Search results
Results from the WOW.Com Content Network
Select all the rows from the beginning of the table to the last row to display ({begin_base_0 + rows}) Read the {begin_base_0 + rows} rows but send to display only when the row_number of the rows read is greater than {begin_base_0}
The query is unselective The number of return rows is too large and takes nearly 100% in the whole table. These rows are unselective. The table statistics does not update The number of rows in the table is higher than before, but table statistics haven't been updated yet. The optimizer can't correctly estimate that using the index is faster.
In situations where the number of unique values of a column is far less than the number of rows in the table, column-oriented storage allow significant savings in space through data compression. 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.)
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 , views, and inline user-defined functions.
A database index is a data structure that improves the speed of data retrieval operations on a database table at the cost of additional writes and storage space to maintain the index data structure. Indexes are used to quickly locate data without having to search every row in a database table every time said table is accessed.
In the above table, the next query extracts for each row the values of a window with one preceding and one following row: SELECT LAG ( name , 1 ) OVER ( ORDER BY name ) "prev" , name , LEAD ( name , 1 ) OVER ( ORDER BY name ) "next" FROM people ORDER BY name
After all conditions are evaluated, the bitmap contains a "1" for rows that matched the expression. Finally, the bitmap is traversed and matching rows are retrieved. In addition to efficiently combining indexes, this also improves locality of reference of table accesses, because all rows are fetched sequentially from the main table. [31]
A derived table is the use of referencing an SQL subquery in a FROM clause. Essentially, the derived table is a subquery that can be selected from or joined to. The derived table functionality allows the user to reference the subquery as a table. The derived table is sometimes referred to as an inline view or a subselect.