Collections
The online Oracle 9i PL/SQL User Guide introduces collections as; "A collection is an ordered group of elements, all of the same type. It is a general concept that encompasses lists, arrays, and other familiar datatypes. Each element has a unique subscript that determines its position in the collection."
Retrieving Values
When you first start using PL/SQL, you generally end up declaring variables, then retrieving a value from a table to go into the variable. If you're going to retrieve a range of values from a table, you declare a cursor, retrieve the values one at a time from the database, and process them sequentially. Sometimes though, you want to retrieve a load of values all in one go, and load them into an array, so that you can carry out some sort of operation on the group of values as a whole.
PL/SQL Table
With Oracle 7, you could create an index-by table, or 'PL/SQL Table', that consist of a series of value pairs; an index value, and a scalar datatype (such as varchar2, or number). You referred to an individual PL/SQL Table entry by using the index, i.e. CUSTOMER_NAME(10). What made it interesting was that, as well as using scalar datatypes, you could also create PL/SQL tables using PL/SQL records, which could consist of a number of individual columns. By creating a PL/SQL record type based off of an existing table (for example, by using the SCOTT.EMP%ROWTYPE), you could load a table row, or an entire table, into a variable and process it within your PL/SQL package.
For example:
DECLARE
TYPE EmpTabTyp IS TABLE OF emp%ROWTYPE
INDEX BY BINARY_INTEGER;
emp_tab EmpTabTyp;
BEGIN
/* Retrieve employee record. */
SELECT * INTO emp_tab(7468) FROM emp WHERE empno = 7468;
...
END;
The above example would set up a PL/SQL table type that consists of PL/SQL records with the same definition as the columns in SCOTT.EMP, and then create a variable based on this type. It would then retrieve a single row from the EMP table, and then place it into the EMP_TAB variable, using the index value of 7468. Note that you don't need to initialize the variable before it's used and you can randomly put values into the variable using any valid BINARY_INTEGER value.
Collections and its composite types
As of Oracle 8, PL/SQL Tables are renamed 'Collections' and supplemented by two new composite types: Nested Tables, and VARRAYs
Nested tables
Nested tables extend the functionality of index-by tables by adding extra collection methods (known as table attributes for index-by tables), and, in a new development, nested tables can also be store in database tables and can be directly manipulated using SQL. Collectively, both types are known known as PL/SQL Tables.
To declare a nested table, you use the syntax (note the lack of 'INDEX BY BINARY INTEGER')
TYPE type_name IS TABLE OF element_type [NOT NULL];
Nested Tables and Index-By Tables
In practical terms, one major difference between nested tables and index-by tables, is that you have to initialize nested tables, using a constructor, definining how many elements can initially be stored in it (although you can later EXTEND the nested table); however, as mentioned above, you can store nested tables within the database (embedded in database columns), which is a discrete advantage over index-by tables. So, if you want to put together a database that is object orientated, and you need the data to be persistent, nested tables are the way to go.
VARRAYs
A VARRAY (variable length array) is a data type that would be familiar to java or C programmers. A varray has a maximum size, which you must specify in its type definition. Its index has a fixed lower bound of 1 and an extensible upper bound. Thus, a varray can contain a varying number of elements, from zero (when empty) to the maximum specified in its type definition.
Index-By Tables - Nested Tables - VARRAYS
So, how do you choose which of these three types (index-by tables, nested tables and varrays) to use. Arrays in other languages become VARRAYs in PL/SQL. Sets and bags in other languages become nested tables in PL/SQL. Hash tables and other kinds of unordered lookup tables in other languages become associative arrays in PL/SQL.
Loading Table
The original method for loading the table as put forward in the question was:
CREATE TABLE t1 AS
SELECT *
FROM all_objects
WHERE 1=0;
CREATE OR REPLACE PROCEDURE test_proc IS
BEGIN
FOR x IN (SELECT * FROM all_objects)
LOOP
INSERT INTO t1
(owner, object_name, subobject_name, object_id,
data_object_id, object_type, created, last_ddl_time,
timestamp, status, temporary, generated, secondary)
VALUES
(x.owner, x.object_name, x.subobject_name, x.object_id,
x.data_object_id, x.object_type, x.created,
x.data_object_id, x.object_type, x.created,
x.last_ddl_time, x.timestamp, x.status, x.temporary,
x.generated, x.secondary);
END LOOP;
COMMIT;
END test_proc;
test_proc producedure
The test_proc producedure declares a cursor that points to the resultset from SELECT * FROM ALL_OBJECTS It then starts at record one, and inserts into the t1 table the columns from the first row in the cursor Then, it loops back and gets the next row of data, until all rows from the cursor have been retrieved. The data is then committed, and the procedure ends.
Nested Table to hold data - bulk collect to load Data
The first solution put forward uses a nested table to hold the data from the ALL_OBJECTS table, and does something called BULK COLLECT to load all of the source tables' data into the nested table.
CREATE OR REPLACE PROCEDURE fast_proc (p_array_size IN PLS_INTEGER DEFAULT 100)
IS
TYPE ARRAY IS TABLE OF all_objects%ROWTYPE;
l_data ARRAY;
CURSOR c IS
SELECT *
FROM all_objects;
BEGIN
OPEN c;
LOOP
FETCH c BULK COLLECT INTO l_data LIMIT p_array_size;
FORALL i IN 1..l_data.COUNT
INSERT INTO t2 VALUES l_data(i);
EXIT WHEN c%NOTFOUND;
END LOOP;
CLOSE c;
END fast_proc;
/
Ignore the fact that the table's called ARRAY - it's a nested table, not a varray.
Bulk Loading Data into nested table
The second example answer is a variation on this, that does much the same thing with slightly more compact code;
SQL> create or replace procedure fast_proc is
type TObjectTable is table of ALL_OBJECTS%ROWTYPE;
ObjectTable$ TObjectTable;
begin
select * BULK COLLECT INTO ObjectTable$
from ALL_OBJECTS;
forall x in ObjectTable$.First..ObjectTable$.Last
insert into t1 values ObjectTable$(x) ;
end;
/
Again, a nested table is declared, but this time the cursor is dispensed with, and the data is just bulk loaded directly into the nested table. Again, the FORALL statement is used afterwards to run through the nested table. If you need to process lots of rows in one go, loading the data into memory first, use a collection and BULK COLLECT the data into them.
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