14 Mar 2017

Using pg_dumpall with Google Cloud Postgres

Luckily the recent update to pg_dumpall works (without any extra effort) to dump Globals off the recently launched Google Cloud PostgreSQL service well.

This is because this Postgres service seems to be using the same limitation (as AWS RDS Postgres does) in that none of the user accounts are SuperUser in the Postgres sense. Therefore, this platform too disallows accessing pg_authid.

The fallback here too, is to use pg_roles to read (as much as is possible) to dump Role related Globals. Read here for more info on the recent changes to pg_dumpall that makes this work.

12 Mar 2017

Redshift Limitations

Recently have been toying around with Redshift (a popular Data-Warehouse tool by AWS) a little bit and see some minor but obvious limitations in SQL limitations:

FULL OUTER JOIN + ON Func(x) = Func(y) : Not Supported

Although Redshift supports most variants of FULL OUTER JOIN, however, when the JOIN condition contains a function on both sides, it gives up complaining "ERROR:  XX000: could not devise a query plan for the given query".

CREATE TABLE x (a integer);

SELECT x1.a, x2.a
FROM x x1
     ON LOWER(x1.a) = LOWER(x2.a); -- Fails
  -- ON LOWER(x1.a) = x2.a; -- Works
  -- ON x1.a = LOWER(x2.a); -- Works
  -- ON x1.a = x2.a; -- Obviously Works

Workaround: The obvious way around it is to move one of the functions to a separate SQL (for e.g create a Temp Table and use that in the final computation), something like this:


SELECT x1.a, x2.a
ON x1.a = LOWER(x2.a); -- Works

7 Mar 2017

Using pg_dumpall with AWS RDS Postgres

My patch (allowing a non-super-user to dump Globals) just got committed to Postgres 10.0.

Besides the use mentioned above, this patch allows one to dump Globals from their AWS RDS Postgres Database and have a complete backup as a part of their Disaster Recovery (DR) solution. This earlier required an inelegant hack (detailed here in another post of mine) that wasn't very convenient for a regular user.

For those interested, RDS Postgres (by design) doesn't allow you to read pg_authid, which was earlier necessary for pg_dumpall to work. With this patch checked-in, pg_dumpall now uses a workaround (pg_roles) to read the same data (except passwords) and generate the same SQL Script.

With that mentioned, let's get our hands dirty and see a working solution:

# Take RDS Postgres Dump
# We store the pgdb database in pgdb.sql and globals in pgdb_globals.sql
pg_dumpall --globals-only --no-role-password \
  -U rdssuperuser -h rdsinstance \
  2>stderr.txt > pgdb_globals.sql
pg_dump -U rdssuperuser -h rdsinstance pgdb 2>stderr.txt > pgdb.sql

# Restore on Local Machine
psql -U postgres -c "CREATE DATABASE pgdb;"

# rdsadmin database (even if empty) is required for the next script
psql -U postgres -c "CREATE DATABASE rdsadmin;" 

psql -U postgres pgdb < pgdb_globals.sql
psql -U postgres pgdb < pgdb.sql

Note: The --no-role-password flag is still necessary since AWS RDS doesn't allow you to read Passwords. Therefore, the above generates a script that resets all users to be password-less, but that's still better than having to restore Globals (like 'CREATE USER ... ') manually by hand!

Big thanks to Simon for ironing out a few issues, Stephen, David & Robert for their (critical) reviews.

23 Dec 2016

Watch an Online Movie: (Wget -c || Deluge) && Chrome > Chromecast

For all those who are in an odd situation where:

  • They have a paid account to a Movie site (like Netflix etc.)
  • Are unable to watch movies online, just because the video-streaming is just too slow
    • Either because your convenient times are 'peak' times for the server
    • Or, you are behind a painfully bad ISP
  • And are able to download the movie, as an option.

To such customers, downloading the movie overnight (using for e.g. wget) would be a big help!

I regularly use this, to download the movie, and watch with non-tech people (my kids) who can't be explained why the movie keeps 'Buffering'!

c:\bin\wget \
  -O KD1242.mp4 \ # Output filename
  -t 0 \ # Retrying indefinitely
  -c -T 10 \ # Reconnect + Timeouts are 10 seconds
  -w 10 \ # wait 10 seconds before retrying a disconnection

Windows binaries for GPL'ed GNU software (such as Wget) are heaven sent here:

And if you need a Big-TV experience (for e.g. if you have a Chromecast), you could stitch things together by using the Google Cast extension for your Chrome Browser, and open up "c:\" on the browser to play the movie directly in the browser (since VLC stream is still in Beta):

If by chance you're downloading videos via torrents (For e.g. NASA videos), here are my GPL recommendations for Windows:

  • Deluge: If you haven't seen this, you should really replace your uTorrent etc. clients with this one
  • Use the Streaming Extension (Github Link)
  • Copy the URL that the Extension provides + Paste to Chrome
  • Cast your tab to your Chromecast + Enjoy!

Have Fun!

9 Dec 2016

Custom pg_dumpall now works with AWS Redshift

While trying to work with AWS Redshift, it was interesting to see pg_dumpall failing to dump databases in my cluster! 

Delving further, for obvious reasons a managed service like this hides some global-information, which pg_dump(all) needs ... and no, this post is NOT about circumventing that.

This branch, gives a 'near' workaround for those who are okay with being able to extract all databases + (almost*) all Global information (Users etc.) in a single command. 

There are caveats though:

  • Redshift doesn't support COPY TO, so the best workaround is using INSERT. Painful, but works.
  • Barring passwords, all Globals can be dumped. The script just resets all users to be password-less, but that's better than having to do 'CREATE USER ... ' commands for your users by hand!

For some people, these caveats are going to be okay, considering that they get a scriptable way of taking a dump of all databases at one go, along with User information. 

If you're interested in a similar hack for AWS Postgres, you're in luck!

Down the line, I'll try to push this to the core, but for now, this works!

Go Play :) !!