Posts tagged ‘fdw’

20 January, 2016

Postgres FDW, one year later

by gorthx

I’ve been using Postgres’ foreign data wrappers for about a year now.

Stuff I’ve learned/done:
– Those of you on 9.5 can use the new IMPORT FOREIGN SCHEMA. If you’re not quite there yet, Leo and Regina’s function to script table creation as been saving me a ton of time.

I modified it slightly (gist here):
– I name my foreign servers after their source database, and organize the foreign tables into schemas named for the source db/foreign server, so I tweaked the function to handle that
– added a parameter to specify updateable true/false, since I explicitly set that for each table
– changed the output to my preferred formatting.

Updating a server definition is much easier than I expected:

ALTER SERVER my_fdw_server
SET dbname 'new_db_name'
, SET host 'new_host_name'

You can do the same thing with the table definitions;  no need to drop & re-create them.

– Handling a serial field on the source table is interesting.
Say I have a table that looks like this:

id serial primary key
, name text
, color text

I create a foreign table on that, including all fields (exercise left to the reader). Then try to update it as I normally would, allowing Pg to handle the incrementing of the id field:

INSERT INTO table1 (name, color)
VALUES ('bob', 'blue');
ERROR: null value in column "id" violates not-null constraint

I don’t want to pass in a value for id, because that negates the entire purpose of have a serial datatype.

My solution was to leave the id field out of the foreign table definition.

Watch your datatypes, as CREATE FOREIGN TABLE doesn’t do any verification. “Fun” things can happen when you define your foreign table with timestamp without tz (to match the source table), then finally get approval to correct that on the source table, and don’t also change it in your foreign table definition.

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9 March, 2015

PDXPUG Lab Recap: postgres_fdw

by gorthx

Back in January, PDXPUG had a lab night to try out the postgres_fdw.

Our labs are even more casual than our meetings: I don’t set an agenda, I invite the attendees to choose specific questions or topics they want to investigate. Here’s what we came up with for our FDW lab:

– What is it
– Use cases; who has one?
– Best practices?
– Security concerns?
– Performance concerns? (We didn’t get to this.)
– Any diffs between 9.3 and 9.4? (… or this either.)
– Test drive!

Our main focus was the Test Drive. Setup went pretty well for most folks, though one attendee had permissions issues & fell back to file_fdw.

There are four steps (outlined in the docs)
0. Install the extension ;)
1. Create a foreign server
2. Create a user mapping
3. Create your foreign table(s)

Useful commands:
\des to describe foreign servers
\deu to describe user mappings
\det to describe foreign tables

My use case: quick & dirty ETL solution. I pull data from several postgres datasources1 into a staging database (for data conformance and other tasks), and then push it out to a data mart.

To keep it all reasonably organized, I put the foreign tables for each data source in a schema named for that datasource, e.g. the data from HR goes to schema ‘hr’, data from Accounting goes to ‘accounting’, etc. (This has tripped me up a couple of times, which I’ll cover in a later post.)

Here’s a quick example: I have a ‘users’ table in the accounting database, in the public schema. I want to put it in the ‘accounting’ schema locally.

dbname ‘accounting’
, host ‘’
, port ‘5432’
, updatable ‘false’

password ‘[password]’
, user ‘[username]’

CREATE FOREIGN TABLE accounting.users (
id integer
, username text
SERVER accounting
schema_name ‘public’
, table_name ‘users’
, updatable ‘false’

Note that you don’t get an error if the schema & table you specify doesn’t exist in the remote database, so check your work. I just run a quick SELECT … LIMIT 1 on the foreign table. This will also help pick up permissions problems.

According to the docs, the default for updatable is ‘true’, and a table setting overrides a server setting. I originally intepreted that to mean that a table’s default setting of true would override a server’s explicit setting of false, which seemed odd, and my tests don’t bear that out. (Nice to see that Pg once again does the sensible thing.) Regardless, I explicitly set this value at both the server and table level, enabling it only on tables I want to update. Of course, you should also set appropriate permissions on the remote table itself.

If you have a lot of tables you need to create fdw for, script the process! This looks like a good option:

You can ALTER FOREIGN TABLE for the field names as you would a regular table, but I haven’t figured out how/if you can update the options (like, if somebody changes the target hostname or dbname2).

The credentials associated with user mappings are pg_dumped as cleartext. Ideally, I guess you’d use .pgpass to manage the passwords for you, but I don’t have that option on RDS. To avoid accidentally committing schema dumps that include credentials to my git repo, I added this to my pre-commit hook:

sed -i .bak "s/password '.*',/password '[pwd]',/" schema_dump.sql

(The .bak is required on OSX.)

As usual, I picked up some additional “off-topic” info: check out SELECT * [sequence] or \d [sequence]!

Thanks to Matt S for hanging out afterwards and introducing me to the differences between Oracle’s and Pg’s query planners.

1 – and a few others that are being a pain in my ass :koff:salesforce:koff:
2 – of course, why worry about that, because that NEVER happens, right?

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