Search results
Results From The WOW.Com Content Network
PostgreSQL includes built-in support for regular B-tree and hash table indexes, and four index access methods: generalized search trees , generalized inverted indexes (GIN), Space-Partitioned GiST (SP-GiST) [42] and Block Range Indexes (BRIN). In addition, user-defined index methods can be created, although this is quite an involved process.
In PostgreSQL, a useful partial index might be defined as: create index partial_status on txn_table ( status ) where status in ( 'A' , 'P' , 'W' ); This index would not bother storing any of the millions of rows that have reached "final" status, 'F', and would allow queries looking for transactions that still "need work" to efficiently search ...
So if two tables are T1 and T2, SELECT * FROM T1, T2 will result in every combination of T1 rows with every T2 rows. E.g., if T1 has 3 rows and T2 has 5 rows, then 15 rows will result. Although not in standard, most DBMS allows using a select clause without a table by pretending that an imaginary table with one row is used.
In relational databases a virtual column is a table column whose value(s) is automatically computed using other columns values, or another deterministic expression. Virtual columns are defined of SQL:2003 as Generated Column, [1] and are only implemented by some DBMSs, like MariaDB, SQL Server, Oracle, PostgreSQL, SQLite and Firebird (database server) (COMPUTED BY syntax).
A common table expression, or CTE, (in SQL) is a temporary named result set, derived from a simple query and defined within the execution scope of a SELECT, INSERT, UPDATE, or DELETE statement. CTEs can be thought of as alternatives to derived tables ( subquery ), views , and inline user-defined functions.
In relational databases, the information schema (information_schema) is an ANSI-standard set of read-only views that provide information about all of the tables, views, columns, and procedures in a database. [1] It can be used as a source of the information that some databases make available through non-standard commands, such as:
This enables much more efficient access, at the cost of extra storage and of some data being potentially out-of-date. Materialized views find use especially in data warehousing scenarios, where frequent queries of the actual base tables can be expensive. [citation needed] In a materialized view, indexes can be built on any column. In contrast ...
This allows indexes to be defined for common query conditions that depend on data in a table, but are not actually stored in that table. A common use for an expression index is to support case-insensitive searching or constraints. For example, if a web site wants to make user names case-insensitive, but still preserve the case as originally ...