Practitioner reference for Elasticsearch 9.x: index and document operations, Query DSL, aggregations, vector / kNN search, ESQL, cluster management, version compatibility notes, and the gotchas that bite first-time operators.
MySQL cheat sheet covering CLI commands, database and table operations, joins, indexes, backups, user management, and transactions, with version notes for 5.7, 8.0, and 8.4.
MySQL 8.4 is the new LTS branch through April 2032. The 8.0 to 8.4 upgrade is much smaller than 5.7 to 8.0, but removed options and the new authentication_policy variable still bite. Full procedure with rollback.
MySQL 5.7 has been past its EOL since October 2023. Here is the migration to 8.0: prerequisites, dry-run with mysqlcheck, in-place upgrade, the authentication-plugin change that breaks old clients, and rollback if it goes sideways.
Format a MySQL DATETIME in a SELECT using DATE_FORMAT, common patterns for ISO-8601 and human-readable output, and CONVERT_TZ for timezone-correct dashboards.
MariaDB started as a MySQL fork and is binary-compatible up to a point. Where the wire protocol stays the same, where the SQL dialect diverges, and how to migrate a real database without losing anything.
Change a MySQL column type with ALTER TABLE MODIFY or CHANGE. Covers data preservation, CAST behavior, INT to BIGINT, VARCHAR widening, and how ALGORITHM applies.
Find rows in one MySQL table that do not exist in another using LEFT JOIN IS NULL, NOT EXISTS, or NOT IN. Each method's NULL behaviour and performance trade-off.
Reset a forgotten MySQL root password using --skip-grant-tables, an init-file, or my.cnf. Step-by-step for MySQL 5.7, 8.0, and the changes that landed in 8.4.
Add a column to a MySQL table with ALTER TABLE ADD COLUMN. Covers DEFAULT values, NOT NULL on existing rows, AFTER positioning, and ALGORITHM=INSTANT on MySQL 8.0.12+.
The alias-swap pattern I use in production to change Elasticsearch mappings without taking the search down. Walks through the five steps, gives you a parameterized bash script that runs the reindex and rolls back if needed, and verifies the technique against Elasticsearch 8.x and 9.x.
Delete duplicate rows in MySQL while keeping one per group, using DELETE JOIN, ROW_NUMBER with CTE, or the safe temp-table swap. With dry-run, transactions, and rollback.
Find duplicate rows in MySQL with GROUP BY HAVING, a ROW_NUMBER window function, or a self-join. Includes NULL behaviour, soft duplicates, and the right index.
MySQL 8.0 made many ALTER TABLE operations instant. The ones that aren't (column type changes, charset conversion, adding indexes on large tables) still need an external tool. Native online DDL vs gh-ost vs pt-online-schema-change, compared on a real production schema.
JOIN two MySQL tables with INNER, LEFT, and RIGHT JOIN. With a real example, when each one matches, and the duplicate-row multiplication mistake everyone makes once.
Import a CSV into MySQL using LOAD DATA INFILE, LOAD DATA LOCAL INFILE, or the mysqlimport command. Covers header rows, encoding, the FILE privilege, and broken CSVs.
Export a MySQL table to CSV using SELECT INTO OUTFILE, mysqldump --tab, the mysql client with a piped redirect, or a no-privilege one-liner that works on hosts where FILE is disabled.
MySQL utf8 is a 3-byte alias for utf8mb3 that cannot store emoji or many CJK characters. utf8mb4 is the real UTF-8 and has been the default since MySQL 8.0. The migration looks like ALTER TABLE but bites on index length and ROW_FORMAT.
MyISAM has no transactions, no foreign keys, and corrupts on crash. InnoDB has been the MySQL default since 5.5 and is the only engine that gets new features. Here is how to convert every MyISAM table in a MySQL database with a single SQL script.
Back up every MySQL database in one command with mysqldump, including --single-transaction for InnoDB consistency, gzip compression, restore steps, and the modern alternatives (MySQL Shell, Percona XtraBackup).
Four ways to store an MD5 hash in MySQL: CHAR(32), VARCHAR(32), BINARY(16), and the two-BIGINT split. Storage cost, index speed, and when to use each (plus when to use a stronger hash instead).
Every MySQL data type with exact storage size, value range, and when to use it. Numeric, string, date/time, JSON, spatial, binary, and the new VECTOR type added in MySQL 9.0.