PostgreSQL - Query data from another database/server

Assume that, you have two databases:
1. db1 with two schemas: public and sch1
2. db2 with two schemas: public and sch2

In PostgreSQL, you can access a table from a different schema by using SELECT * FROM schema_name.table_name such as SELECT * FROM db1_sch.employee;
However, you cannot access a table on the different database with explicit way like SELECT * FROM db2.sch2.employee;

The question is how can we access like SQL Server or some other DBMS.
Fortunately, PostgreSQL support dblink to do that.

First, install extension dblink on the database you want to be able to access to other
CREATE EXTENSION dblink;

If db2 is in the same server, you can query table in db2 like that:
SELECT *
FROM dblink('dbname=db2', 'SELECT employee_id, employee_name FROM sch2.employee')
           AS employee(employee_id integer, employee_name text);

If db2 is in the different server, you can query:
SELECT *
FROM dblink('hostaddr=172.x.x.x dbname=db2 user=admin password=pwd port=5432', 'SELECT employee_id, employee_name FROM sch2.employee')
           AS employee(employee_id integer, employee_name text);

PostgreSQL - get table structure

SELECT ordinal_position
, column_name
, CASE data_type WHEN 'character varying' THEN data_type || '('||character_maximum_length || ')' ELSE data_type END data_type
, column_default
, is_nullable
, character_maximum_length
, numeric_precision
FROM information_schema.columns
WHERE table_catalog = 'database_name'
AND table_schema = 'schema_name'
AND table_name = 'table_name'
ORDER BY ordinal_position;

PostgreSQL: check the core settings of server

SELECT name, context, unit, setting, short_desc

FROM pg_settings

WHERE name in ('listen_addresses' ,'max_connections' ,'shared_buffers' ,'effective_cache_size' , 'work_mem' , 'maintenance_work_mem')

ORDER BY context,name;

If context is set to postmaster, it means changing this parameter requires a restart of the postgresql service. If context is set to user, changes require at least a reload. Furthermore, these settings can be            overridden at the database, user, session, or function levels.

unit tells you the unit of measurement that the setting is reported in. This is very important for memory settings since, as you can see, some are reported in 8 kB and some in kB

PosgreSQL: get main configuration files

SELECT name, setting FROM pg_settings WHERE category = 'File Locations';

postgresql.conf controls general settings, such as how much memory to allocate, default storage location for new databases, which IPs PostgreSQL listens on, where logs are stored, and so forth.

pg_hba.conf controls security. It manages access to the server, dictating which users can login into which databases, which IPs or groups of IPs are permitted to connect and the authentication scheme expected.

pg_ident.conf is the mapping file that maps an authenticated OS login to a PostgreSQL user. This file is used less often, but allows you to map a server account to a PostgreSQL account. For example, people sometimes map the OS root account to the postgre’s super user account. Each authentication line in pg_hba.conf can use a different pg_ident.conf file

PostgreSQL quiz: stored procedure execution

Assume that you have a table contact and a stored procedure called insert_contact as the following screenshots

Which one below is correct?

A. CALL insert_contact(‘John’,'Tran’);

B. EXECUTE insert_contact(‘John’,’Tran’);

C. SELECT insert_contact(‘John’,’Tran’)

D. Both A & B

E. Error in the store procedure code

F. None of the above

 

Answer: [C]

Highlight to find out the answer.

PostgreSQL license

Select the most suitable statement about the PostgreSQL license from below.

A. PostgreSQL is distributed under the GPL license.
B. PostgreSQL is distributed under the PostgreSQL license.
C. PostgreSQL is distributed under the LGPL license.
D. PostgreSQL is distributed under the BSD license.
E. PostgreSQL is distributed under the X11(MIT) license.

 

Answer: [D]

Highlight to find out the answer.

Convert date time values to string in PostgreSQL

This is a view extracted from OpenERP.

CREATE OR REPLACE VIEW crm_claim_report
AS
SELECT min(c.id) AS id
, to_char(c.date, 'YYYY'::text) AS name
, to_char(c.date, 'MM'::text) AS month
, to_char(c.date, 'YYYY-MM-DD'::text) AS day
, to_char(c.date_closed, 'YYYY-MM-DD'::text) AS date_closed
, to_char(c.date_deadline::timestamp with time zone, 'YYYY-MM-DD'::text) AS date_deadline, c.state, c.user_id
, c.stage_id, c.section_id, c.partner_id, c.company_id, c.categ_id, count(*) AS nbr, c.priority
, c.type_action, date_trunc('day'::text, c.create_date) AS create_date
, avg(date_part('epoch'::text, c.date_closed - c.create_date)) / (3600 * 24)::double precision AS delay_close
, ( SELECT count(mailgate_message.id) AS count FROM mailgate_message
WHERE mailgate_message.model::text = 'crm.claim'::text AND mailgate_message.res_id = c.id AND mailgate_message.history = true) AS email
, ( SELECT avg(crm_case_stage.probability) AS avg FROM crm_case_stage
WHERE crm_case_stage.type::text = 'claim'::text AND crm_case_stage.id = c.stage_id) AS probability
, date_part('epoch'::text, c.date_deadline::timestamp without time zone - c.date_closed) / (3600 * 24)::double precision AS delay_expected
FROM crm_claim c
GROUP BY to_char(c.date, 'YYYY'::text), to_char(c.date, 'MM'::text), to_char(c.date, 'YYYY-MM-DD'::text), c.state, c.user_id, c.section_id
, c.stage_id, c.categ_id, c.partner_id, c.company_id, c.create_date, c.priority, c.type_action, c.date_deadline, c.date_closed, c.id;

PostgreSQL Contactenation

Which query is correct in PostgreSQL?

A. SELECT 'Test' . 'query';
B. SELECT cat('Test','query') FROM dual;
C. SELECT 'Test' + 'query';
D. SELECT 'Test' + 'query' FROM dual;
E. SELECT 'Test' || 'query';

Answer:[E]
Highlight to find out the answer.

Same query for Fiscal and Normal calendar

tblFiscalMapping

Assume that you have a mapping table between Fiscal and Normal calendar above.

Sometimes, you need to write one stored procedure with a param @IsFiscal to determine when to return order info on Fiscal year or Normal calendar year.

You can implement as the following

DECLARE @StartMonth SMALLDATETIME;   
DECLARE @EndMonth SMALLDATETIME;

IF ISNULL(@IsFiscal,0) = 0
    SELECT @StartMonth = CAST(CAST(@Year AS VARCHAR) + '-01-01' AS SMALLDATETIME);
ELSE
    SELECT @StartMonth = CAST(CAST([Year] AS VARCHAR) + '-' + CAST(NormalMonth AS VARCHAR) + '-01' AS SMALLDATETIME)
    FROM dbo.tblFiscalMapping
    WHERE FiscalYear = @Year;

SET @EndMonth = DATEADD(m, 11, @StartMonth);

SELECT OrderID, OrderNumber, OrderDate, CustomerID, TotalAmount

FROM dbo.tblOrder

WHERE DATEDIFF(m, @StartMonth, OrderDate) >= 0
                                AND DATEDIFF(m, @EndMonth, OrderDate) <= 0

Create a Virtual Active Directory Domain Controller with VirtualBox

I need to query Active Directory by using SQL Server, so I created virtual Active Directory Domain Controller to do that.

1. Install Windows Server virtual machine, such as Windows Server 2003

2. Install Client virtual machine, such as Windows XP

3. Set up network

4. Set up new Adapter for each machine, use the adapter set up on Step 3

5. Run Windows Server machine

6. Install Active Directory by following steps after running dcpromo

7. Set up IP Address on Windows Server, note that IP address must be in the range set up in Step 3

8. Set up IP Address on Client machine

9. Create a domain user for client machine in Domain Controller

To allow domain user as local admin, please visit this link

10. Register client machine in domain controller which is defined in Step 6, then input user name/pwd defined in Step 9