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.
51 lines
1.4 KiB
Markdown
51 lines
1.4 KiB
Markdown
---
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title: Use UPSERT for Insert-or-Update Operations
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impact: MEDIUM
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impactDescription: Atomic operation, eliminates race conditions
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tags: upsert, on-conflict, insert, update
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---
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## Use UPSERT for Insert-or-Update Operations
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Using separate SELECT-then-INSERT/UPDATE creates race conditions. Use INSERT ... ON CONFLICT for atomic upserts.
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**Incorrect (check-then-insert race condition):**
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```sql
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-- Race condition: two requests check simultaneously
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select * from settings where user_id = 123 and key = 'theme';
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-- Both find nothing
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-- Both try to insert
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insert into settings (user_id, key, value) values (123, 'theme', 'dark');
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-- One succeeds, one fails with duplicate key error!
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```
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**Correct (atomic UPSERT):**
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```sql
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-- Single atomic operation
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insert into settings (user_id, key, value)
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values (123, 'theme', 'dark')
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on conflict (user_id, key)
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do update set value = excluded.value, updated_at = now();
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-- Returns the inserted/updated row
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insert into settings (user_id, key, value)
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values (123, 'theme', 'dark')
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on conflict (user_id, key)
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do update set value = excluded.value
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returning *;
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```
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Insert-or-ignore pattern:
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```sql
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-- Insert only if not exists (no update)
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insert into page_views (page_id, user_id)
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values (1, 123)
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on conflict (page_id, user_id) do nothing;
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```
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Reference: [INSERT ON CONFLICT](https://www.postgresql.org/docs/current/sql-insert.html#SQL-ON-CONFLICT)
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