41 lines
1.3 KiB
Markdown
41 lines
1.3 KiB
Markdown
---
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title: Use Covering Indexes to Avoid Table Lookups
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impact: MEDIUM-HIGH
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impactDescription: 2-5x faster queries by eliminating heap fetches
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tags: indexes, covering-index, include, index-only-scan
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---
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## Use Covering Indexes to Avoid Table Lookups
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Covering indexes include all columns needed by a query, enabling index-only scans that skip the table entirely.
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**Incorrect (index scan + heap fetch):**
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```sql
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create index users_email_idx on users (email);
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-- Must fetch name and created_at from table heap
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select email, name, created_at from users where email = 'user@example.com';
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```
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**Correct (index-only scan with INCLUDE):**
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```sql
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-- Include non-searchable columns in the index
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create index users_email_idx on users (email) include (name, created_at);
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-- All columns served from index, no table access needed
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select email, name, created_at from users where email = 'user@example.com';
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```
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Use INCLUDE for columns you SELECT but don't filter on:
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```sql
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-- Searching by status, but also need customer_id and total
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create index orders_status_idx on orders (status) include (customer_id, total);
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select status, customer_id, total from orders where status = 'shipped';
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```
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Reference: [Index-Only Scans](https://www.postgresql.org/docs/current/indexes-index-only-scans.html)
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