The Oracle database has been the gold standard for enterprise applications for a long time now. It has great performance, solid reliability and most of the features that you could want are available. The big problem is that it is expensive. And I mean REALLY expensive. That’s just for the base product as well. All of the extra features that you might want are chargeable extras which means that wench developing for Oracle, you often have to work without some of the more advanced features because they would cost too much.
PostgreSQL
Postgres Vacuum and AutoVacuum.
Basics of vacuum Postgres maintains multi version consistency by keeping old versions of changes tuples instead of actually deleting them. Eventually, keeping all of those out of date versions becomes big burden in terms of storage and performance. Eventually, you end up with bloated tableland indexes. If not felt with, eventually they would fill up tour disks but they would probably make the database unusable before then so we have a handy process to clean it all up. That is Vacuum. Postgres Vacuum goes through your tables and indexes an cleans out had tuples – that is tuples that can no longer be needed by a transaction.
Backing up and Restoring Postgres using pg_basebackup
There are several great tools available that handle backing up and managing the backups of your PostgresQL database. It is really important to understand the underlying process that these tools use though as well as the standard postgres commands are that you would need to run in case you ever need to do it manually.
There are 2 types of backups that you can take in Posgres, logical and physical. Logical backups are in the form if the SQL statements necessary to recreate the database (not necessarily in a human readable form). The 2 tools to take logical backups are pg_dump and pg_dumpall.
What is the pg_clog and the clog
There are several directories named log in a Postgres installation.
You have pg_xlog, pg_log and pg_clog.
These are all important but I’ll talk about the others another time.
Pg_clog is the commit log. It is generally a small folder that you should never have a reason to look at. (note that from version 10 of postgres the pg_clog directory is being renamed to pg_xact I will continue to refer to it as pg_clog in this document but the functioning of both is the same)
Importantly, you can never delete anything from that directory. If you do your database will become unusable and you will need to recreate it from a backup.
Setting up a Postgres test cluster in vagrant
Most of the time its fine to test things out on your local machine and local installation of Postgres but often, you want more flexibility, the ability to quickly reset to a known starting point and the ability to try out different and more complex server architectures.
That is where Vagrant comes in. I use it to quickly set up Postgres clusters at different versions and different configurations.
You can find all of these in my repo at https://github.com/philmcc/postgres_clusters
For example here is a set up to allow you to set up a 3 node cluster with 1 leader and 2 followers all on postgres 9.4 and using replication slots:
Failover testing of a Postgres cluster
Testing failover to one of 2 slaves and reattaching to the new master.
Starting config
master | | slave1 slave2
Post failover config
slave1 | | slave2 master
Get all of the tables linked by a Foreign Key
Here is a handy little PostgreSQL query that will list all of the tables linked by a foreign key to the table YOUR_TABLE. The output will include the keys on each side. This can be very useful if you want to build up a map of your database through the relationships between the tables. select …
How to kill all sessions in Postgres so that they stay dead.
Have you ever tried to drop or rename a postgres database or even create another database from one as a template, only to be confronted by the dreaded message: postgres=# create database test with template a_database; ERROR: source database “a_database” is being accessed by other users DETAIL: There are 40 other sessions using the database. …