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Danylo Hlynskyi danbst

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View postgresql-vacuum-freeze.md

VACUUM FREEZE

Our database has fast rate of transactions. I did think, that such rate is impossible, until I've seen it with my eyes. I've been instinctively insisting on fixing code to reduce number of transactions (sometimes successful), but never checked what are consequences for database.

One of consequences is fast rate of autovacuum VACUUM FREEZE.

VACUUM is essential for Postgresql to mark non-visible rows (those which were normal rows, but became invisible because of update or delete) as visible to PG. Note, that they become visible only to PG, previously PG didn't know they are free for use, and thus had allocated extra buffers in the end of a table for any new rows (inserts or updates). PG not knowing which rows are free for use is another problem, which causes table bloat, which deserves another post.

VACUUM FREEZE is special kind of VACUUM. From resource usage perspective, it differs from VACUUM by that it has to scan full table (normal VACUUM processes only buffers known to contain at

@danbst
danbst / iphone.nix
Created Sep 8, 2018
iPhone pairing for NixOS
View iphone.nix
# First add this module to your /etc/nixos/configuration.nix
# ...
# imports = [ /path/to/iphone.nix ];
# iphone.enable = true;
# iphone.user = "yourusername";
# ...
# Then rebuild system. Attach iPhone via cable, open terminal and run command `iphone`
# It will fail, but there will occure a dialog on your iPhone to "trust this computer"
# Press OK there and run `iphone` again. If it succeeds it will open a freshly mounted folder
@danbst
danbst / bash-curl-pup-jq-notifysend-nix.md
Created Jul 19, 2018
Bash. Curl. Pup. Jq. Notify-send. Nix
View bash-curl-pup-jq-notifysend-nix.md

Цей список складений з технологій (утиліт Linux), які я використаю для однієї життєвої ситуації - пошуку аренди квартири по риночній ціні.

Знайомим з поняттям "скрейпінг" мабуть уже зрозуміло, про що буде пост. Тим не менше, запрошую під кат.

Постановка задачі

OLX є, здається, найактивнішою площадкою для пошуку аренди квартири у Києві. Але просто зайти на OLX, задати фільтри (район, кількість кімнат) недостатньо. Справа в тім, що є 3 типи аренд:

  • аренди від ріелторів - ціни на ці квартири майже завжди завищені, тому-що від ціни квартири залежить дохід ріелтора. Через завищену вартість (неринкову) афіші висять відносно довго
  • аренди від хазяїв - ціни на квартири нижчі, щоб швидше знаходити арендаторів
@danbst
danbst / default.nix
Created Jul 13, 2018
Proof-of-Concept running PostgreSQL tests inside Nix package build
View default.nix
with import <nixpkgs> { };
runCommand "some-test" {
buildInputs = [ postgresql ];
preCheck = ''
set -e
export PGDATA=$TMP/db
export PGHOST=$TMP/socketdir
mkdir $PGDATA $PGHOST
pg_ctl initdb
echo "unix_socket_directories = '$PGHOST'" >> $PGDATA/postgresql.conf
View nixos-infect.sh
#! /usr/bin/env bash
# More info at: https://github.com/elitak/nixos-infect
set -o pipefail
set -e
makeConf() {
# Skip everything if main config already present
[[ -e /etc/nixos/configuration.nix ]] && return 0
# NB <<"EOF" quotes / $ ` in heredocs, <<EOF does not
@danbst
danbst / README.md
Last active Jun 17, 2019
Old Firefox with latest Java plugin. For the upcoming in August Java plugin deprecation in Firefox
View README.md

What is this for

Firefox and Oracle deprecated Java plugin. But some online banking systems rely on that plugin very much. Soon the only solution would be to use old browser version.

Here I try to solve the yet-to-come problem using Nix.

  1. I do use an old firefox-esr from 18.03 NixOS release (52.9.0esr as of writing)
  2. However Java plugin is used from bleeding edge nixpkgs-usntable
  3. Oracle makes some complications for automated JDK fetchers, but we overcome them with some hacky curl | sh from some random script in Internet. We accept Oracle license by using Oracle JDK
  4. But thanks to Nix build environment, that curl | sh hardly can harm our system,
View prefetch-github.sh
#!/usr/bin/env nix-shell
#!nix-shell -i bash -p gitMinimal nix-prefetch-git jq
set -e
project="$1"
branch="${2:-master}"
owner=$(echo "$project" | cut -d/ -f1)
repo=$(echo "$project" | cut -d/ -f2)
rev=$(git ls-remote "git://github.com/$project.git" "$branch" | head -n1 | awk '{print $1}')
rev=${rev:-$2}
@danbst
danbst / default.nix
Created Mar 27, 2018
Nix + Python + nanomsg
View default.nix
####################################################################################
# Nix shell for nix-powered environments
# This is like `virtualenv`, but with fine-defined system dependencies
####################################################################################
rec {
nanomsg-py = pythonPackages.buildPythonPackage {
name = "nanomsg-1.0";
src = pkgs.fetchurl { url = "https://pypi.python.org/packages/0b/b1/3037e0e380d5fab2824e69a59bd374da402c0cef264ccb3fe5d71c230c4b/nanomsg-1.0.tar.gz"; sha256 = "843be41258219d9d319cf434a68cac7669834ab9c993ea4bab5b3d87f62a7a13"; };
doCheck = false;
buildInputs = [ pkgs.nanomsg ];
@danbst
danbst / README.md
Last active Dec 5, 2018
nixq - jq but with Nix syntax
View README.md

nixq - jq but with Nix syntax

Familliar syntax and all of <nixpkgs/lib> and builtins in your hands.

Now going through jq tutorial:

Identity filter

$ # jq
@danbst
danbst / nixeval.md
Last active Nov 8, 2017
Is there simpler way to extract string from Nix string?
View nixeval.md
function nixeval { eval echo -en "$(nix-instantiate --eval --strict "$@" | sed 's/${/\\${/g')"; }

Evaluate Nix expression and remove quotes from resulting expresssion:

$ nixeval -E 'builtins.toJSON __nixPath' | jq
[             
  {
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