面试题：Objective-C下SQLite数据库复杂查询与性能调优

1. 编写Objective-C代码实现查询

#import <Foundation/Foundation.h>
#import <sqlite3.h>

void executeQuery() {
    sqlite3 *db;
    const char *databaseName = "your_database.db";
    if (sqlite3_open(databaseName, &db) != SQLITE_OK) {
        NSLog(@"Failed to open database: %s", sqlite3_errmsg(db));
        return;
    }
    
    const char *query = "WITH RankedOrders AS ( "
                        "SELECT "
                        "o.order_id, "
                        "o.customer_id, "
                        "o.order_date, "
                        "o.total_amount, "
                        "c.customer_name, "
                        "c.customer_city, "
                        "RANK() OVER (PARTITION BY c.customer_city ORDER BY o.total_amount DESC) AS ranking "
                        "FROM orders o "
                        "JOIN customers c ON o.customer_id = c.customer_id "
                        ") "
                        "SELECT "
                        "customer_name, "
                        "customer_city, "
                        "order_id, "
                        "order_date, "
                        "total_amount "
                        "FROM RankedOrders "
                        "WHERE ranking = 1;";
    
    sqlite3_stmt *stmt;
    if (sqlite3_prepare_v2(db, query, -1, &stmt, NULL) != SQLITE_OK) {
        NSLog(@"Failed to prepare statement: %s", sqlite3_errmsg(db));
        sqlite3_close(db);
        return;
    }
    
    while (sqlite3_step(stmt) == SQLITE_ROW) {
        const char *customerName = (const char *)sqlite3_column_text(stmt, 0);
        const char *customerCity = (const char *)sqlite3_column_text(stmt, 1);
        int orderId = sqlite3_column_int(stmt, 2);
        const char *orderDate = (const char *)sqlite3_column_text(stmt, 3);
        double totalAmount = sqlite3_column_double(stmt, 4);
        
        NSLog(@"Customer Name: %s, City: %s, Order ID: %d, Order Date: %s, Total Amount: %.2f",
              customerName, customerCity, orderId, orderDate, totalAmount);
    }
    
    sqlite3_finalize(stmt);
    sqlite3_close(db);
}

2. 性能调优分析与优化措施

索引优化：
- 在orders表的customer_id和total_amount字段上创建复合索引。这可以加快连接操作以及按total_amount排序的速度。
```
CREATE INDEX idx_orders_customer_amount ON orders (customer_id, total_amount);
```
- 在customers表的customer_id字段上确保有主键索引（通常主键会自动创建索引），如果没有，可以手动创建：
```
CREATE INDEX idx_customers_customer_id ON customers (customer_id);
```
查询优化：
- 使用WITH子句（CTE）来提高查询的可读性和执行效率。CTE允许将复杂查询分解为更简单的部分，并且数据库优化器可以对其进行更有效的优化。
- 如果数据量非常大，可以考虑分页处理，避免一次性加载过多数据到内存中。在SQL查询中，可以使用LIMIT和OFFSET关键字来实现分页。
数据库配置优化：
- 适当调整SQLite的缓存大小，可以提高读写性能。可以通过sqlite3_db_cache_size函数来设置缓存大小。
- 确保数据库文件存储在高速存储设备上，如SSD，以减少I/O延迟。

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面试题：Objective-C下SQLite数据库复杂查询与性能调优

知识考点

面试题答案

1. 编写Objective-C代码实现查询

2. 性能调优分析与优化措施