CoreData总性能 [英] CoreData Sum Performance

问题描述

对于核心数据和求和函数，我有一些理论上的问题。

I have some theoretical question to ask about Core Data and sum function.

我尝试对核心数据中的值求和表以三种方式。

I try to sum values from Core Data table with three ways.

1. 全部获取并使用表达式求和：

1. fetch all and use expression to sum it up :

NSArray * array1 = [self getAll:self.managedObjectContext];
int sum = [[array1 valueForKeyPath:@"@sum.sum"] intValue];

全部获取并用于循环：

fetch all and use for loop:

int sum2 = 0;
NSArray * array2 = [self getAll:self.managedObjectContext];

for (Test * t in array2) {
    sum2 = sum2 + [t.sum intValue];
}

让Core Data对其求和。

let Core Data sum it.

NSArray * array = [self getAllGroupe:self.managedObjectContext];
NSDictionary * i = [array objectAtIndex:0];
id j = [i objectForKey:@"sum"];

(NSArray *)getAllGroupe:(NSManagedObjectContext*)Context{

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Test"  
                                   inManagedObjectContext:Context];

    NSExpressionDescription* ex = [[NSExpressionDescription alloc] init];
    [ex setExpression:[NSExpression expressionWithFormat:@"@sum.sum"]];
    [ex setExpressionResultType:NSDecimalAttributeType];
    [ex setName:@"sum"];


    [fetchRequest setPropertiesToFetch:[NSArray arrayWithObjects:ex, nil]];
    [fetchRequest setResultType:NSDictionaryResultType ];


    NSError *error;
    [fetchRequest setEntity:entity];
    NSArray *fetchedObjects = [Context executeFetchRequest:fetchRequest error:&error];

return fetchedObjects;
}

令人惊讶的是

1. 方式最慢（对于1.000.000数据<< c $ c>-> 19.2 s ） ，


2. 的速度更快（对于1.000.000数据，-> 3.54 s ）和


3. 方式最快（对于1.000.000数据<< c $ c>-> 0.3 s ）

1. way was the slowest (for 1.000.000 data --> 19.2 s), the
2. way was faster (for 1.000.000 data --> 3.54 s) and the
3. way was the fastest (for 1.000.000 data --> 0.3 s)

为什么这样？

如果我理解正确，甚至核心数据都需要遍历所有 1.000.000 数据并将其求和。这是因为要使用更多的内核（如果有）？

If I understand right even core data need to go through all 1.000.000 datas and sum it. Is this because use more cores if there are available?

推荐答案

没有CoreData自己不进行求和-委托它支持sqllite数据库，该数据库针对此类事情进行了优化。
基本上，CoreData从表中发送 select SUM（sum）; 到它的数据库，并在那里执行。

No CoreData doesn't do the summing on it's own - it delegates that to it's backing sqllite database which is optimized for things like that. Basically CoreData sends a select SUM(sum) from table; to it's db and it's performed there.

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