Postgresql on Jake Worth

Why Is It Called PostgreSQL?

Wed, 10 Nov 2021 11:04:17 -0500

PostgreSQL was originally named “POSTGRES”— all caps. The name was chosen because it was the successor (“post-”) to the “Ingres” database. As the system adopted SQL standards and features, the “ql” as added and capitalization refined.

Query the Size of Postgres Array

Sun, 14 Feb 2016 16:57:20 -0600

Today I learned how to query the size of a Postgres array. There are (at least) two methods that work.

The first is array_length. This requires you to know the array dimension you seek; a useful filter, although most of the time this will probably be 1:

hr-til_development=# select title from posts
hr-til_development=# where array_length(slack_notified_at_likes_threshold, 1) = 1;
 title
-------------------------------------------
 Because Javascript
 Percent Notation
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(3 rows)

When the dimension is 1, a more terse solution is cardinality:

PostgreSQL Empty Array as Default Value

Sun, 14 Feb 2016 15:46:54 -0600

We can add a PostgreSQL array column with a default empty array using:

add column things integer[] not null default '{}';

Cleanup PostgreSQL Databases

Mon, 02 Nov 2015 16:36:56 -0600

Today I learned that I have thirty-nine Postgres databases on my computer, after running this command inside psql:

\l ;

Each one is small, but I don’t like to carrying around old data. I ended up dropping nine of them, with:

drop database foo_development;

For a lighter storage and cognitive load.

Postgres Unlogged

Wed, 23 Sep 2015 07:37:27 -0500

Using a Postgres table for caching? You might want to try making it unlogged.

unlogged tables are not written to the write-ahead log, which makes them much faster. This also means they are not crash-safe, and are truncated when a crash or unclean shutdown occurs. For caching purposes, that’s likely to be an acceptable tradeoff.

Documentation

Prepare / Execute

Tue, 08 Sep 2015 18:51:01 -0500

You can store a PostgreSQL query with prepare.

db=# prepare posts_search as select title from posts limit 5;
PREPARE

Call the method with execute.

db=# execute posts_search;
title
-----------------------------------
Hello World!
My First Pull Request: HAML
My First Pull Request: Sinatra
My First Pull Request: Capistrano
My First Pull Request: SASS
(5 rows)

Deallocate the query with deallocate, and you can set it again.

db=# deallocate posts_search;
DEALLOCATE
db=# prepare posts_search as select title from posts limit 10;
PREPARE

Psql Connect

Tue, 08 Sep 2015 18:51:01 -0500

Want to change database connections from inside psql? You can!

Psql, the REPL for Postgres, has a useful meta-command called \connect, or \c. This lets you establish a new connection to a Postgres server, while closing the current connection.

Here is the required format:

\c or \connect [ dbname [ username ] [ host ] [ port ] ] | conninfo

Only \c [my_database] is required; omitted parameters are taken from the previous connection.

Terminate Database Connections

Tue, 08 Sep 2015 18:51:01 -0500

To maintain data integrity, certain actions like rake db:drop will not execute when there are active sessions on a Postgres database. You’ll get an error like this:

ERROR: database "database_name" is being accessed by other users
DETAIL: There are 2 other session(s) using the database.

Sessions can be killed from the command line, but a safer route is to kill them with a database method. First, revoke all public access from the database: