Read a PostgreSQL EXPLAIN plan before adding an index
Find the expensive step and distinguish estimates from measurements.
PostgreSQL / SQL / Performance
TECHNOLOGY / KNOWLEDGE / PRACTICE
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Practical guides to AI, Python, Git, SQL, Web/API, SAP/ERP and mathematical models, with explanations, code examples, sources and limitations.
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Why JOIN Multiplies Rows: Matching Keys, Constraints and Output Grain
Diagnose repeated JOIN rows by counting matching keys, checking constraints and choosing whether the result should contain orders, items or summaries.
SQL / JOIN / PostgreSQL
Why SQL comparisons with NULL do not behave like zero
Understand IS NULL, three-valued logic and the difference between COUNT(*) and COUNT(column).
SQL / PostgreSQL / NULL