This section explains how to use PL/Proxy to proxy queries across a set of remote databases. For purposes of this intro we assume each remote database has a "user" table that contains a username and an email column.
We also assume that the data is partitioned across remote databases by taking a hash of the username and assigning it to one of 2 servers. Real applications should use a partitioning scheme that is appropriate to their requirements.
For the purposes of this example assume that the partition databases part00 and part01 both contain a table resembling
CREATE TABLE users ( username text, email text );
Download PL/Proxy from http://pgfoundry.org/projects/plproxy and extract.
Build PL/Proxy by running make and make install inside of the plproxy directory. If your having problems make sure that pg_config from the postgresql bin directory is in your path.
To install PL/Proxy in a database execute the commands in the plproxy.sql file. For example psql -f $SHAREDIR/contrib/plproxy.sql mydatabase
Steps 1 and 2 can be skipped if your installed pl/proxy from a packaging system such as RPM.
Simple remote function call
Here we will create a plproxy function that will run on the proxy database which will connect to a remote database named part00 and return a users email address.
This example uses plproxy in CONNECT mode, it will connect to dbname=part00 and run following SQL there:
CREATE FUNCTION get_user_email(i_username text) RETURNS SETOF text AS $$ CONNECT 'dbname=part00'; SELECT email FROM users WHERE username = $1; $$ LANGUAGE plproxy; SELECT * from get_user_email($1);
The above example uses plproxy to proxy the query to the remote database but doesn’t handle partitioning of data. It assumes that the entire users table is in the remote users database. The next few steps will describe how to partition data with PL/Proxy.
Create configuration functions
Using PL/Proxy for partitioning requires setting up some configuration functions. Alternatively, if you are running PostgreSQL 8.4 or above you can take advantage of the SQL/MED connection management facilities. See below.
When a query needs to be forwarded to a remote database the function plproxy.get_cluster_partitions(cluster) is invoked by plproxy to get the connection string to use for each partition.
The following is an example
CREATE OR REPLACE FUNCTION plproxy.get_cluster_partitions(cluster_name text) RETURNS SETOF text AS $$ BEGIN IF cluster_name = 'usercluster' THEN RETURN NEXT 'dbname=part00 host=127.0.0.1'; RETURN NEXT 'dbname=part01 host=127.0.0.1'; RETURN; END IF; RAISE EXCEPTION 'Unknown cluster'; END; $$ LANGUAGE plpgsql;
A production application might query some configuration tables to return the connstrings. The number of partitions must be a power of 2.
Next define a plproxy.get_cluster_version(cluster_name) function. This is called on each request and determines if the output from a cached result from plproxy.get_cluster_partitions can be reused.
CREATE OR REPLACE FUNCTION plproxy.get_cluster_version(cluster_name text) RETURNS int4 AS $$ BEGIN IF cluster_name = 'usercluster' THEN RETURN 1; END IF; RAISE EXCEPTION 'Unknown cluster'; END; $$ LANGUAGE plpgsql;
We also need to provide a plproxy.get_cluster_config function, ours will provide a value for the connection lifetime. See the configuration section for details on what this function can do.
CREATE OR REPLACE FUNCTION plproxy.get_cluster_config( in cluster_name text, out key text, out val text) RETURNS SETOF record AS $$ BEGIN -- lets use same config for all clusters key := 'connection_lifetime'; val := 30*60; -- 30m RETURN NEXT; RETURN; END; $$ LANGUAGE plpgsql;
The config section contains more information on all of these functions.
Configuring Pl/Proxy clusters with SQL/MED
First we need a foreign data wrapper. This is mostly a placeholder, but can be extended with a validator function to verify the cluster definition. See http://www.postgresql.org/docs/8.4/static/sql-createforeigndatawrapper.html for additional details of how to manage the SQL/MED catalog.
CREATE FOREIGN DATA WRAPPER plproxy;
Then the actual cluster with its configuration options and partitions:
CREATE SERVER usercluster FOREIGN DATA WRAPPER plproxy OPTIONS (connection_lifetime '1800', p0 'dbname=part00 host=127.0.0.1', p1 'dbname=part01 host=127.0.0.1' );
We also need a user mapping that maps local PostgreSQL users to remote partitions. It is possible to create PUBLIC mapping that applies for all users in the local system:
CREATE USER MAPPING FOR PUBLIC SERVER usercluster;
Or a private mapping that can only be used by specific users:
CREATE USER MAPPING FOR bob SERVER usercluster OPTIONS (user 'plproxy', password 'salakala');
Finally we need to grant USAGE on the cluster to specific users:
GRANT USAGE ON SERVER usercluster TO bob;
Partitioned remote call
Here we assume that the user table is spread over several databases based on a hash of the username. The connection string for the partitioned databases are contained in the get_cluster_partitions function described above.
Below is a get_user_email function that is executed on the proxy server,which will make a remote connection to the appropriate partitioned database. The user’s email address will be returned.
This function should be created in the proxy database.
CREATE OR REPLACE FUNCTION get_user_email(i_username text) RETURNS SETOF text AS $$ CLUSTER 'usercluster'; RUN ON hashtext(i_username) ; SELECT email FROM users WHERE username = i_username; $$ LANGUAGE plproxy;
Inserting into the proper partition
Next we provide a simple INSERT function.
Inserting data through plproxy requires functions to be defined on the proxy databases that will perform the insert.
We define this function on both part00 and part01
CREATE OR REPLACE FUNCTION insert_user(i_username text, i_emailaddress text) RETURNS integer AS $$ INSERT INTO users (username, email) VALUES ($1,$2); SELECT 1; $$ LANGUAGE SQL;
Now we define a proxy function inside the proxy database to send the INSERT’s to the appropriate target.
CREATE OR REPLACE FUNCTION insert_user(i_username text, i_emailaddress text) RETURNS integer AS $$ CLUSTER 'usercluster'; RUN ON hashtext(i_username); $$ LANGUAGE plproxy;
Putting it all together
Connect to the proxy database (The one we installed plproxy and the plproxy functions on).
SELECT insert_user('Sven','email@example.com'); SELECT insert_user('Marko', 'firstname.lastname@example.org'); SELECT insert_user('Steve','email@example.com');
Now connect to the plproxy_1 and plproxy_2 databases. Sven and Marko should be in plproxy_2 , and Steve should be in plproxy_1.
When connected to the proxy user you can obtain data by doing
SELECT get_user_email('Sven'); SELECT get_user_email('Marko'); SELECT get_user_email('Steve');
When used in partitioned setup, PL/Proxy somewhat wastes connections as it opens connection to each partition from each backend process. So it’s good idea to use a pooler that can take queries from several connections and funnel them via smaller number of connections to actual database. We use and recommend PgBouncer for that.
Kristo Kaiv has his own take on tutorial here: http://kaiv.wordpress.com/category/plproxy/