postgresql/contrib/dblink/README.dblink

/*
 * dblink
 *
 * Functions returning results from a remote database
 *
 * Copyright (c) Joseph Conway <joe.conway@mail.com>, 2001;
 * 
 * Permission to use, copy, modify, and distribute this software and its
 * documentation for any purpose, without fee, and without a written agreement
 * is hereby granted, provided that the above copyright notice and this
 * paragraph and the following two paragraphs appear in all copies.
 * 
 * IN NO EVENT SHALL THE AUTHOR OR DISTRIBUTORS BE LIABLE TO ANY PARTY FOR
 * DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, INCLUDING
 * LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS
 * DOCUMENTATION, EVEN IF THE AUTHOR OR DISTRIBUTORS HAVE BEEN ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 * 
 * THE AUTHOR AND DISTRIBUTORS SPECIFICALLY DISCLAIMS ANY WARRANTIES,
 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS FOR A PARTICULAR PURPOSE.  THE SOFTWARE PROVIDED HEREUNDER IS
 * ON AN "AS IS" BASIS, AND THE AUTHOR AND DISTRIBUTORS HAS NO OBLIGATIONS TO
 * PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
 *
 */


Version 0.3 (14 June, 2001):
  Function to test returning data set from remote database
  Tested under Linux (Red Hat 6.2 and 7.0) and PostgreSQL 7.1 and 7.2devel

Release Notes:

  Version 0.3
    - fixed dblink invalid pointer causing corrupt elog message
    - fixed dblink_tok improper handling of null results
    - fixed examples in README.dblink

  Version 0.2
    - initial release    

Installation:
  Place these files in a directory called 'dblink' under 'contrib' in the PostgreSQL source tree. Then run:

    make
    make install

  You can use dblink.sql to create the functions in your database of choice, e.g.

    psql -U postgres template1 < dblink.sql

  installs following functions into database template1:

     dblink() - returns a pointer to results from remote query
     dblink_tok() - extracts and returns individual field results

Documentation
==================================================================
Name

dblink -- Returns a pointer to a data set from a remote database

Synopsis

dblink(text connstr, text sql)

Inputs

  connstr

    standard libpq format connection srting, 
    e.g. "hostaddr=127.0.0.1 port=5432 dbname=mydb user=postgres password=mypasswd"

  sql

    sql statement that you wish to execute on the remote host
    e.g. "select * from pg_class"

Outputs

  Returns setof int (pointer)

Example usage

  select dblink('hostaddr=127.0.0.1 port=5432 dbname=mydb user=postgres password=mypasswd','select f1, f2 from mytable');


==================================================================

Name

dblink_tok -- Returns individual select field results from a dblink remote query

Synopsis

dblink_tok(int pointer, int fnumber)

Inputs

  pointer

    a pointer returned by a call to dblink()

  fnumber

    the ordinal position (zero based) of the field to be returned from the dblink result set

Outputs

  Returns text

Example usage

  select dblink_tok(t1.dblink_p,0) as f1, dblink_tok(t1.dblink_p,1) as f2
  from (select dblink('hostaddr=127.0.0.1 port=5432 dbname=mydb user=postgres password=mypasswd'
                     ,'select f1, f2 from mytable') as dblink_p) as t1;


==================================================================

NOTE: If you need to provide selection criteria in a WHERE clause, it is necessary
to 'fake' a UNION, e.g.

  select
     dblink_tok(t1.dblink_p,0) as f1
    ,dblink_tok(t1.dblink_p,1) as f2
  from
   (
    select dblink(
                   'hostaddr=127.0.0.1 port=5432 dbname=mydb user=postgres password=mypasswd'
                  ,'select f1, f2 from mytable'
                 ) as dblink_p
    union all
    select null where false
   ) as t1
  where
    dblink_tok(t1.dblink_p,0) = 'mycriteria';

in order to work around an issue with the query optimizer. A more convenient way to approach
this problem is to create a view:

  create view myremotetable as
  select
     dblink_tok(t1.dblink_p,0) as f1
    ,dblink_tok(t1.dblink_p,1) as f2
  from
   (
    select dblink(
                   'hostaddr=127.0.0.1 port=5432 dbname=mydb user=postgres password=mypasswd'
                  ,'select f1, f2 from mytable'
                 ) as dblink_p
    union all
    select null where false
   ) as t1;

Then you can simply write:

   select f1,f2 from myremotetable where f1 = 'mycriteria';

==================================================================

-- Joe Conway