5 Data
Every entry filed under Data and its subtopics.
What kind of help
Works on
What do these mean?
- Beginner
- You are new to this topic.
- Intermediate
- You use it, and want to get better.
- Advanced
- You know it well and need the details.
- Guided project
- Build something small from start to finish, step by step.
- Explains an idea
- Plain words on how something works and why.
- Step by step
- Numbered steps to get one task done.
- Cheat sheet
- Commands and options in a table, to look up fast.
- Fixes an error
- An error message, what it means, and how to solve it.
27 entries match these filters
- 0421 grep, sed, awk and jq: the four text tools you will use every week A working reference for the four commands that handle most text on the command line: grep finds lines, sed edits them, awk… Intermediate · Cheat sheet · Linux, macOS Article jq & JSON on the command line · Modern command-line tools · Text processing 15 Oct 2026
- 0364 Schedule PostgreSQL and MySQL dumps with systemd timers Write one dump script for PostgreSQL and MySQL, run it from a systemd service and timer instead of cron, keep two weeks… Intermediate · Step by step · Linux Article Backup tools · PostgreSQL 15 Oct 2026
- 0324 Fix: psql: FATAL: Peer authentication failed On local socket connections PostgreSQL checks that your OS user name equals the database role. When they differ you get this error.… Intermediate · Fixes an error · Linux Article Install & connect · Security & access 15 Oct 2026
- 0326 Fix: too many open files A process hit its file descriptor limit, usually 1024. Raise it for the shell, the systemd unit or the user, fix a… Intermediate · Fixes an error · Linux, macOS Article Performance & tuning · Processes & ports · Services & systemd 15 Oct 2026
- 0331 docker compose for an app with a PostgreSQL database A compose.yaml that runs your application next to a PostgreSQL container, with a named volume for the data, a health check so… Intermediate · Step by step · Linux, macOS, Windows Article Docker Compose · Install & connect 15 Oct 2026
- 0233 Turn on MongoDB authentication and create least-privilege users A fresh MongoDB install accepts connections without a password. Create a user administrator, enable authorization, then give each application and backup job… Intermediate · Step by step · Linux Article Administration · Security & access 15 Oct 2026
- 0235 Fix: MongoServerSelectionError: connect ECONNREFUSED 127.0.0.1:27017 Your app or tool tried to reach MongoDB on port 27017 and nothing accepted the connection. Usually mongod is not running, crashed… Intermediate · Fixes an error · Linux, macOS, Windows Article Administration · Basics 15 Oct 2026
- 0238 Redis persistence and high availability: RDB, AOF, replicas and Sentinel Redis keeps data in memory and writes it to disk with RDB snapshots, an append-only file, or both. Replicas copy data to… Intermediate · Explains an idea · Linux Article Administration & persistence 15 Oct 2026
- 0240 Fix: MISCONF Redis is configured to save RDB snapshots, but it’s currently unable to persist to disk Redis blocks writes because its last background snapshot failed. Find the cause in the log (full disk, permissions, failed fork), fix it,… Intermediate · Fixes an error · Linux Article Administration & persistence 15 Oct 2026
- 0243 Database backup strategy: the 3-2-1 rule and testing restores Keep three copies of your data on two kinds of storage with one copy offsite, choose dump or snapshot methods per database,… Intermediate · Step by step · Linux Article Database operations · Security & access 15 Oct 2026
- 0250 Containerize a Node.js app and run it with PostgreSQL using Docker Compose Package a small Node.js web app in a multi-stage Docker image, then run it next to PostgreSQL with Docker Compose, named volumes,… Intermediate · Guided project · Linux, macOS, Windows Article Docker basics · Docker Compose · PostgreSQL 15 Oct 2026
- 0257 Run PostgreSQL properly on a Linux server: install, lock down, back up, monitor and patch Set up PostgreSQL 18 from the official repository for one application: a dedicated role and database, SCRAM logins from one app server… Intermediate · Guided project · Linux Article Backup & restore · Install & connect · Roles & administration 15 Oct 2026
- 0259 Speed up a slow database query with a Redis cache: cache-aside in Python, measured and locked down Put Redis in front of a slow query in a small Python app: cache-aside reads, TTLs with jitter, invalidation on writes, an… Intermediate · Guided project · Linux Article Administration & persistence · Basics 15 Oct 2026
- 0204 VACUUM, autovacuum and table bloat in PostgreSQL Updates and deletes leave dead row versions behind. VACUUM makes that space reusable, keeps statistics fresh and prevents transaction ID wraparound; autovacuum… Intermediate · Explains an idea · Linux Article Performance & tuning · Roles & administration 15 Oct 2026
- 0209 Fix: FATAL: password authentication failed for user PostgreSQL reached the server and found a matching pg_hba.conf rule, but the password check failed. Usually a wrong password, a role without… Intermediate · Fixes an error · Any system Article Roles & administration · Security & access 15 Oct 2026
- 0210 Fix: FATAL: no pg_hba.conf entry for host The server received the connection but no pg_hba.conf rule matches this client address, user, database and SSL mode. Add or correct a… Intermediate · Fixes an error · Linux Article Install & connect · Security & access 15 Oct 2026
- 0216 MySQL vs MariaDB: what actually differs MariaDB started as a fork of MySQL and still speaks the same client protocol, but GTIDs, JSON storage, authentication and tooling have… Intermediate · Explains an idea · Linux Article Choosing a database · MySQL & MariaDB 15 Oct 2026
- 0222 Connecting to Oracle: SQL*Plus, SQLcl, EZConnect, tnsnames.ora and the listener Clients reach an Oracle database through the listener, using an Easy Connect string or a tnsnames.ora alias. Here are the formats, the… Intermediate · Cheat sheet · Linux, macOS, Windows Article Administration · Concepts & tools 15 Oct 2026
- 0225 Oracle Data Pump: export and import schemas with expdp and impdp Data Pump moves schemas, tables or whole databases between Oracle databases as dump files. Set up a directory object, export with expdp… Intermediate · Step by step · Linux, Windows Article Administration · Backup with RMAN 15 Oct 2026
- 0226 Fix: ORA-12541: TNS:no listener The client reached the host but nothing was listening on that port. Start or fix the Oracle listener, or correct the host… Intermediate · Fixes an error · Linux, Windows Article Administration · Concepts & tools 15 Oct 2026
- 0227 Fix: ORA-01017: invalid username/password; logon denied Oracle rejected the login. Check the password (case-sensitive), that you connect to the right PDB service, and whether the account is locked… Intermediate · Fixes an error · Any system Article Administration · Security & access 15 Oct 2026
- 0201 pg_hba.conf and PostgreSQL authentication methods pg_hba.conf decides who may connect from where and how they must prove who they are. Rules are checked top to bottom and… Intermediate · Explains an idea · Linux Article Install & connect · Security & access 15 Oct 2026
- 0200 PostgreSQL roles, privileges and default privileges Create login and group roles, grant access to databases, schemas and tables, and use ALTER DEFAULT PRIVILEGES so tables created later get… Intermediate · Step by step · Linux Article Roles & administration · Security & access 15 Oct 2026
- 0068 SQL joins explained: INNER, LEFT and finding rows with no match A join combines rows from two tables using a matching condition. INNER JOIN keeps only matches, LEFT JOIN keeps every row from… Intermediate · Explains an idea · Any system Article Joins 15 Oct 2026
- 0069 NULL in SQL: why = NULL never matches and how to handle missing values NULL means "unknown", so comparisons with it are neither true nor false. Use IS NULL, COALESCE and careful aggregates to avoid rows… Intermediate · Explains an idea · Any system Article Queries 15 Oct 2026
- 0071 Fix: column “x” must appear in the GROUP BY clause or be used in an aggregate function PostgreSQL rejects a grouped query that selects a column with more than one possible value per group. Add the column to GROUP… Intermediate · Fixes an error · Any system Article Queries 15 Oct 2026
- 0074 pandas basics: read a CSV, filter rows and summarize with groupby Load a CSV into a pandas DataFrame, inspect it, filter rows with conditions, and compute totals per group, with the SQL equivalent… Intermediate · Step by step · Any system Article Data analysis · Files & automation 15 Oct 2026