connected logically, working together as part of a unified application or flexible framework. For example, the INFORMATION_SCHEMA  and  performance_schema  databases use “schema”  in their names to emphasize the close relationships between the tables and columns they contain. In MySQL, physically, a  schema  is synonymous with a  database . You can substitute the keyword  SCHEMA instead of  DATABASE  in MySQL SQL syntax, for example using  CREATE SCHEMA  instead of  CREATE DATABASE . Some other database products draw a distinction. For example, in the Oracle Database product, a  schema represents only a part of a database: the tables and other objects owned by a single user. See Also database, INFORMATION_SCHEMA, Performance Schema.  In MySQL,  full-text search  queries use a special kind of index, the  FULLTEXT index . In MySQL 5.6.4 and up, InnoDB  and  MyISAM  tables both support  FULLTEXT  indexes; formerly, these indexes were only available for MyISAM  tables.  Acronym for “serialized dictionary information” . See Also serialized dictionary information (SDI). See Also full-text search, FULLTEXT index.  A type of  InnoDB index  that represents a subset of table columns. An  InnoDB  table can have zero, one, or many secondary indexes. (Contrast with the  clustered index , which is required for each  InnoDB  table, and stores the data for all the table columns.) A secondary index can be used to satisfy queries that only require values from the indexed columns. For more complex queries, it can be used to identify the relevant rows in the table, which are then retrieved through lookups using the clustered index. Creating and dropping secondary indexes has traditionally involved significant overhead from copying all the data in the  InnoDB  table. The  fast index creation  feature makes both  CREATE INDEX  and  DROP INDEX  statements much faster for  InnoDB  secondary indexes. See Also clustered index, Fast Index Creation, index.  A division within an  InnoDB tablespace . If a tablespace is analogous to a directory, the segments are analogous to files within that directory. A segment can grow. New segments can be created. For example, within a  file-per-table  tablespace, table data is in one segment and each associated index is in its own segment. The  system tablespace  contains many different segments, because it can hold many tables and their associated indexes. Prior to MySQL 8.0, the system tablespace also includes one or more  rollback segments  used for  undo logs . Segments grow and shrink as data is inserted and deleted. When a segment needs more room, it is extended by one  extent  (1 megabyte) at a time. Similarly, a segment releases one extent’s worth of space when all the data in that extent is no longer needed. See Also extent, file-per-table, rollback segment, system tablespace, tablespace, undo log.  A property of data distribution, the number of distinct values in a column (its  cardinality ) divided by the number of records in the table. High selectivity means that the column values are relatively unique, and can retrieved efficiently through an index. If you (or the query optimizer) can predict that a test in a  WHERE  clause only matches a small number (or proportion) of rows in a table, the overall  query  tends to be efficient if it evaluates that test first, using an index. 6185 SDI search index secondary index segment selectivity