29b45dd234
- Adds `skills/postgres-best-practices/`: A new skill containing comprehensive Postgres performance optimization rules and guidelines from Supabase. - The skill includes rules for query performance, connection management, security (RLS), and schema design. - Updates `README.md` to include the new skill in the directory.
45 lines
1.4 KiB
Markdown
45 lines
1.4 KiB
Markdown
---
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title: Create Composite Indexes for Multi-Column Queries
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impact: HIGH
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impactDescription: 5-10x faster multi-column queries
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tags: indexes, composite-index, multi-column, query-optimization
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---
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## Create Composite Indexes for Multi-Column Queries
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When queries filter on multiple columns, a composite index is more efficient than separate single-column indexes.
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**Incorrect (separate indexes require bitmap scan):**
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```sql
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-- Two separate indexes
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create index orders_status_idx on orders (status);
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create index orders_created_idx on orders (created_at);
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-- Query must combine both indexes (slower)
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select * from orders where status = 'pending' and created_at > '2024-01-01';
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```
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**Correct (composite index):**
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```sql
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-- Single composite index (leftmost column first for equality checks)
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create index orders_status_created_idx on orders (status, created_at);
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-- Query uses one efficient index scan
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select * from orders where status = 'pending' and created_at > '2024-01-01';
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```
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**Column order matters** - place equality columns first, range columns last:
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```sql
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-- Good: status (=) before created_at (>)
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create index idx on orders (status, created_at);
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-- Works for: WHERE status = 'pending'
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-- Works for: WHERE status = 'pending' AND created_at > '2024-01-01'
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-- Does NOT work for: WHERE created_at > '2024-01-01' (leftmost prefix rule)
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```
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Reference: [Multicolumn Indexes](https://www.postgresql.org/docs/current/indexes-multicolumn.html)
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