# PostgreSQL Riferimento Rapido

*Tabelle, query, join, indici, JSON, ruoli*

> Source: PostgreSQL Documentation (postgresql.org/docs) · MIT

## Connessione

### Riga di Comando

```
psql -U postgres
psql -h localhost -p 5432 -U user -d mydb
psql "postgresql://user:pass@host:5432/mydb"
```

### Meta-Comandi psql

| Command | Description |
|---------|-------------|
| `\l` | Elenca i database |
| `\c dbname` | Connetti al database |
| `\dt` | Elenca le tabelle |
| `\d tablename` | Descrive la struttura della tabella |
| `\dn` | Elenca gli schema |
| `\du` | Elenca i ruoli |
| `\q` | Esci da psql |
| `\i file.sql` | Esegue un file SQL |

## Tabelle e Schema

### Crea Tabella

```
CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  name VARCHAR(100) NOT NULL,
  email TEXT UNIQUE,
  created_at TIMESTAMPTZ DEFAULT NOW()
);
```

### Operazioni sullo Schema

```
CREATE SCHEMA app;
CREATE TABLE app.users (id SERIAL PRIMARY KEY);
SET search_path TO app, public;
DROP SCHEMA app CASCADE;
```

### Modifica Tabella

```
ALTER TABLE users ADD COLUMN age INT;
ALTER TABLE users ALTER COLUMN name TYPE TEXT;
ALTER TABLE users DROP COLUMN age;
ALTER TABLE users RENAME TO customers;
```

## Tipi di Dato

### Numerici

| Command | Description |
|---------|-------------|
| `INTEGER / INT` | Intero a 4 byte |
| `BIGINT` | Intero a 8 byte |
| `SERIAL` | Intero auto-incrementante |
| `NUMERIC(p,s)` | Numerico esatto (es. NUMERIC(10,2)) |
| `REAL / DOUBLE PRECISION` | Virgola mobile (4 / 8 byte) |
| `BOOLEAN` | true / false / null |

### Stringhe e Binari

| Command | Description |
|---------|-------------|
| `TEXT` | Testo variabile senza limite |
| `VARCHAR(n)` | Testo variabile fino a n caratteri |
| `CHAR(n)` | Testo a lunghezza fissa |
| `BYTEA` | Dati binari |
| `UUID` | Identificatore unico universale a 128 bit |

### Date, JSON e Array

| Command | Description |
|---------|-------------|
| `DATE` | Data del calendario |
| `TIMESTAMPTZ` | Timestamp con fuso orario |
| `INTERVAL` | Intervallo di tempo (es. '2 days') |
| `JSONB` | JSON binario (indicizzabile) |
| `INT[] / TEXT[]` | Tipi array |

## Query

### Insert

```
INSERT INTO users (name, email)
VALUES ('Alice', 'alice@example.com')
RETURNING id;

INSERT INTO users (name, email) VALUES
('Bob', 'bob@ex.com'),
('Carol', 'carol@ex.com');
```

### Select

```
SELECT * FROM users WHERE id = 1;
SELECT name, email FROM users
ORDER BY name LIMIT 10 OFFSET 20;
```

### Update

```
UPDATE users SET email = 'new@ex.com'
WHERE id = 1 RETURNING *;
```

### Upsert

```
INSERT INTO users (email, name)
VALUES ('a@ex.com', 'Alice')
ON CONFLICT (email) DO UPDATE
SET name = EXCLUDED.name;
```

### Delete

```
DELETE FROM users WHERE id = 1 RETURNING *;
TRUNCATE TABLE users RESTART IDENTITY;
```

## Join e Sottoquery

### Tipi di Join

| Command | Description |
|---------|-------------|
| `INNER JOIN` | Righe corrispondenti in entrambe le tabelle |
| `LEFT JOIN` | Tutte le righe sinistra + corrispondenti destra |
| `RIGHT JOIN` | Tutte le righe destra + corrispondenti sinistra |
| `FULL OUTER JOIN` | Tutte le righe da entrambe le tabelle |
| `CROSS JOIN` | Prodotto cartesiano |
| `LATERAL JOIN` | Sottoquery che fa riferimento alla riga esterna |

### CTE (Common Table Expression)

```
WITH active AS (
  SELECT * FROM users WHERE active = true
)
SELECT a.name, o.total
FROM active a
JOIN orders o ON a.id = o.user_id;
```

### Sottoquery

```
SELECT name FROM users
WHERE id IN (
  SELECT user_id FROM orders
  WHERE total > 100
);
```

## Indici

### Crea e Rimuovi

```
CREATE INDEX idx_name ON users(name);
CREATE UNIQUE INDEX idx_email ON users(email);
CREATE INDEX idx_gin ON posts USING GIN(tags);
DROP INDEX idx_name;
```

### Tipi di Indice

| Command | Description |
|---------|-------------|
| `B-tree` | Default, ottimo per =, <, >, BETWEEN |
| `Hash` | Solo confronti di uguaglianza |
| `GIN` | Invertito generalizzato — array, JSONB, full-text |
| `GiST` | Ricerca generalizzata — geometrica, range |
| `BRIN` | Intervallo blocco — tabelle ordinate grandi |

### Analisi delle Query

```
EXPLAIN ANALYZE
SELECT * FROM users WHERE name = 'Alice';
```

## Funzioni e Procedure

### Funzione SQL

```
CREATE FUNCTION active_count()
RETURNS INTEGER AS $$
  SELECT COUNT(*)::INT FROM users
  WHERE active = true;
$$ LANGUAGE sql;
SELECT active_count();
```

### Funzione PL/pgSQL

```
CREATE FUNCTION greet(name TEXT)
RETURNS TEXT AS $$
BEGIN
  RETURN 'Hello, ' || name;
END;
$$ LANGUAGE plpgsql;
```

### Funzioni Integrate Utili

| Command | Description |
|---------|-------------|
| `NOW() / CURRENT_TIMESTAMP` | Timestamp corrente con TZ |
| `AGE(ts1, ts2)` | Intervallo tra timestamp |
| `COALESCE(a, b)` | Primo valore non-null |
| `NULLIF(a, b)` | NULL se a = b |
| `GENERATE_SERIES(1,10)` | Genera righe di valori sequenziali |
| `STRING_AGG(col, ',')` | Concatena valori con separatore |

## Ruoli e Permessi

### Gestione dei Ruoli

```
CREATE ROLE app LOGIN PASSWORD 'secret';
ALTER ROLE app SET search_path TO myapp;
DROP ROLE app;
```

### Grants

```
GRANT ALL ON DATABASE mydb TO app;
GRANT SELECT, INSERT ON users TO reader;
GRANT USAGE ON SCHEMA public TO app;
REVOKE INSERT ON users FROM reader;
```

### Row-Level Security

```
ALTER TABLE users ENABLE ROW LEVEL SECURITY;
CREATE POLICY user_own ON users
FOR ALL USING (id = current_setting('app.uid')::INT);
```

## Supporto JSON

### Operatori JSONB

| Command | Description |
|---------|-------------|
| `->'key'` | Ottieni valore JSON per chiave (come JSON) |
| `->>'key'` | Ottieni valore JSON per chiave (come testo) |
| `#>'{a,b}'` | Ottieni valore annidato per percorso |
| `@>` | Contiene (sinistro include destro) |
| `?` | La chiave esiste |
| `\|\|` | Concatena valori JSONB |

### Query JSONB

```
SELECT data->>'name' FROM profiles
WHERE data @> '{"active": true}';

SELECT * FROM profiles
WHERE data ? 'email';
```

### Funzioni JSONB

```
SELECT jsonb_each(data) FROM profiles;
SELECT jsonb_array_elements('[1,2,3]');
SELECT jsonb_set(data, '{name}', '"Alice"')
FROM profiles WHERE id = 1;
```

## Pattern Comuni

### Transazioni

```
BEGIN;
UPDATE accounts SET balance = balance - 100
WHERE id = 1;
UPDATE accounts SET balance = balance + 100
WHERE id = 2;
COMMIT;  -- oppure ROLLBACK;
```

### Window Function

```
SELECT name, salary,
  RANK() OVER (ORDER BY salary DESC),
  AVG(salary) OVER (PARTITION BY dept)
FROM employees;
```

### Copia Dati

```
COPY users TO '/tmp/users.csv'
WITH (FORMAT csv, HEADER);
COPY users FROM '/tmp/users.csv'
WITH (FORMAT csv, HEADER);
```

### Backup con pg_dump

```
pg_dump -U postgres mydb > backup.sql
pg_dump -Fc mydb > backup.dump
pg_restore -d mydb backup.dump
```