REF CURSOR

REF CURSOR游标:
动态游标,在运行的时候才能确定游标使用的查询。分类:
1.强类型(限制)REF CURSOR,规定返回类型
2.弱类型(非限制)REF CURSOR,不规定返回类型,可以获取任何结果集。

### Refcursor vs Cursor in PostgreSQL Differences and Usage In PostgreSQL, both `REFCURSOR` and cursors serve similar purposes but differ significantly in their implementation and use cases. #### Definition and Basic Characteristics A **cursor** allows retrieval of multiple rows from a query one at a time or in blocks. This feature is particularly useful when dealing with large datasets that cannot fit into memory all at once[^1]. Cursors can be declared within PL/pgSQL functions or procedures directly using the `DECLARE CURSOR` syntax. On the other hand, a **REFCURSOR** represents an abstract handle to a cursor rather than being a cursor itself. It acts more like a pointer or reference to a cursor object created elsewhere. Unlike regular cursors, `REFCURSOR`s do not require explicit declaration inside procedural code; instead, they get associated dynamically through function calls returning such references. #### Syntax Examples Here’s how each type gets defined: For standard cursors: ```sql BEGIN; DECLARE my_cursor SCROLL CURSOR FOR SELECT * FROM employees WHERE department_id = 50; FETCH ALL IN my_cursor; CLOSE my_cursor; COMMIT; ``` Using REFCURSORS involves creating stored procedures/functions that open cursors internally before passing back handles via OUT parameters. ```plpgsql CREATE OR REPLACE FUNCTION fetch_employee_data(dept integer, refcur refcursor) RETURNS refcursor AS $$ BEGIN OPEN refcur FOR SELECT * FROM employees WHERE department_id = dept; RETURN refcur; END; $$ LANGUAGE plpgsql; -- Calling the above-defined procedure would look something like this: BEGIN; SELECT fetch_employee_data(50, 'my_ref_cursor'); FETCH ALL IN "my_ref_cursor"; CLOSE "my_ref_cursor"; COMMIT; ``` #### Key Distinctions Between REF_CURSOR and Regular Cursors - **Scope**: Standard cursors exist only during transaction boundaries while `REFCURSOR`s persist beyond transactions since these act merely as pointers passed around between different parts of applications. - **Usage Contexts**: While direct manipulation (open/fetch/close) works fine for simple scripts or interactive sessions involving plain SQL commands, complex logic requiring dynamic queries benefits greatly from employing `REFCURSOR`. - **Portability Across Sessions/Connections**: Since `REFCURSOR` essentially serves as opaque identifiers pointing towards actual server-side resources allocated upon invocation, sharing across separate client connections becomes feasible without exposing underlying details about internal structures involved. #### Practical Implications on Application Design Choices Choosing whether to implement traditional cursors versus leveraging `REFCURSOR` depends largely on specific requirements surrounding data access patterns alongside architectural considerations regarding session management strategies employed throughout software stacks interfacing with relational databases managed under PostgreSQL environments. --related questions-- 1. How does Spring Batch address performance issues related to handling massive volumes of records? 2. What changes were made to alter column definitions specifically concerning storage characteristics in Oracle tables? 3. Can DelimitedLineTokenizer process CSV files containing quoted strings properly?
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