# pgx PostgreSQL Driver Development Guide

**Library**: `github.com/jackc/pgx/v5`  
**Used In**: MVP - PostgreSQL + PostGIS for geospatial queries  
**Purpose**: High-performance PostgreSQL driver with native PostGIS support

---

## Where It's Used

- **PostGIS spatial queries** (distance calculations, radius searches)
- **Site location data** (synced from Neo4j)
- **Raw SQL queries** when needed
- **Connection pooling** for database operations

---

## Official Documentation

- **GitHub**: https://github.com/jackc/pgx
- **GoDoc**: https://pkg.go.dev/github.com/jackc/pgx/v5
- **PostgreSQL Docs**: https://www.postgresql.org/docs/
- **PostGIS Docs**: https://postgis.net/documentation/

---

## Installation

```bash
go get github.com/jackc/pgx/v5
go get github.com/jackc/pgx/v5/pgxpool
```

---

## Key Concepts

### 1. Connection Pool Setup

```go
import (
    "context"
    "github.com/jackc/pgx/v5/pgxpool"
)

func NewPostgresPool(ctx context.Context, connString string) (*pgxpool.Pool, error) {
    config, err := pgxpool.ParseConfig(connString)
    if err != nil {
        return nil, err
    }
    
    // Configure connection pool
    config.MaxConns = 25
    config.MinConns = 5
    config.MaxConnLifetime = time.Hour
    config.MaxConnIdleTime = 30 * time.Minute
    
    pool, err := pgxpool.NewWithConfig(ctx, config)
    if err != nil {
        return nil, err
    }
    
    // Verify connection
    if err := pool.Ping(ctx); err != nil {
        return nil, err
    }
    
    return pool, nil
}

// Connection string format
// postgres://user:password@localhost:5432/dbname?sslmode=disable
```

### 2. Basic Queries

```go
// Single row query
func GetSite(ctx context.Context, pool *pgxpool.Pool, id string) (*Site, error) {
    var site Site
    err := pool.QueryRow(ctx,
        "SELECT id, business_id, latitude, longitude, address FROM sites WHERE id = $1",
        id,
    ).Scan(&site.ID, &site.BusinessID, &site.Latitude, &site.Longitude, &site.Address)
    
    if err == pgx.ErrNoRows {
        return nil, ErrNotFound
    }
    if err != nil {
        return nil, err
    }
    
    return &site, nil
}

// Multiple rows query
func ListSites(ctx context.Context, pool *pgxpool.Pool, limit, offset int) ([]Site, error) {
    rows, err := pool.Query(ctx,
        "SELECT id, business_id, latitude, longitude, address FROM sites ORDER BY created_at DESC LIMIT $1 OFFSET $2",
        limit, offset,
    )
    if err != nil {
        return nil, err
    }
    defer rows.Close()
    
    var sites []Site
    for rows.Next() {
        var site Site
        if err := rows.Scan(&site.ID, &site.BusinessID, &site.Latitude, &site.Longitude, &site.Address); err != nil {
            return nil, err
        }
        sites = append(sites, site)
    }
    
    return sites, rows.Err()
}
```

### 3. Transactions

```go
// Transaction
func CreateSiteWithBusiness(ctx context.Context, pool *pgxpool.Pool, site Site) error {
    tx, err := pool.Begin(ctx)
    if err != nil {
        return err
    }
    defer tx.Rollback(ctx)
    
    // Insert business
    _, err = tx.Exec(ctx,
        "INSERT INTO businesses (id, name, email) VALUES ($1, $2, $3)",
        site.BusinessID, "Business Name", "email@example.com",
    )
    if err != nil {
        return err
    }
    
    // Insert site
    _, err = tx.Exec(ctx,
        "INSERT INTO sites (id, business_id, latitude, longitude, location) VALUES ($1, $2, $3, $4, ST_MakePoint($4, $3))",
        site.ID, site.BusinessID, site.Latitude, site.Longitude,
    )
    if err != nil {
        return err
    }
    
    return tx.Commit(ctx)
}
```

### 4. PostGIS Spatial Queries

```go
import "github.com/twpayne/go-geom"

// Find sites within radius (PostGIS)
func FindSitesWithinRadius(ctx context.Context, pool *pgxpool.Pool, lat, lon float64, radiusMeters float64) ([]Site, error) {
    query := `
        SELECT 
            site_id,
            business_id,
            latitude,
            longitude,
            ST_Distance(
                location,
                ST_MakePoint($1, $2)::geography
            ) AS distance_meters
        FROM sites_geo
        WHERE ST_DWithin(
            location::geography,
            ST_MakePoint($1, $2)::geography,
            $3
        )
        ORDER BY distance_meters ASC
        LIMIT 50
    `
    
    rows, err := pool.Query(ctx, query, lon, lat, radiusMeters)
    if err != nil {
        return nil, err
    }
    defer rows.Close()
    
    var sites []SiteGeo
    for rows.Next() {
        var site SiteGeo
        var distanceMeters float64
        err := rows.Scan(
            &site.SiteID,
            &site.BusinessID,
            &site.Latitude,
            &site.Longitude,
            &distanceMeters,
        )
        if err != nil {
            return nil, err
        }
        site.DistanceKm = distanceMeters / 1000
        sites = append(sites, site)
    }
    
    return sites, rows.Err()
}

// Spatial index is critical for performance
// CREATE INDEX idx_sites_location ON sites_geo USING GIST(location);
```

### 5. Batch Operations

```go
// Batch insert (faster for multiple rows)
func BatchInsertSites(ctx context.Context, pool *pgxpool.Pool, sites []Site) error {
    batch := &pgx.Batch{}
    
    for _, site := range sites {
        batch.Queue(
            "INSERT INTO sites_geo (site_id, business_id, latitude, longitude, location) VALUES ($1, $2, $3, $4, ST_MakePoint($4, $3))",
            site.ID, site.BusinessID, site.Latitude, site.Longitude,
        )
    }
    
    results := pool.SendBatch(ctx, batch)
    defer results.Close()
    
    for i := 0; i < len(sites); i++ {
        _, err := results.Exec()
        if err != nil {
            return err
        }
    }
    
    return nil
}
```

### 6. Prepared Statements

```go
// Prepare statement for reuse (performance)
func (s *SiteService) prepareStatements(ctx context.Context) error {
    var err error
    
    s.findSiteStmt, err = s.pool.Prepare(ctx, "find_site",
        "SELECT id, business_id, latitude, longitude FROM sites WHERE id = $1",
    )
    if err != nil {
        return err
    }
    
    s.insertSiteStmt, err = s.pool.Prepare(ctx, "insert_site",
        "INSERT INTO sites (id, business_id, latitude, longitude, location) VALUES ($1, $2, $3, $4, ST_MakePoint($4, $3))",
    )
    if err != nil {
        return err
    }
    
    return nil
}

// Use prepared statement
func (s *SiteService) FindSite(ctx context.Context, id string) (*Site, error) {
    var site Site
    err := s.pool.QueryRow(ctx, "find_site", id).Scan(
        &site.ID, &site.BusinessID, &site.Latitude, &site.Longitude,
    )
    // ...
}
```

---

## MVP-Specific Patterns

### Site Geo Service

```go
type SiteGeoService struct {
    pool *pgxpool.Pool
}

type SiteGeo struct {
    SiteID     uuid.UUID
    BusinessID uuid.UUID
    Latitude   float64
    Longitude  float64
    DistanceKm float64 // For query results
}

// Sync from Neo4j (event-driven)
func (s *SiteGeoService) SyncFromNeo4j(ctx context.Context, site SiteGeo) error {
    query := `
        INSERT INTO sites_geo (site_id, business_id, latitude, longitude, location)
        VALUES ($1, $2, $3, $4, ST_MakePoint($4, $3))
        ON CONFLICT (site_id) 
        DO UPDATE SET 
            latitude = EXCLUDED.latitude,
            longitude = EXCLUDED.longitude,
            location = ST_MakePoint(EXCLUDED.longitude, EXCLUDED.latitude),
            updated_at = NOW()
    `
    
    _, err := s.pool.Exec(ctx, query,
        site.SiteID, site.BusinessID, site.Latitude, site.Longitude,
    )
    return err
}

// Spatial pre-filter for matching algorithm
func (s *SiteGeoService) FindWithinRadius(ctx context.Context, lat, lon, radiusKm float64) ([]uuid.UUID, error) {
    radiusMeters := radiusKm * 1000
    query := `
        SELECT site_id 
        FROM sites_geo
        WHERE ST_DWithin(
            location::geography,
            ST_MakePoint($1, $2)::geography,
            $3
        )
    `
    
    rows, err := s.pool.Query(ctx, query, lon, lat, radiusMeters)
    if err != nil {
        return nil, err
    }
    defer rows.Close()
    
    var siteIDs []uuid.UUID
    for rows.Next() {
        var siteID uuid.UUID
        if err := rows.Scan(&siteID); err != nil {
            return nil, err
        }
        siteIDs = append(siteIDs, siteID)
    }
    
    return siteIDs, rows.Err()
}
```

---

## PostGIS Schema

```sql
-- Create table with PostGIS geometry
CREATE EXTENSION IF NOT EXISTS postgis;

CREATE TABLE sites_geo (
    site_id UUID PRIMARY KEY,
    business_id UUID NOT NULL,
    latitude FLOAT NOT NULL,
    longitude FLOAT NOT NULL,
    location GEOMETRY(POINT, 4326) NOT NULL,
    created_at TIMESTAMP DEFAULT NOW(),
    updated_at TIMESTAMP DEFAULT NOW()
);

-- Spatial index (critical for performance)
CREATE INDEX idx_sites_location ON sites_geo USING GIST(location);

-- Business ID index (for joins)
CREATE INDEX idx_sites_business_id ON sites_geo(business_id);

-- Update location from lat/lon trigger (optional)
CREATE OR REPLACE FUNCTION update_location()
RETURNS TRIGGER AS $$
BEGIN
    NEW.location = ST_MakePoint(NEW.longitude, NEW.latitude);
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER trigger_update_location
BEFORE INSERT OR UPDATE ON sites_geo
FOR EACH ROW
EXECUTE FUNCTION update_location();
```

---

## Performance Tips

1. **Use GIST index** for spatial queries (PostGIS)
2. **Use prepared statements** for repeated queries
3. **Batch operations** for bulk inserts
4. **Connection pooling** - configure appropriately
5. **Geography vs Geometry** - use geography for distance calculations on Earth
6. **Use LIMIT** - always limit query results

---

## Error Handling

```go
err := pool.QueryRow(ctx, query, args...).Scan(...)
if err == pgx.ErrNoRows {
    // Not found
    return nil, ErrNotFound
}
if err != nil {
    // Other error
    return nil, err
}
```

---

## Tutorials & Resources

- **pgx Examples**: https://github.com/jackc/pgx/tree/master/examples
- **PostGIS Documentation**: https://postgis.net/documentation/
- **PostgreSQL Spatial**: https://www.postgresql.org/docs/current/postgis.html