For a long time I have been working only with Oracle Databases and I haven't had much contact with PostgreSQL.
So now, I have a few questions for people who are closer to Postgres.
Is it possible to create a connection from Postgres to Oracle (oracle_fdw?) and perform selects on views in a different schema than the one you connected to?
Is it possible to create a connection from Postgres to Oracle (oracle_fdw?) and perform inserts on tables in the same schema as the one you connected to?
Ad 1:
Yes, certainly. Just define the foreign table as
CREATE FOREIGN TABLE view_1_r (...) SERVER ...
OPTIONS (table 'VIEW_1', schema 'USERB');
Ad 2:
Yes, certainly. Just define a foreign table on the Oracle table and insert into it. Note that bulk inserts work, but won't perform well, since there will be a round trip between PostgreSQL and Oracle for each row inserted.
Both questions indicate a general confusion between a) the Oracle user that you use to establish the connection and b) the schema of the table or view that you want to access. These things are independent: The latter is determined by the schema option of the foreign table definition, while the former is determined by the user mapping.
I tried searching for it but couldn't find out
What is the best way to copy data from Redshift to Postgresql Database ?
using Talend job/any other tool/code ,etc
anyhow i want to transfer data from Redshift to PostgreSQL database
also,you can use any third party database tool if it has similar kind of functionality.
Also,as far as I know,we can do so using AWS Data Migration Service,but not sure our source db and destination db matches that criteria or not
Can anyone please suggest something better ?
The way I do it is with a Postgres Foreign Data Wrapper and dblink,
This way, the redshift table is available directly within Postgres.
Follow the instructions here to set it up https://aws.amazon.com/blogs/big-data/join-amazon-redshift-and-amazon-rds-postgresql-with-dblink/
The important part of that link is this code:
CREATE EXTENSION postgres_fdw;
CREATE EXTENSION dblink;
CREATE SERVER foreign_server
FOREIGN DATA WRAPPER postgres_fdw
OPTIONS (host '<amazon_redshift _ip>', port '<port>', dbname '<database_name>', sslmode 'require');
CREATE USER MAPPING FOR <rds_postgresql_username>
SERVER foreign_server
OPTIONS (user '<amazon_redshift_username>', password '<password>');
For my use case I then set up a postgres materialised view with indexes based upon that.
create materialized view if not exists your_new_view as
SELECT some,
columns,
etc
FROM dblink('foreign_server'::text, '
<the redshift sql>
'::text) t1(some bigint, columns bigint, etc character varying(50));
create unique index if not exists index1
on your_new_view (some);
create index if not exists index2
on your_new_view (columns);
Then on a regular basis I run (on postgres)
REFRESH MATERIALIZED VIEW your_new_view;
or
REFRESH MATERIALIZED VIEW CONCURRENTLY your_new_view;
In the past, I managed to transfer data from one PostgreSQL database to another by doing a pg_dump and piping the output as an SQL command to the second instance.
Amazon Redshift is based on PostgreSQL, so this method should work, too.
You can control whether pg_dump should include the DDL to create tables, or whether it should just load the data (--data-only).
See: PostgreSQL: Documentation: 8.0: pg_dump
I am migrating this Oracle command to PostgreSQL:
CREATE SYNONYM &user..emp FOR &schema..emp;
Please suggest to me how I can migrate the above command.
PostgreSQL does not support SYNOSYM or ALIAS. Synonym is a non SQL 2003 feature implemented by Microsoft SQL 2005 (I think). While it does provide an interesting abstraction, but due to lack of relational integrity, it can be considered a risk.
That is, you can create a synonym, advertise it to you programmers, the code is written around it, including stored procedures, then one day the backend of this synonym (or link or pointer) is changed/deleted/etc leading to a run time error. I don't even think a prepare would catch that.
It is the same trap as the symbolic links in unix and null pointers in C/C++.
But rather create a DB view and it is already updateable as long as it contains one table select only.
CREATE VIEW schema_name.synonym_name AS SELECT * FROM schema_name.table_name;
You don't need synonyms.
There are two approaches:
using the schema search path:
ALTER DATABASE xyz SET search_path = schema1, schema2, ...;
Put the schema that holds the table on the search_path of the database (or user), then it can be used without schema qualification.
using a view:
CREATE VIEW dest_schema.tab AS SELECT * FROM source_schema.tab;
The first approach is good if you have a lot of synonyms for objects in the same schema.
Is it possible to JOIN rows from two separate postgres databases?
I am working with system with couple databases in one server and sometimes I really need such a feature.
According to http://wiki.postgresql.org/wiki/FAQ
There is no way to query a database other than the current one.
Because PostgreSQL loads database-specific system catalogs, it is
uncertain how a cross-database query should even behave.
contrib/dblink allows cross-database queries using function calls. Of
course, a client can also make simultaneous connections to different
databases and merge the results on the client side.
EDIT: 3 years later (march 2014), this FAQ entry has been revised and is more helpful:
How do I perform queries using multiple databases?
There is no way to directly query a database other than the current
one. Because PostgreSQL loads database-specific system catalogs, it is
uncertain how a cross-database query should even behave.
The SQL/MED support in PostgreSQL allows a "foreign data wrapper" to
be created, linking tables in a remote database to the local database.
The remote database might be another database on the same PostgreSQL
instance, or a database half way around the world, it doesn't matter.
postgres_fdw is built-in to PostgreSQL 9.3 and includes read/write
support; a read-only version for 9.2 can be compiled and installed as
a contrib module.
contrib/dblink allows cross-database queries using function calls and
is available for much older PostgreSQL versions. Unlike postgres_fdw
it can't "push down" conditions to the remote server, so it'll often
land up fetching a lot more data than you need.
Of course, a client can also make simultaneous connections to
different databases and merge the results on the client side.
Forget about dblink!
Say hello to Postgres_FDW:
To prepare for remote access using postgres_fdw:
Install the postgres_fdw extension using CREATE EXTENSION.
Create a foreign server object, using CREATE SERVER, to represent each remote database you want to connect to. Specify connection
information, except user, and password, as options of the server
object.
Create a user mapping, using CREATE USER MAPPING, for each database user you want to allow to access each foreign server. Specify
the remote user name and password to use as user and password options
of the user mapping.
Create a foreign table, using CREATE FOREIGN TABLE or IMPORT FOREIGN SCHEMA, for each remote table you want to access. The columns
of the foreign table must match the referenced remote table. You can,
however, use table and/or column names different from the remote
table's, if you specify the correct remote names as options of the
foreign table object.
Now you need only SELECT from a foreign table to access the data
stored in its underlying remote table.
It's really useful even on large data.
Yes, it is possible to do this using dblink albeit with significant performance considerations.
The following example will require the current SQL user to have permissions on both databases. If db2 is not located on the same cluster, then you will need to replace dbname=db2 with the full connection string defined in the dblink documentation.
SELECT *
FROM table1 tb1
LEFT JOIN (
SELECT *
FROM dblink('dbname=db2','SELECT id, code FROM table2')
AS tb2(id int, code text);
) AS tb2 ON tb2.column = tb1.column;
If table2 is very large, you could have performance issues because the sub-query loads up the entire table2 before performing the join.
No you can't. You could use dblink to connect from one database to another database, but that won't help if you're looking for JOIN's.
You can't use different SCHEMA's within a single database to store all you data?
Just a few steps and You can reach the goal:
follow this reference step by step
WE HAVE BEEN CONNECTED TO DB2 WITH TABLE TBL2 AND COLUMN COL2
ALSO THERE IS DB1 WITH TBL1 AND COLUMN COL1
*** connecting to second db ie db2
Now just **copy paste the 1-7 processes** (make sure u use correct username and password and ofcourse db name)
1.**CREATE EXTENSION dblink;**
2.**SELECT pg_namespace.nspname, pg_proc.proname
FROM pg_proc, pg_namespace
WHERE pg_proc.pronamespace=pg_namespace.oid
AND pg_proc.proname LIKE '%dblink%';**
3.**SELECT dblink_connect('host=localhost user=postgres password=postgres dbname=db1');**
4.**CREATE FOREIGN DATA WRAPPER postgres VALIDATOR postgresql_fdw_validator;**
5.**CREATE SERVER postgres2 FOREIGN DATA WRAPPER postgres OPTIONS (hostaddr '127.0.0.1', dbname 'db1');**
6.**CREATE USER MAPPING FOR postgres SERVER postgres2 OPTIONS (user 'postgres', password 'postgres');**
7.**SELECT dblink_connect('postgres2');**
---Now, you can SELECT the data of Database_One from Database_Two and even join both db results:
**SELECT * FROM public.dblink
('postgres2','SELECT col1,um_name FROM public.tbl1 ')
AS DATA(um_userid INTEGER),tbl2 where DATA.col1=tbl2.col2;**
You can also Check this :[How to join two tables of different databases together in postgresql [\[working finely in version 9.4\]][1]
You need to use dblink...as araqnid mentioned above, something like this works fine:
select ST.Table_Name, ST.Column_Name, DV.Table_Name, DV.Column_Name, *
from information_schema.Columns ST
full outer join dblink('dbname=otherdatabase','select Table_Name,
Column_Name from information_schema.Columns') DV(Table_Name text,
Column_Name text)
on ST.Table_Name = DV.Table_name
and ST.Column_Name = DV.Column_Name
where ST.Column_Name is null or DV.Column_Name is NULL
You have use dblink extension of postgresql.
Reference take from this Article:
DbLink extension of PostgreSQL which is used to connect one database to another database.
Install DbLink extension.
CREATE EXTENSION dblink;
Verify DbLink:
SELECT pg_namespace.nspname, pg_proc.proname
FROM pg_proc, pg_namespace
WHERE pg_proc.pronamespace=pg_namespace.oid
AND pg_proc.proname LIKE '%dblink%';
I have already prepared full demonstration on this. Please visit my post to learn step by step for executing cross database query in Postgresql.
Cannot be done? Of course we can, without special extensions. In our case, we had to compare two tables from different database servers, e.g. ACC and PROD, hence an even harder case than from most answers. Especially because ACC and PROD are deliberately on different servers to create a barrier, so you will not easily gain enough rights to perform a GRANT USAGE ON FOREIGN SERVER.
The obvious solution is to export both tables, and import both in the same database, e.g. DEV, or your own local db, under appropriate names, e.g. table1_acc and table1_prod, or schemas like acc and prod. Then, you may JOIN those with no special problems.
I have a schema called application. Is there a way that, rather than using the syntax
SELECT * FROM application.table
I can set the default schema so that I can just use
SELECT * FROM table
It would be the same idea as a using statement I suppose.
The default schema for all sql server users is "dbo", You can alter the default schema for a user by using commands ALTER USER
ALTER USER UserName WITH DEFAULT_SCHEMA = application;
It will be great to have a use statement for schemas.
You can vote the feature suggestion on Microsoft connect.