xmlagg in oracle 

Database Virtual Box Appliance / Virtual Machine - 12c for Practice

Database Virtual Box Appliance / Virtual Machine - 12c for Practice

Download it from here : click me

The pre-installed Oracle database 12c in Oracle Linux environment can be downloaded from the above link.

It will be very helpful for practicing 👍👍👍

Oracle Tuning Approach

I see a very good picture detailing the steps to tune a sql and here is the picture for you.

Useful SQL Info - Part-2

1. If we want to generate random numbers, random strings from Oracle (let say for the sake of testing etc...) then use DBMS_RANDOM package.

select dbms_random.value , dbms_random.value (1,100), trunc(dbms_random.value (1,100)), dbms_random.string ('A',4), dbms_random.string ('U',4), dbms_random.string ('L',4),

 dbms_random.string ('X',4), sysdate + trunc(dbms_random.value (1,100))

from dual

connect by level <=100

2. Materialize hint

The Oracle materialize hint is used to ensure that the Oracle cost-based optimizer materializes the temporary tables that are created inside the "WITH" clause.

MATERIALIZE hint in with clause converts result set into global temporary table

WITH data AS (

  SELECT /*+ MATERIALIZE */ level AS id

  FROM   dual

  CONNECT BY level <= 10000

)

SELECT rownum AS id

FROM   data, data, data

WHERE  rownum <= 1000000;

3.Dynamic Sampling & gather_plan_statistics Hint

CREATE TABLE CUST

(

  CUST_ID     NUMBER,

  LAST_NAME   VARCHAR2(30 BYTE),

  FIRST_NAME  VARCHAR2(30 BYTE)

)

Insert into CUST

   (CUST_ID, LAST_NAME, FIRST_NAME)

 Values

   (1028, 'RDRIZTFLNZWA', 'UPGGKGGDYF');

Insert into CUST

   (CUST_ID, LAST_NAME, FIRST_NAME)

 Values

   (1029, 'DQDQNU', 'TZSHGTTKUOV');

Insert into CUST

   (CUST_ID, LAST_NAME, FIRST_NAME)

 Values

   (1030, 'JZSOW', 'BMU');

commit;

If we do the explain plan for this below SQL

select * from cust

We notice below NOTE.

As stats were not gathered on CUST table, default dynamic sampling level=2 to used to calculate the stats on the fly..

Note

-----

   - dynamic sampling used for this statement (level=2)

   - Warning: basic plan statistics not available. These are only collected when:

       * hint 'gather_plan_statistics' is used for the statement or

       * parameter 'statistics_level' is set to 'ALL', at session or system level

Execute below to get rid of the above warning

First, gather table stats using dbms_stats package

Second,  use below commands to see expected & actual cardinality values.

alter session set statistics_level = all

select /*+ gather_plan_statistics  */  * from cust

4. Dynamic Sampling levels

Unnecessary Locking on Parent Table due to missing index on foreign key column in Child Table

If index not exists on foreign key column of a child table then it will create locks on the parent table when DML operation performed on child table.

Demonstration:

Create following sample tables. order_header is parent and order_detail is child table.


create table order_header (header_id number, customer varchar2(20))

alter table order_header add constraint pk_order_header  primary key (header_id)

create table order_detail (header_id number references order_header (header_id) on delete cascade , line_id number);

insert into order_header values (1,'a1');
insert into order_header values  (2,'a2');
insert into order_header values  (3,'a3');

insert into order_detail values  (1,1);
insert into order_detail values  (2,1);

commit;


Now, open two sessions to the database,

From session-1, delete a record from child table.
From session-2, delete a record from parent table. This transaction is going to be locked until the transaction in session-1 is either committed or rolled back.

To avoid this lock it is recommend to create a index on child table column.

create index order_detail_idx on order_detail (header_id);

Now, if we try the same delete statements, we wont see locking any more....

PIPELINED Function vs Normal Function Use Case in Oracle

Suppose we wanted to return values from Function in a nested table type format then pipelined table functions will give us better performance when compared with normal functions.

PGA memory consumption will be less in case of pipelined functions when compared it with normal functions


Example:
Create Object Type

create or replace type test_obj as object (id number, dsc varchar2(100));

Create Table Type

create or replace type test_obj_tab as table of test_obj;

Create Normal Function

CREATE OR REPLACE FUNCTION test_normal (p_val IN NUMBER)
   RETURN test_obj_tab
AS
   v_test_obj_tab   test_obj_tab := test_obj_tab ();
BEGIN
   FOR i IN 1 .. p_val
   LOOP
      v_test_obj_tab.EXTEND;
      v_test_obj_tab (v_test_obj_tab.LAST) := test_obj (i, 'Number-' || i);
   END LOOP;

   RETURN v_test_obj_tab;
EXCEPTION
   WHEN OTHERS
   THEN
      DBMS_OUTPUT.put_line (' error-  ' || SQLERRM);

END;

Create PIPELINED Function

CREATE OR REPLACE FUNCTION test_pipe (p_val IN NUMBER)

   RETURN test_obj_tab pipelined 

AS

   v_test_obj_tab   test_obj_tab := test_obj_tab ();

BEGIN

   FOR i IN 1 .. p_val

   LOOP

      pipe row (test_obj (i, 'Number-' || i));

   END LOOP;

EXCEPTION

   WHEN OTHERS

   THEN

      DBMS_OUTPUT.put_line (' error-  ' || SQLERRM);

END;

Testing

select * from
table(test_normal (100));

select * from
table (test_pipe(100));

PIPELINED Table Functions To load Data Files into Oracle Table

To load the data files (.csv) into Oracle Tables we have following options

1) Use SQLLOADER
2) Use External Table
3) Use UTL_FILE

The traditional or old approach is using SQLLOADER to perform the file loads.

External Table is the new concept and using pipelined table functions in combination with external tables will give huge performance benefit while loading huge data files.

Code is created to load the file using either of these methods and posted in here.

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