Using bulk collect clause- bulk bind performance improvement, PL-SQL Programming

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Using the BULK COLLECT Clause

The keywords BULK COLLECT specify the SQL engine to bulk-bind output collections before returning them to the PL/SQL engine. You can use these keywords in the FETCH INTO, SELECT INTO, and RETURNING INTO clauses. The syntax for the above is shown below:

... BULK COLLECT INTO collection_name[, collection_name] ...

The SQL engine bulk-binds all the collections referenced in the INTO list. The parallel columns should store the scalar (not composite) values. In the illustration, the SQL engine loads the whole empno and ename database columns into the nested tables before returning the tables to the PL/SQL engine:

DECLARE

TYPE NumTab IS TABLE OF emp.empno%TYPE;

TYPE NameTab IS TABLE OF emp.ename%TYPE;

enums NumTab; -- no need to initialize

names NameTab;

BEGIN

SELECT empno, ename BULK COLLECT INTO enums, names FROM emp;

...

END;

The SQL engine initializes and then expands the collections for you. (Though, it cannot expands the varrays beyond their maximum size.) Then, starting at index 1, it inserts the elements successively and overwrites any pre-existent elements.

The SQL engine bulk-binds the whole database columns. Therefore, if a table has 50,000 rows, then the engine loads 50,000 column values into the target collection. Though, you can use the pseudocolumn ROWNUM to limit the number of rows processed. In the illustration below, you limit the number of rows to 100:

DECLARE

TYPE NumTab IS TABLE OF emp.empno%TYPE;

sals NumTab;

BEGIN

SELECT sal BULK COLLECT INTO sals FROM emp WHERE ROWNUM <= 100;

...

END;


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