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  2. Database index - Wikipedia

    en.wikipedia.org/wiki/Database_index

    To process this statement without an index the database software must look at the last_name column on every row in the table (this is known as a full table scan). With an index the database simply follows the index data structure (typically a B-tree) until the Smith entry has been found; this is much less computationally expensive than a full ...

  3. Full table scan - Wikipedia

    en.wikipedia.org/wiki/Full_table_scan

    Full table scan occurs when there is no index or index is not being used by SQL. And the result of full scan table is usually slower that index table scan. The situation is that: the larger the table, the slower of the data returns. Unnecessary full-table scan will lead to a huge amount of unnecessary I/O with a process burden on the entire ...

  4. DBSCAN - Wikipedia

    en.wikipedia.org/wiki/DBSCAN

    Consider a set of points in some space to be clustered. Let ε be a parameter specifying the radius of a neighborhood with respect to some point. For the purpose of DBSCAN clustering, the points are classified as core points, (directly-) reachable points and outliers, as follows:

  5. Determining the number of clusters in a data set - Wikipedia

    en.wikipedia.org/wiki/Determining_the_number_of...

    The average silhouette of the data is another useful criterion for assessing the natural number of clusters. The silhouette of a data instance is a measure of how closely it is matched to data within its cluster and how loosely it is matched to data of the neighboring cluster, i.e., the cluster whose average distance from the datum is lowest. [8]

  6. Extensible Storage Engine - Wikipedia

    en.wikipedia.org/wiki/Extensible_Storage_Engine

    Indexes are defined to persist interesting orders of data, and allow both sequential access to records in index order, and direct access to records by index column values. Clustered indexes in ESE must also be primary, meaning that the index key must be unique. Clustered and non-clustered indexes are represented using B+ trees. If an insert or ...

  7. Comparison of relational database management systems - Wikipedia

    en.wikipedia.org/wiki/Comparison_of_relational...

    Note (4): Used for InMemory ColumnStore index, temporary hash index for hash join, Non/Cluster & fill factor. Note (5): InnoDB automatically generates adaptive hash index [125] entries as needed. Note (6): Can be implemented using Function-based Indexes in Oracle 8i and higher, but the function needs to be used in the sql for the index to be used.

  8. Block Range Index - Wikipedia

    en.wikipedia.org/wiki/Block_Range_Index

    A large database index would typically use B-tree algorithms. BRIN is not always a substitute for B-tree, it is an improvement on sequential scanning of an index, with particular (and potentially large) advantages when the index meets particular conditions for being ordered and for the search target to be a narrow set of these values.

  9. Query plan - Wikipedia

    en.wikipedia.org/wiki/Query_plan

    This plan table will return the cost and time for executing a query. Oracle offers two optimization approaches: CBO or Cost Based Optimization; RBO or Rule Based Optimization; RBO is slowly being deprecated. For CBO to be used, all the tables referenced by the query must be analyzed. To analyze a table, a DBA can launch code from the DBMS_STATS ...