speed up access, but indexes are a more common mechanism." (Peter Gulutzan & Trudy Pelzer, "SQL Performance Tuning", 2002)

"A set of characters generated by running text data through certain algorithms. Often used to create digital signatures and compare changes in content." (Tom Petrocelli, "Data Protection and Information Lifecycle Management", 2005)

"Hash, a mathematical method for creating a numeric signature based on content; these days, often unique and based on public key encryption technology." (Bo Leuf, "The Semantic Web: Crafting infrastructure for agency", 2006)

[hash code:] "An integer calculated from an object. Identical objects have the same hash code. Generated by a hash method." (Michael Fitzgerald, "Learning Ruby", 2007)

"An unordered collection of data where keys and values are mapped. Compare with array." (Michael Fitzgerald, "Learning Ruby", 2007)

"A cryptographic hash is a fixed-size bit string that is generated by applying a hash function to a block of data. Secure cryptographic hash functions are collision-free, meaning there is a very small possibility of generating the same hash for two different blocks of data. A secure cryptographic hash function should also be one-way, meaning it is infeasible to retrieve the original text from the hash." (Michael Coles & Rodney Landrum, "Expert SQL Server 2008 Encryption", 2008)

"A hash is the result of applying a mathematical function or transformation on data to generate a smaller 'fingerprint' of the data. Generally, the most useful hash functions are one-way collision-free hashes that guarantee a high level of uniqueness in their results." (Michael Coles, "Pro T-SQL 2008 Programmer's Guide", 2008)

"The output of a hash function." (Mark S Merkow & Lakshmikanth Raghavan, "Secure and Resilient Software Development", 2010)

"A number based on the hash value of a string." (DAMA International, "The DAMA Dictionary of Data Management", 2011)

"1.Data allocated in an algorithmically randomized fashion in an attempt to evenly distribute data and smooth access patterns. 2.Verb. To calculate a hash key for data." (DAMA International, "The DAMA Dictionary of Data Management", 2011)

"A hash is the result of applying a mathematical function or transformation on data to generate a smaller 'fingerprint' of the data. Generally, the most useful hash functions are one-way collision-free hashes that guarantee a high level of uniqueness in their results." (Jay Natarajan et al, "Pro T-SQL 2012 Programmer's Guide" 3rd Ed., 2012)

"An unordered association of key/value pairs, stored such that you can easily use a string key to look up its associated data value. This glossary is like a hash, where the word to be defined is the key and the definition is the value. A hash is also sometimes septisyllabically called an “associative array”, which is a pretty good reason for simply calling it a 'hash' instead." (Jon Orwant et al, "Programming Perl" 4th Ed., 2012)

"In a hash cluster, a unique numeric ID that identifies a bucket. Oracle Database uses a hash function that accepts an infinite number of hash key values as input and sorts them into a finite number of buckets. Each hash value maps to the database block address for the block that stores the rows corresponding to the hash key value (department 10, 20, 30, and so on)." (Oracle, "Database SQL Tuning Guide Glossary", 2013)

"The result of applying a mathematical function or transformation to data to generate a smaller 'fingerprint' of the data. Generally, the most useful hash functions are one-way, collision-free hashes that guarantee a high level of uniqueness in their results." (Miguel Cebollero et al, "Pro T-SQL Programmer’s Guide" 4th Ed., 2015)

[hash code:] "The output of the hash function that is associated with the input object" (Nell Dale et al, "Object-Oriented Data Structures Using Java" 4th Ed., 2016)

"A numerical value produced by a mathematical function, which generates a fixed-length value typically much smaller than the input to the function. The function is many to one, but generally, for all practical purposes, each file or other data block input to a hash function yields a unique hash value." (William Stallings, "Effective Cybersecurity: A Guide to Using Best Practices and Standards", 2018)

"The number generated by a hash function to indicate the position of a given item in a hash table." (IEEE 610.5-1990)

🛢DBMS: Hash Table (Definitions)

"A data structure used internally by Perl for implementing associative arrays (hashes) efficiently. See also bucket." (Jon Orwant et al, "Programming Perl" 4th Ed., 2012)

[hash cluster:] "A type of table cluster that is similar to an indexed cluster, except the index key is replaced with a hash function. No separate cluster index exists. In a hash cluster, the data is the index." (Oracle, "Database SQL Tuning Guide Glossary", 2013)

"An in-memory data structure that associates join keys with rows in a hash join. For example, in a join of the employees and departments tables, the join key might be the department ID. A hash function uses the join key to generate a hash value. This hash value is an index in an array, which is the hash table." (Oracle, "Database SQL Tuning Guide Glossary", 2013)

"The data structure used to store elements using hashing" (Nell Dale et al, "Object-Oriented Data Structures Using Java" 4th Ed., 2016)

"An object that is like a dictionary or an associative array. A hash table stores and retrieves elements using key values called hashcodes. See also hashcode." (Daniel Leuck et al, "Learning Java" 5th Ed., 2020)

[sorted hash cluster:] "A hash cluster that stores the rows corresponding to each value of the hash function in such a way that the database can efficiently return them in sorted order. The database performs the optimized sort internally." (Oracle, "Oracle Database Concepts")

"An in-memory data structure that associates join keys with rows in a hash join. For example, in a join of the employees and departments tables, the join key might be the department ID. A hash function uses the join key to generate a hash value. This hash value is an index in an array, which is the hash table." (Oracle, "Oracle Database Concepts")

"A two-dimensional table of items in which a hash function is applied to the key of each item to determine its hash value. The hash value identifies each item's primary position in the table, and if this position is already occupied, the item is inserted either in an overflow table or in another available position in the table." (IEEE 610.5-1990)

🛢DBMS: Hash Index (Definitions)

"A hashing algorithm is used to organize an index into a sequence, where each indexed value is retrievable based on the result of the hash key value. Hash indexes are efficient with integer values, but are usually subject to overflow as a result of changes." (Gavin Powell, "Beginning Database Design", 2006)

"An index based on an ordered list of hash values." (Carlos Coronel et al, "Database Systems: Design, Implementation, and Management" 9th Ed., 2011)

"An index based on an ordered list of hash values." (Carlos Coronel & Steven Morris, "Database Systems: Design, Implementation, & Management" 11th Ed., 2014)

 "A type of index intended for queries that use equality operators, rather than range operators such as greater-than or BETWEEN. It is available for MEMORY tables. Although hash indexes are the default for MEMORY tables for historic reasons, that storage engine also supports B-tree indexes, which are often a better choice for general-purpose queries. MySQL includes a variant of this index type, the adaptive hash index, that is constructed automatically for InnoDB tables if needed based on runtime conditions." (MySQL, "MySQL 8.0 Reference Manual Glossary")

[adaptive hash index:] "An optimization for InnoDB tables that can speed up lookups using - and IN operators, by constructing a hash index in memory. MySQL monitors index searches for InnoDB tables, and if queries could benefit from a hash index, it builds one automatically for index pages that are frequently accessed." (MySQL, "MySQL 8.0 Reference Manual Glossary")

"Hash indexes are file structures that can be used either to resolve queries by accessing the index instead of its underlying base table or to enhance access performance when they do not cover a query by providing a secondary access path to requested base table rows. They can either substitute for or point to base table rows." (Teradata)

🛢DBMS: Hash Joins (Definitions)

"A sophisticated join algorithm that builds an interim structure to derive result sets." (Microsoft Corporation, "SQL Server 7.0 System Administration Training Kit", 1999)

"A method for producing a joined table. Given two input tables Table1 and Table2, processing is as follows: (a) For each row in Table1, produce a hash. Assign the hash to a hash bucket. (b) For each row in Table2, produce a hash. Check if the hash is already in the hash bucket. If it is: there's a join. If it is not: there's no join." (Peter Gulutzan & Trudy Pelzer, "SQL Performance Tuning", 2002)

"An efficient method of searching two tables to be joined when they have very low selectivity (i.e., very few matching values). Common values are matched in fast memory, then the rest of the data record is obtained using hashing mechanisms to access the disk only once for each record." (Sam Lightstone et al, "Physical Database Design: The Database Professional’s Guide to Exploiting Indexes, Views, Storage, and More", 2007)

"A method for joining large data sets. The database uses the smaller of two data sets to build a hash table on the join key in memory. It then scans the larger data set, probing the hash table to find the joined rows." (Oracle, "Database SQL Tuning Guide Glossary", 2013)

"The hash join is based on a hash function that provides access to items in the joining data structure in constant time. A hash function maps arbitrary inputs to fixed length keys, even though the inputs might have variable lengths. The joining data structure for the hash join is a so-called hash map, which implements an associative array that maps keys to values." (Hasso Plattner, "A Course in In-Memory Data Management: The Inner Mechanics of In-Memory Databases" 2nd Ed., 2014)

 "A join in which the database uses the smaller of two tables or data sources to build a hash table in memory. The database scans the larger table, probing the hash table for the addresses of the matching rows in the smaller table." (Oracle, "Oracle Database Concepts")