How to Reduce DB File Sequential Read Wait

DB File Sequential Read wait event occurs when we are trying to access data using index and oracle is waiting for the read of index block from disk to buffer cache to complete.  A sequential read is a single-block read.Single block I/Os are usually the result of using indexes. Rarely, full table scan calls could get truncated to a single block call due to extent boundaries, or buffers already present in the buffer cache.Db file sequential read wait events may also appear when undo blocks are read from disk in order to provide a consistent get(rarely).

To determine the actual object being waited can be checked by the p1, p2, p3 info in v$session_wait .  A sequential read is usually a single-block read, although it is possible to see sequential reads for more than one block (See P3). This wait may also be seen for reads from datafile headers (P2 indicates a file header read) ,where p1,p2 and p3 gives the the absolute file number ,the block being read ,and  the number of blocks (i.e, P3 should be 1) respectively. 

Block reads are fairly inevitable so the aim should be to minimise un-necessary IO. This is best achieved by good application design and efficient execution plans. Changes to execution plans can yield orders of magnitude changes in performance.Hence to reduce this wait event follow the below points .

1.) Tune Oracle - tuning SQL statements to reduce unnecessary I/O request is the only guaranteed way to reduce "db file sequential read" wait time.
2.) Tune Physical Devices - Distribute(stripe) the data on diferent disk to reduce the i/o . Logical distribution is useless. "Physical" I/O performance is only governed by "independency of devices".
3.) Faster Disk - Buy the faster disk to reduce the unnecessary I/O request .
4.) Increase db_block_buffers - A larger buffer cache can (not will, "might") help .


Reference :: From  Asktom site


Enjoy     :-) 

Cannot Load OCI.DLL : While Connecting

Sometimes the error "cannot load OCI.DLL" occur whenever we try to connect with the oracle database by using the third-party tools(i.e, toad,sqltools and others) or command prompt . This error may occur because of the following reason .

1.) The oci.dll error may occur because you have not set the correct ORACLE_HOME and path in environment variables . 

2.) It might be possible that the oci.dll file may be corrupt or may not exist on the correct path .

3.) May be possible that oci.dll may not be correct version. (e.g. 32bit s/w will load a 32bit DDL - we cannot for example use a 64bit DLL for a 32bit executable) . 

To solve this issue , consider the below points .

1.) Check the ORACLE_HOME and Path setting in the envirnoment variable.
2.) Check the correct location of the oci.dll path . The path of the oci.dll file is $ORACLE_HOME\bin\oci.dll
3.) Check the oci.dll correct version .

In my case , i am facing this issue because the $ORACLE_HOME is not correctly set in the environment variables . So setting the correct path in environment variables, we find not any error while connecting the  database .

Enjoy     :-) 

Dbms_Metadata Package : To Extract all DDL

There  are various method available to extract the DDL from the oracle database . We can extract the DDL either by  export/import utility , dbms_metadata package ,or by using others third party tools . The best ways to extract the DDL is by using the DBMS_METADATA pacakage if don't have any tools . Here we will discuss the various DDL extracting method to extract the DDL from the Oracle .

The DBMS_METADATA package provides a way to retrieve metadata from the database dictionary as XML or  DDL . We can retrive either a particular object type (such as a table, index, or procedure) or a heterogeneous collection of object types that form a logical unit (such as a database export or schema export) 

One of the very useful function i.e, GET_xxxx functions are used to extract the DDL .  The following GET_xxxx functions fetches the metadata for objects with a single call . They encapsulate calls to OPEN, SET_FILTER, and so on. The function we use depends on the characteristics of the object type and on whether we want XML or DDL.

1.) GET_DDL( )   :  This function is used to fetch named objects, especially schema objects (tables, views). They can also be used with nameless objects, such as resource_cost .

Snytax   :
DBMS_METADATA.GET_DDL (
object_type     IN VARCHAR2,
name            IN VARCHAR2,
schema          IN VARCHAR2 DEFAULT NULL,
version         IN VARCHAR2 DEFAULT 'COMPATIBLE',
model           IN VARCHAR2 DEFAULT 'ORACLE',
transform       IN VARCHAR2 DEFAULT 'DDL')
RETURN CLOB;

Example  : Here we will extract the DDL for the "employees"  table .

SQL> select dbms_metadata.get_ddl('TABLE', 'EMP')   from dual ;

DBMS_METADATA.GET_DDL('TABLE','EMP')
--------------------------------------------------------------------------------
   CREATE TABLE "ALEX"."EMP"
   (    "EMPNO" NUMBER(4,0),
        "ENAME" VARCHAR2(10),
        "JOB" VARCHAR2(9),
        "MGR" NUMBER(4,0),
        "HIREDATE" DATE,
        "SAL" NUMBER(7,2),
        "COMM" NUMBER(7,2),
        "DEPTNO" NUMBER(2,0),
         CONSTRAINT "EMP_DEPT_FK" FOREIGN KEY ("DEPTNO")
          REFERENCES "ALEX"."DEPT" ("DEPTNO") ENABLE
   ) PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255 NOCOMPRESS LOGGING
  STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
  PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
  TABLESPACE "USERS"

Similary , we also extract the DDL of others objects like tablespace,views,sequence and others .

If we want only the DDL  not the storage clause and constraints ,then we need to modify some transformation parameters as

SQL> begin
  2     dbms_metadata.set_transform_param (dbms_metadata.session_transform,'STORAGE',false);
  3     dbms_metadata.set_transform_param (dbms_metadata.session_transform,'TABLESPACE',false);
  4     dbms_metadata.set_transform_param (dbms_metadata.session_transform,'SEGMENT_ATTRIBUTES', false);
  5     dbms_metadata.set_transform_param (dbms_metadata.session_transform,'REF_CONSTRAINTS', false);
  6     dbms_metadata.set_transform_param (dbms_metadata.session_transform,'CONSTRAINTS', false);
  7  end;
  8  /
 PL/SQL procedure successfully completed.

Now on running the same above statements give the following results .

SQL> select dbms_metadata.get_ddl ('TABLE', 'EMP')  from dual ;
 DBMS_METADATA.GET_DDL('TABLE','EMP')
--------------------------------------------------------------------------------
   CREATE TABLE "ALEX"."EMP"
   (    "EMPNO" NUMBER(4,0),
        "ENAME" VARCHAR2(10),
        "JOB" VARCHAR2(9),
        "MGR" NUMBER(4,0),
        "HIREDATE" DATE,
        "SAL" NUMBER(7,2),
        "COMM" NUMBER(7,2),
        "DEPTNO" NUMBER(2,0)
   )

One of the nice things is that we don't need to modify all the transformation parameters again to go back to the default. They made it really easy to return to the default settings:

SQL> begin
  2     dbms_metadata.set_transform_param (dbms_metadata.session_transform, 'DEFAULT');
  3  end;
  4  /
 PL/SQL procedure successfully completed.

2.) GET_DEPENDENT_DDL( ) : This function is used to fetch dependent objects (audits, object grants).
 Syntax :
DBMS_METADATA.GET_DEPENDENT_DDL (
object_type         IN VARCHAR2,
base_object_name    IN VARCHAR2,
base_object_schema  IN VARCHAR2 DEFAULT NULL,
version             IN VARCHAR2 DEFAULT 'COMPATIBLE',
model               IN VARCHAR2 DEFAULT 'ORACLE',
transform           IN VARCHAR2 DEFAULT 'DDL',
object_count        IN NUMBER   DEFAULT 10000)
RETURN CLOB ;

Example :   In this example, we will extract the reference constraints which is dependent on another tables .

SQL > select dbms_metadata.get_dependent_ddl( 'REF_CONSTRAINT', table_name) DDL
              FROM USER_TABLES WHERE table_name = 'EMPLOYEES' ;
ALTER  TABLE  "HR"."EMPLOYEES" ADD CONSTRAINT  "EMP_DEPT_FK"  FOREIGN KEY ("DEPARTMENT_ID")  REFERENCES  "HR"."DEPARTMENTS"  ("DEPARTMENT_ID") ENABLE

ALTER  TABLE "HR"."EMPLOYEES"  ADD  CONSTRAINT "EMP_JOB_FK"  FOREIGN KEY ("JOB_ID") REFERENCES  "HR"."JOBS"  ("JOB_ID") ENABLE

ALTER TABLE "HR"."EMPLOYEES" ADD CONSTRAINT "EMP_MANAGER_FK" FOREIGN KEY ("MANAGER_ID")  REFERENCES "HR"."EMPLOYEES" ("EMPLOYEE_ID") ENABLE

3.) GET_GRANTED_DDL( ) : This function  is used to fetch granted objects (system grants, role grants ) to the users of the database .

Syntax : 


DBMS_METADATA.GET_GRANTED_DDL (
object_type     IN VARCHAR2,
grantee         IN VARCHAR2 DEFAULT NULL,
version         IN VARCHAR2 DEFAULT 'COMPATIBLE',
model           IN VARCHAR2 DEFAULT 'ORACLE',
transform       IN VARCHAR2 DEFAULT 'DDL',
object_count    IN NUMBER   DEFAULT 10000)
RETURN CLOB;

Example :  Here we will extract the "system grant" assigned to the user "SYSTEM "

SQL> select dbms_metadata.get_granted_ddl('SYSTEM_GRANT','SYSTEM') from dual;

DBMS_METADATA.GET_GRANTED_DDL('SYSTEM_GRANT','SYSTEM')
--------------------------------------------------------------------------------
  GRANT GLOBAL QUERY REWRITE TO "SYSTEM"
  GRANT CREATE MATERIALIZED VIEW TO "SYSTEM"
  GRANT SELECT ANY TABLE TO "SYSTEM"
  GRANT CREATE TABLE TO "SYSTEM"
  GRANT UNLIMITED TABLESPACE TO "SYSTEM" WITH ADMIN OPTION

To extract all the grants to all the user we can use the below statements  .

SQL> SELECT DBMS_METADATA.GET_GRANTED_DDL(‘ROLE_GRANT’, USERNAME) || ‘/’ DDL FROM DBA_USERS where exists (select ‘x’ from dba_role_privs drp where drp.grantee = dba_users.username)
UNION ALL
SELECT DBMS_METADATA.GET_GRANTED_DDL(‘SYSTEM_GRANT’, USERNAME) || ‘/’ DDL FROM DBA_USERS where exists (select ‘x’ from dba_role_privs drp where drp.grantee = dba_users.username)
UNION ALL
SELECT DBMS_METADATA.GET_GRANTED_DDL(‘OBJECT_GRANT’, USERNAME) || ‘/’ DDL FROM  DBA_USERS where exists (select ‘x’ from dba_tab_privs dtp where  dtp.grantee = dba_users.username);


Enjoy   :-)