Given a table users(id, name, email) with an index on (id, email), which query can use an index-only scan?

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PostgreSQL - Indexing Strategies

Given a table users(id, name, email) with an index on (id, email), which query can use an index-only scan?

ASELECT id, email FROM users WHERE id = 10;

BSELECT name FROM users WHERE id = 10;

CSELECT email FROM users WHERE name = 'Alice';

DSELECT * FROM users WHERE email = 'a@example.com';

Step-by-Step Solution

Solution:

Step 1: Check index columns and query columns
The index covers columns id and email. Query C requests only id and email.
Step 2: Determine if index-only scan is possible
Query C can use index-only scan because all requested columns are in the index. Others request columns not in the index.
Final Answer:
SELECT id, email FROM users WHERE id = 10; -> Option A
Quick Check:
Query columns ⊆ index columns = index-only scan [OK]

Quick Trick: Index-only scan if query columns are subset of index columns [OK]

Common Mistakes:

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