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The files have names that begin with the table name and have an extension to indicate the file type. MySQL uses a .frm file to store the definition of the table, but this file is not a part of the MyISAM engine; instead it is a part of the server. The data file has a .MYD (MYData) extension. The index file has a .MYI (MYIndex) extension. The ...
Shadow tables are often used with DBMS' to improve efficiency by preventing redundant operations being performed by the DBM. Shadow tables are also easy to implement in most modern DBMS' because they do not affect the original data, so the way the databases and applications accessing them work together is not affected, unless desired. [6]
Columnar storage also allows fast execution of range queries (e.g., show all records where a particular column is between X and Y, or less than X.) In all these cases, however, the database designer does not have to perform 6NF normalization manually by creating separate tables.
In order not to fill the data store with useless information, there is a policy to remove tombstones completely. For this, the system checks the age of the tombstone and removes it after a prescribed time has elapsed. In Apache Cassandra, this elapsed time is set with the GCGraceSeconds parameter [1] and the process is named Compaction. [2]
MySQL (/ ˌ m aɪ ˌ ɛ s ˌ k juː ˈ ɛ l /) [6] is an open-source relational database management system (RDBMS). [6] [7] Its name is a combination of "My", the name of co-founder Michael Widenius's daughter My, [1] and "SQL", the acronym for Structured Query Language.
TiDB (/’taɪdiːbi:/, "Ti" stands for Titanium) is an open-source NewSQL database that supports Hybrid Transactional and Analytical Processing workloads. [3] Designed to be MySQL compatible, it is developed and supported primarily by PingCAP and licensed under Apache 2.0.
In summary, PostgreSQL fully supports schemas and multi-tenancy by strictly separating databases from each other and thus lacks some functionality MySQL has with databases, while MySQL does not even attempt to support standard schemas. Oracle has its own spin where creating a user is synonymous with creating a schema.
In SQL, the unique keys have a UNIQUE constraint assigned to them in order to prevent duplicates (a duplicate entry is not valid in a unique column). Alternate keys may be used like the primary key when doing a single-table select or when filtering in a where clause, but are not typically used to join multiple tables.