Question

是否有特定情况，我应该使用RETURN AS VALUE？通常我只使用NESTED TABLE xxx STORE AS xxx

例如：

    CREATE OR REPLACE TYPE address_t AS OBJECT (
    ADDID      NUMBER(10,0),
    STREET     VARCHAR2(40),
    ZIP        VARCHAR2(5),
    CITY       VARCHAR2(40)
)
/

CREATE OR REPLACE TYPE addresses_nt AS TABLE OF address_t
/

CREATE OR REPLACE TYPE invoicepos_t AS OBJECT (
    ARTID      NUMBER(10,0),
    AMOUNT     NUMBER(10,0)
)
/

CREATE OR REPLACE TYPE invoicepos_nt AS TABLE OF invoicepos_t
/

CREATE OR REPLACE TYPE customer_t AS OBJECT (
    CUSID      NUMBER(10,0),
    FIRSTNAME  VARCHAR2(30),
    LASTNAME   VARCHAR2(30),
    ADDRESSES  addresses_nt
)
/

CREATE OR REPLACE TYPE invoice_t AS OBJECT (
    INVOICEID  NUMBER(10,0),
    CUSTOMER   REF customer_t,
    ADDID      NUMBER(10,0),
    POSITIONS  invoicepos_nt
)
/

CREATE TABLE customer OF customer_t
    NESTED TABLE ADDRESSES STORE AS all_adresses RETURN AS VALUE    
/

CREATE TABLE invoices OF invoice_t
    NESTED TABLE POSITIONS STORE AS all_invoicepos RETURN AS VALUE
/

Answer 1

据我所知，唯一的区别是LOCATOR比VALUE快一点。但这没有意义，我希望有人会证明我错了;几乎从来没有“快速=真实”的转换。

根据SQL Language Reference：

RETURN [AS] Specify what Oracle Database returns as the result of a query.

    VALUE returns a copy of the nested table itself.

    LOCATOR returns a collection locator to the copy of the nested table.

    The locator is scoped to the session and cannot be used across sessions. Unlike a LOB locator, the collection locator cannot be used to modify the collection instance.

这意味着LOCATOR是只读的。但是在11gR2上，LOCATOR仍然可以被修改。

Object Relational Developer's Guide也讨论了LOCATOR，但没有提到使用它们的任何缺点。

示例架构

CREATE OR REPLACE TYPE invoicepos_t AS OBJECT (
    ARTID      NUMBER(10,0),
    AMOUNT     NUMBER(10,0)
)
/

CREATE OR REPLACE TYPE invoicepos_nt AS TABLE OF invoicepos_t
/

create table invoices_val
(
    INVOICEID  NUMBER,
    POSITIONS  invoicepos_nt
)
    NESTED TABLE POSITIONS STORE AS all_invoicepos_val RETURN AS VALUE
/

create table invoices_loc
(
    INVOICEID  NUMBER,
    POSITIONS  invoicepos_nt
)
    NESTED TABLE POSITIONS STORE AS all_invoicepos_loc RETURN AS locator
/

insert into invoices_val values(1, invoicepos_nt(invoicepos_t(1,1)));
insert into invoices_loc values(1, invoicepos_nt(invoicepos_t(1,1)));
insert into invoices_def values(1, invoicepos_nt(invoicepos_t(1,1)));
commit;

比较效果和功能

--Value: 1.0 seconds
declare
    v_positions invoicepos_nt;
begin
    for i in 1 .. 10000 loop
        select positions
        into   v_positions
        from   invoices_val;
    end loop;

    v_positions.extend;
    v_positions(2) := invoicepos_t(3,3);
    update invoices_val set positions = v_positions;
end;
/

--Locator: 0.8 seconds
declare
    v_positions invoicepos_nt;
begin
    for i in 1 .. 10000 loop
        select positions
        into   v_positions
        from   invoices_loc;
    end loop;

    v_positions.extend;
    v_positions(2) := invoicepos_t(3,3);
    update invoices_loc set positions = v_positions;
end;
/

嵌套表中“返回值”的目的是什么（Oracle 9i）

1 个答案: