postgresql（PostgreSQL 数据库）

PostgreSQL

Data Type Defaults

Schema Rules

• Every FK column gets an index (PG does NOT auto-create these)
• NOT NULL on every column unless NULL has business meaning
• CHECK constraints for domain rules at DB level
• EXCLUDE constraints for range overlaps: EXCLUDE USING gist (room WITH =, during WITH &&)
• Default created_at TIMESTAMPTZ NOT NULL DEFAULT now()
• Separate updated_at with trigger, never trust app layer alone
• Use BIGINT PKs -- cheaper JOINs than UUID, better index locality
• Safe migrations: CREATE INDEX CONCURRENTLY, add columns with DEFAULT (instant add). Never ALTER TYPE on large tables in-place.
• NULLS NOT DISTINCT on unique indexes (PG15+) -- treats NULLs as equal for uniqueness
• Revoke default public schema access: REVOKE ALL ON SCHEMA public FROM public

Migration Safety

Core rules:

• Every schema change is a migration. No ad-hoc DDL in production.
• Migrations are immutable once deployed -- never edit a migration that has run in any shared environment.
• Schema migrations and data migrations are separate files. Schema changes are fast and transactional; data backfills are slow and may need batching.
• Forward-only in production. Rollback = a new forward migration that reverses the change.

Expand-contract pattern for zero-downtime renames and removals:

1. Expand: add the new column/table, backfill data, update writes to populate both old and new
2. Migrate: switch reads to the new column/table, verify in production
3. Contract: remove the old column/table in a later deploy

Never rename or remove a column in a single migration -- callers reading the old name will break between deploy and code rollout.

Dangerous operations:

• NOT NULL without a DEFAULT on an existing table locks and rewrites every row. Add the column nullable first, backfill, then add the constraint.
• CREATE INDEX (without CONCURRENTLY) locks writes for the duration. Always use CONCURRENTLY, which cannot run inside a transaction block -- keep it in its own migration.
• Large data backfills: batch with FOR UPDATE SKIP LOCKED to avoid locking the entire table:

UPDATE target SET new_col = compute(old_col)
WHERE id IN (
  SELECT id FROM target
  WHERE new_col IS NULL
  LIMIT 1000
  FOR UPDATE SKIP LOCKED
);

Run in a loop until zero rows affected.

Index Strategy

Index rules:

• Composite: most selective column first, max 3-4 columns
• Partial: WHERE status = 'active' -- smaller, faster
• Covering: INCLUDE (col) -- avoids heap lookup
• Expression: ON (lower(email)) -- for function-based WHERE
• fillfactor = 70-90 on write-heavy tables -- reserves space for HOT updates, reducing index bloat
• Drop unused indexes (only after one full business cycle since last restart -- check pg_stat_database.stats_reset first, otherwise you may drop a primary key on a freshly restarted DB or read replica): SELECT * FROM pg_stat_user_indexes WHERE idx_scan = 0

Detect unindexed foreign keys:

SELECT conrelid::regclass, a.attname
FROM pg_constraint c
JOIN pg_attribute a ON a.attrelid = c.conrelid AND a.attnum = ANY(c.conkey)
WHERE c.contype = 'f'
  AND NOT EXISTS (
    SELECT 1 FROM pg_index i
    WHERE i.indrelid = c.conrelid AND a.attnum = ANY(i.indkey)
  );

JSONB Patterns

-- GIN index for containment queries
CREATE INDEX ON items USING gin (metadata);
SELECT * FROM items WHERE metadata @> '{"status": "active"}';

-- Expression index for specific key access
CREATE INDEX ON items ((metadata->>'category'));
SELECT * FROM items WHERE metadata->>'category' = 'electronics';

Prefer typed columns over JSONB for frequently queried, well-structured data. Use JSONB for truly dynamic/variable attributes.

Use jsonb_path_ops operator class for containment-only (@>) queries -- 2-3x smaller index. Use default jsonb_ops when key-existence (?, ?|) is needed.

Row-Level Security (RLS)

ALTER TABLE orders ENABLE ROW LEVEL SECURITY;
ALTER TABLE orders FORCE ROW LEVEL SECURITY;  -- applies to table owner too

-- Set session context (generic, no extensions needed)
SET app.current_user_id = '123';

CREATE POLICY orders_user_policy ON orders
  FOR ALL
  USING (user_id = current_setting('app.current_user_id')::bigint);

Performance: Policy expressions evaluate per row. Wrap function calls in a scalar subquery so PG evaluates once and caches:

-- BAD: called per row
USING (get_current_user() = user_id)
-- GOOD: evaluated once, cached
USING ((SELECT get_current_user()) = user_id)

Always index columns referenced in RLS policies. For complex multi-table checks, use SECURITY DEFINER helper functions.

Query Optimization

• Always EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT) before optimizing
• Use pg_stat_statements for slow-query detection and pg_stat_user_tables for bloat (see Detection queries below for the full SQL)
• Sequential scan on large table -> add index or check WHERE for function wrapping
• High rows removed by filter -> index doesn't match predicate
• CTEs are inlined by default; use MATERIALIZED/NOT MATERIALIZED hints to control optimization
• Prefer EXISTS over IN for correlated subqueries
• Use LATERAL JOIN when subquery needs outer row reference
• Cursor pagination (WHERE id > $last ORDER BY id LIMIT $n) over OFFSET
• Approximate row counts: SELECT reltuples FROM pg_class WHERE relname = 'table' -- avoids full count(*) on large tables
• Materialized views for expensive aggregations: REFRESH MATERIALIZED VIEW CONCURRENTLY (needs unique index). Schedule refresh, not per-query.

Concurrency Patterns

See concurrency-patterns.md for UPSERT, deadlock prevention, N+1 elimination, batch inserts, and queue processing with SKIP LOCKED.

Partitioning

Use when table exceeds ~100M rows or needs TTL purge:

• RANGE -- time-series (by month/year), most common
• LIST -- categorical (by region, tenant)
• HASH -- even distribution when no natural key

Partition key must be in every unique/PK constraint. Create indexes on partitions, not parent.

Transactions & Locking

• Keep transactions short -- long txns block vacuum and bloat tables
• Advisory locks for application-level mutual exclusion: pg_advisory_xact_lock(key)
• Non-blocking alternative: pg_try_advisory_lock(key) -- returns false instead of waiting
• Check blocked queries: SELECT * FROM pg_stat_activity WHERE wait_event_type = 'Lock'
• Monitor deadlocks: SELECT deadlocks FROM pg_stat_database WHERE datname = current_database()

Full-Text Search

See full-text-search.md for weighted tsvector setup, query syntax, highlighting, and when to use PG full-text vs external search.

Connection Pooling

Always pool in production. Direct connections cost ~10MB each.

• PgBouncer in transaction mode for most workloads
• statement mode if no session-level features (prepared statements, temp tables, advisory locks)

Prepared statement caveat: Named prepared statements are bound to a specific connection. In transaction-mode pooling, the next request may hit a different connection. Use unnamed/extended-query-protocol statements (most ORMs default to this), or deallocate immediately after use.

Operations

See operations.md for performance tuning, maintenance/monitoring, WAL, replication, and backup/recovery.

Vector Search (pgvector)

CREATE EXTENSION vector;
ALTER TABLE items ADD COLUMN embedding vector(1536);  -- match your model's output dimensions

-- HNSW: better recall, higher memory. Default choice.
CREATE INDEX ON items USING hnsw (embedding vector_cosine_ops);

-- IVFFlat: lower memory for large datasets. Set lists = sqrt(row_count).
CREATE INDEX ON items USING ivfflat (embedding vector_cosine_ops) WITH (lists = 1000);

Always filter BEFORE vector search (use partial indexes or CTEs with pre-filtered rows). Distance operators: <=> cosine, <-> L2, <#> inner product.

Anti-Patterns

Detection queries:

-- Slow queries (requires pg_stat_statements)
SELECT query, mean_exec_time, calls
FROM pg_stat_statements
WHERE mean_exec_time > 100
ORDER BY mean_exec_time DESC LIMIT 20;

-- Table bloat (dead tuples awaiting vacuum)
SELECT relname, n_dead_tup, last_vacuum, last_autovacuum
FROM pg_stat_user_tables
WHERE n_dead_tup > 10000
ORDER BY n_dead_tup DESC;

-- Unused indexes (candidates for removal)
SELECT schemaname, relname, indexrelname, idx_scan
FROM pg_stat_user_indexes
WHERE idx_scan = 0 AND indexrelname NOT LIKE '%_pkey'
ORDER BY pg_relation_size(indexrelid) DESC;

Verify

Run EXPLAIN (ANALYZE, BUFFERS) on changed queries. Confirm no sequential scans on large tables and no unindexed FK columns before declaring done.