12cR2 New Feature -sql prompt "history"

12cR2 has really great features to use in day to day life for dba. Here we will be discussing one such feature i.e. SQL prompt "history". Many dba's use history command on UNIX prompt to analyze the things or events, Before 12c, this option was not there to see history of sql's you have executed but from 12cR2 onward this is very much possible.

Lets' try this thing out, below is the demonstration 

Check if history parameter is ON/OFF

    SQL> show history; 

    history is OFF

Set history ON

    SQL> set history ON;

    SQL> show history;

    history is ON and set to "100"

Now execute few queries

    SQL> select name,open_mode from v$database;

    NAME      OPEN_MODE

    --------- --------------------

    TECHCDB   READ WRITE

    SQL> show con_id

    CON_ID

    ------------------------------

    1

    SQL> select count(*) from dba_db_links;

      COUNT(*)

    ----------

             2

    SQL> select sysdate from dual;

    SYSDATE

    ---------

    28-JUL-20

 

Now check the history, you will be able see all the sql's executed.

    SQL> history

      1  show history;

      2  select name,open_mode from v$database;

      3  show con_id

      4  select count(*) from dba_db_links;

      5  select sysdate from dual;

    SQL>

set history OFF

    SQL> set history off

    SQL>

    SQL> history

    SP2-1650: History is off, use "SET HIST[ORY] ON" to enable History.

this is use of "history" command. Suppose if you forget to turn off history after your work, don't worry it will be only available for that session, it will be automatically turned off.

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Physical location of datafiles in PostgreSQL

In this article, we will discuss finding the physical location of data files for a particular database.

In the logical structure, we can see different databases and tables or any other objects under it but as DBA we should also be aware of how to find out the location of data files associated with a database or a table.

In the article table creation in Postgres database we learned to create tables now we use the same tables here to perform a demonstration. Assume we have the below details

Database name- tech_db

Table name - employee

tech_db=# \c tech_db

You are now connected to database "tech_db" as user "postgres".

tech_db=# \dt

          List of relations

 Schema |   Name   | Type  |  Owner

--------+----------+-------+----------

 public | employee | table | postgres

(1 row)

tech_db=# select * from employee;

 emp_id | emp_name | age

--------+----------+-----

    123 | sam      |  25

    124 | John     |  24

(2 rows)

as of now we do not physical location of this database or table, now we will try to find out the same

Read More : Database Creation in PostgreSQL

Data Directory

First of all check data directory using show command

tech_db=# show data_directory;

  data_directory

------------------

 E:/postgres/data

(1 row)

Above we got data directory, as by default folders for each database gets created under data_directory/base location

as we see folder as created with some random numbers which is called as OID , value of OID can be find using below query.

tech_db=# select datname, oid from pg_database where datname='tech_db';

 datname |  oid

---------+-------

 tech_db | 16393

(1 row) 

we can see value of oid for database tech_db is 16393 and in above screenshot we can find the same folder under base directory. Under it you can find number of files and now lets find the file related to our table employee

tech_db=# select relname,oid, relfilenode from pg_class where relname='employee';

 relname  |  oid  | relfilenode

----------+-------+-------------

 employee | 16394 |       16394

(1 row)

pick up the relfilenode from above output, you will be able to see same file under folder 16393

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