数据库操作优化

• 数据库耗时操作

  • 主要是往数据库执行写操作

  // 插入数据
  [db executeUpdate:@"INSERT INTO t_PetInfo(petNo, age, name, weight, hoster) VALUES(?, ?, ?, ?, ?);", @(i), @(2), @"fancy", @(15.0), @"derain"];
  
  // 更新数据
  [db executeUpdate:@"UPDATE t_PetInfo SET name = ?", @"fffffancy"];
  
  // 删除数据
  [db executeUpdate:@"DELETE FROM t_PetInfo WHERE petNo > ?", @(20)];
  

• 耗时操作优化

  • 利用事务提交 可以有效的减少耗时

  • 在子线程处理耗时操作

耗时操作实验

• 插入数据

  • 开启事务耗时: 0.097206
  • 不开启事务耗时:38.051225
  • 插入10000条数据, 不开启事务耗时是开启事务的391倍

  
  // 1.0 开启事务
  CGFloat beginTime = [NSDate timeIntervalSinceReferenceDate];
  
  [db beginTransaction];
  for (NSInteger i = 0; i < 10000; i++) {
      
      NSString *name = (i % 2) ? @"fancy" : nil;
       [db executeUpdate:@"INSERT INTO t_PetInfo(petNo, age, name, weight, hoster) VALUES(?, ?, ?, ?, ?);", @(i), @(2), name, @(15.0), @"derain"];
  }
  [db commit];
   CGFloat endTime = [NSDate timeIntervalSinceReferenceDate];
  
  NSLog(@"%f", endTime - beginTime); // 耗时: 0.097206
  
  // 2.0 不开启事务
  CGFloat beginTime = [NSDate timeIntervalSinceReferenceDate];
  
  for (NSInteger i = 0; i < 10000; i++) {
      
      NSString *name = (i % 2) ? @"fancy" : nil;
       [db executeUpdate:@"INSERT INTO t_PetInfo(petNo, age, name, weight, hoster) VALUES(?, ?, ?, ?, ?);", @(i), @(2), name, @(15.0), @"derain"];
  }
  CGFloat endTime = [NSDate timeIntervalSinceReferenceDate];
  
  NSLog(@"%f", endTime - beginTime); // 耗时: 38.051225
  
  

• 查询耗时实验
  • 开启事务耗时: 0.037018
  • 不开启事务耗时:0.037175
  • 两者基本一样, 因为读操作只需数据库一次, 频繁的开启事务, 提交事务
  • 当然, 当有大量读操作时, 依旧推荐使用事务

    
    // 开启事务
    CGFloat beginTime2 = [NSDate timeIntervalSinceReferenceDate];
    
    [db beginTransaction];
    
    for (NSInteger i = 0; i < 1000; i++) {
        
       FMResultSet *result = [db executeQuery:@"SELECT * FROM t_PetInfo;"];
    }

    [db commit];
    CGFloat endTime2 = [NSDate timeIntervalSinceReferenceDate];
    
    NSLog(@"time2 - %f", endTime2 - beginTime2);
    
    [db close];
    

// 不开启事务
    CGFloat beginTime2 = [NSDate timeIntervalSinceReferenceDate];
      for (NSInteger i = 0; i < 1000; i++) {
        
       FMResultSet *result = [db executeQuery:@"SELECT * FROM t_PetInfo;"];
    }

   CGFloat endTime2 = [NSDate timeIntervalSinceReferenceDate];
    
    NSLog(@"time2 - %f", endTime2 - beginTime2);
    
    [db close];

• 更新操作耗时实验
  • 开启事务耗时: 5.725228
  • 不开启事务耗时:15.122401
  • 因为表内有10000条数据, 又执行修改100次, 耗时都较多

// 开启事务
    // 1.0 创建数据库
    NSString *dbPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES).lastObject stringByAppendingPathComponent:@"tmps.db"];
    
    FMDatabase *db = [FMDatabase databaseWithPath:dbPath];
    
    [db open];

    [db beginTransaction];
    
    CGFloat beginTime2 = [NSDate timeIntervalSinceReferenceDate];
    
    for (NSInteger i = 0; i < 1000; i++) {
        
        [db executeUpdate:@"UPDATE t_PetInfo SET name = ?, age = ?;", @"funny", @(i)];
    }

    [db commit];
    CGFloat endTime2 = [NSDate timeIntervalSinceReferenceDate];
    
    NSLog(@"time2 - %f", endTime2 - beginTime2); // 5.725228
    
    [db close];

   // 不开启事务
    
    NSString *dbPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES).lastObject stringByAppendingPathComponent:@"tmps.db"];
    
    FMDatabase *db = [FMDatabase databaseWithPath:dbPath];
    
    [db open];
    
    CGFloat beginTime2 = [NSDate timeIntervalSinceReferenceDate];
    
    for (NSInteger i = 0; i < 1000; i++) {
        
        [db executeUpdate:@"UPDATE t_PetInfo SET name = ?, age = ?;", @"funny", @(i)];
    }
    
    CGFloat endTime2 = [NSDate timeIntervalSinceReferenceDate];
    
    NSLog(@"time2 - %f", endTime2 - beginTime2); // 15.122401
     
    [db close];

• 删除数据耗时实验
  • 开启事务耗时: 0.608012
  • 不开启事务耗时:4.768991

    // 开启事务
    NSString *dbPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES).lastObject stringByAppendingPathComponent:@"tmps.db"];
    
    FMDatabase *db = [FMDatabase databaseWithPath:dbPath];
    
    [db open];

    [db beginTransaction];
    
    CGFloat beginTime2 = [NSDate timeIntervalSinceReferenceDate];
    
    for (NSInteger i = 0; i < 1000; i++) {
        
        [db executeUpdate:@"DELETE FROM t_PetInfo WHERE petNo = ?", @(i)];
    }
    
    [db commit];
    CGFloat endTime2 = [NSDate timeIntervalSinceReferenceDate];
    
    NSLog(@"time2 - %f", endTime2 - beginTime2);
    
    [db close]; // 0.608012

    NSString *dbPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES).lastObject stringByAppendingPathComponent:@"tmps.db"];
    
    FMDatabase *db = [FMDatabase databaseWithPath:dbPath];
    
    [db open];

   // [db beginTransaction];
    
    CGFloat beginTime2 = [NSDate timeIntervalSinceReferenceDate];
    
    for (NSInteger i = 0; i < 1000; i++) {
        
        [db executeUpdate:@"DELETE FROM t_PetInfo WHERE petNo = ?", @(i)];
    }
    
   //[db commit];
    CGFloat endTime2 = [NSDate timeIntervalSinceReferenceDate];
    
    NSLog(@"time2 - %f", endTime2 - beginTime2);
    
    [db close]; // 4.768991

• 在子线程处理耗时操作

  • FMDatabase是多线程不安全的
  • 推荐使用FMDatabaseQueue, 使用全局单列, 在子线成执行耗时操作

• 耗时来由

  • 数据库以文件的形式存在磁盘中, 每次访问时都要打开一次文件，一切的数据库操作其实都会转化为对文件的操作
  • 如果对数据库进行大量的写操作, 则耗时较大
  • 每次执行sqlite3_exec, 默认都会开启一个隐藏事务, 当执行完操作, 就会提交事务; 每次都会操作文件(数据库)

  int sqlite3_exec(
  sqlite3* ppDb,                             /* An open database */
  const char *sql,                           /* SQL to be evaluated */
  int (*callback)(void*,int,char**,char**),  /* Callback function */
  void *,                                    /* 1st argument to callback */
  char **errmsg                              /* Error msg written here */
  );
  

• 手动开启事务解如何解决耗时操作?

  • 当我们手动开启事务时, 系统就不再默认开启隐藏事务

  • 开始事务后，进行的大量操作语句都保存在内存中，当提交时才全部写入数据库，此时，数据库文件也只用打开一次;