My stuff » Oracle

Authomatic statistics gathering

Administrator — Mon, 29 Oct 2007 17:13:23 +0000

Oracle 10g has a feature where it gathers optimizer statistics automatically at night. This is done through a scheduled job called GATHER_STATS_JOB. If you want to disable this, you have to issue :

  EXEC dbms_scheduler.disable(’GATHER_STATS_JOB’);

To enable it again :

  EXEC dbms_scheduler.enable(’GATHER_STATS_JOB’);

The following query shows when this job was run :

  SELECT * FROM dba_scheduler_job_run_details WHERE job_name = ’GATHER_STATS_JOB’;

There are other tables, all starting with dba_scheduler, that show the schedules and so on. One of the more interesting ones is dba_scheduler_jobs.

NLS_LANG settings

Administrator — Wed, 24 Oct 2007 10:11:19 +0000

To get the language setting of the current session :

  SELECT USERENV ('language') FROM DUAL;

This returns the database character set, not the user’s though.

The NLS settings can be found in the following list, in this order. If a value is specified in NLS_SESSION_PARAMETERS, it has preference over a value in NLS_INSTANCE_PARAMETERS.

NLS_SESSION_PARAMETERS
NLS_INSTANCE_PARAMETERS
NLS_DATABASE_PARAMETERS

TNS_ADMIN

Administrator — Fri, 19 Oct 2007 08:27:05 +0000

By default Oracle expects tnsnames.ora to be in the $ORACLE_HOME\network\admin directory. This is not always the most convenient place, especially because that directory is usually owned by the oracle user.

To get around this, you can set the TNS_ADMIN variable. This points to the directory where you have put your own copy of tnsnames.ora . Like they say on TV : darn useful.

Killing an Oracle session

Administrator — Wed, 26 Sep 2007 10:22:58 +0000

Every time you log into Oracle, you create a session. This is an easy way to kill these Oracle sessions, thanks to this article.

First, select the session ID and serial number as such :

  SELECT s.sid, s.serial#, s.osuser, s.program
    FROM v$session s;

The osuser and program field can be used to identify the session.

Then you can kill the session using :

  ALTER SYSTEM KILL SESSION 'sid,serial#';

It’s also possible to kill the session immediately (but that’s considered rude) :

  ALTER SYSTEM KILL SESSION 'sid,serial#' IMMEDIATE;

Selecting random rows

Administrator — Wed, 01 Aug 2007 13:29:23 +0000

If you want to select random rows from a table, you can order the rows using the dbms_random.value function :

  SELECT COLUMN
    FROM TABLE
   ORDER BY dbms_random.VALUE

To get 20 random rows from the table, we can add a condition on rownum. This selects the first 20 rows :

  SELECT COLUMN
    FROM ( SELECT COLUMN
                 FROM TABLE
                ORDER BY dbms_random.VALUE )
   WHERE rownum < 21

With thanks to this site.

Recompiling database objects

Administrator — Thu, 26 Jul 2007 07:38:35 +0000

The DBMS_UTILITY package has a function called compile_schema that compiles all procedures, functions, packages, and triggers.

  EXEC DBMS_UTILITY.compile_schema(schema => 'SCOTT');

You can also use the UTL_RECOMP package, but I haven’t tried that one yet.

With thanks to this ORACLE-BASE article.

Statspack summary

Administrator — Wed, 04 Jul 2007 10:41:01 +0000

This post is a summary of the Oracle Statspack Survival Guide.

To install statspack :

Create tablespace for it
@?/rdbms/admin/spcreate (run as sysdba)

To take snaps :

exec statspack.snap;

To create the report :

@?/rdbms/admin/spreport.sql

How much space does an Oracle table take up ?

Administrator — Tue, 19 Jun 2007 09:47:56 +0000

The following queries can be used to find out how much space a table takes up. The first query gets the block size used in this database. The second one shows how many blocks are used for a table.

  SELECT VALUE
    FROM v$parameter
  WHERE name = 'db_block_size';
 
  SELECT blocks, last_analyzed
    FROM all_tables
  WHERE owner = 'X'
     AND TABLE_NAME = 'Y';

The blocks field will have a valid value if the table has been analyzed recently, that is why the last_analyzed field is selected as well.

Checking tablespace usage

Administrator — Fri, 08 Jun 2007 09:44:19 +0000

This SQL, inspired by this, shows how full the tablespaces are :

  SELECT tablespace_name                                     ||
         ' is '                                              ||
         TO_CHAR(ROUND(100-(free_bytes*100/total_bytes), 2)) ||
         '% full.' T, total_bytes, free_bytes
   FROM (SELECT a.tablespace_name, a.total_bytes, b.free_bytes
           FROM (SELECT tablespace_name, NVL(SUM(bytes), 1) AS total_bytes
                   FROM dba_data_files
                  GROUP BY tablespace_name) a,
                (SELECT tablespace_name, SUM(bytes) AS free_bytes
                   FROM dba_free_space
                  GROUP BY tablespace_name) b
           WHERE a.tablespace_name = b.tablespace_name
        )
  ORDER BY tablespace_name;

Another way of doing it is like this (courtesy of this site) :

  SELECT a.TABLESPACE_NAME,
           a.BYTES bytes_used,
           b.BYTES bytes_free,
           b.largest,
           round(((a.BYTES-b.BYTES)/a.BYTES)*100,2) percent_used
    FROM
           (
                SELECT TABLESPACE_NAME,
                         SUM(BYTES) BYTES
                  FROM dba_data_files
                 GROUP BY TABLESPACE_NAME
           ) a,
           (
                SELECT TABLESPACE_NAME,
                         SUM(BYTES) BYTES ,
                         MAX(BYTES) largest
                  FROM dba_free_space
                GROUP BY TABLESPACE_NAME
           ) b
  WHERE a.TABLESPACE_NAME=b.TABLESPACE_NAME
   ORDER BY ((a.BYTES-b.BYTES)/a.BYTES) DESC

Check if Java is installed in Oracle

Administrator — Wed, 06 Jun 2007 15:52:49 +0000

If you want to know if Java is installed, use the following SQL :

  select * from dba_registry;

There should be an entry with comp_id equal to ‘JAVAVM’.