table型函数用来返回类似表结构的集合,常规的table型函数需要先填充完集合才能返回,如果集合对象太大会消耗大量内存;pipelined的table型函数能够在边填充时边返回,从而避免占用过多内存。
Table Functions
Table functions are used to return PL/SQL collections that mimic tables. They can be queried like a regular table by using the TABLE
function in the FROM
clause. Regular table functions require collections to be fully populated before they are returned. Since collections are held in memory, this can be a problem as large collections can waste a lot of memory and take a long time to return the first row. These potential bottlenecks make regular table functions unsuitable for large Extraction Transformation Load (ETL) operations. Regular table functions require named row and table types to be created as database objects.
Notice the above output is in reverse order because the query includes a descending order by clause.
Pipelined Table Functions
Pipelining negates the need to build huge collections by piping rows out of the function as they are created, saving memory and allowing subsequent processing to start before all the rows are generated.
Pipelined table functions include the PIPELINED
clause and use the PIPE ROW
call to push rows out of the function as soon as they are created, rather than building up a table collection. Notice the empty RETURN
call, since there is no collection to return from the function.
Once you start working with large warehousing ETL operations the performance improvements can be massive, allowing data loads from external tables via table functions directly into the warehouse tables, rather than loading via a staging area.
FROM & MORE:
http://www.oracle-base.com/articles/misc/pipelined-table-functions.php
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