PostgreSQL & MySQL
The open-source relational databases behind most web applications, and SQL practice.
Advanced
Once the basics are comfortable.
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PostgreSQL EXPLAIN Glossary
Reads like a dictionary for query plans: what each node in EXPLAIN output means and what to look for when a query is slow.
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PostgreSQL Wiki: Don't Do This
A short list of common •••• mistakes and what to do instead, from choosing data types to using timestamps and NULL correctly.
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