我正在运行一个非常基本的NSFetchRequest来获取实体MessageObject。我只有 2000 个消息对象，我想全部检索它们。但是，由于某种奇怪的原因，获取请求花费了 10 秒以上！
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"MessageObject" inManagedObjectContext:appDelegate.managedObjectContext];
[fetchRequest setEntity:entity];
NSSortDescriptor *sort= [[NSSortDescriptor alloc] initWithKey:@"createDate" ascending:NO selector:@selector(compare:)];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];
[fetchRequest setFetchBatchSize:5];
就是这样，这就是我的获取请求。我什至没有使用谓词，它需要 10 多秒。我完全不知道是什么原因导致这种情况。如果有人有任何想法或出发点，请分享。
我也尝试启用SQLite调试日志记录（-com.apple.CoreData.SQLDebug 1），但我只是从这个简单的获取中获得了数千行输出。这正常吗？
2012-06-22 19:39:59.171 myapp[81825:15e03] about to execute fetch
2012-06-22 19:39:59.172 myapp[81825:15e03] CoreData: sql: SELECT 0, t0.Z_PK FROM ZMBNOTEOBJECT t0 ORDER BY t0.ZCREATEDATE DESC
2012-06-22 19:39:59.178 myapp[81825:15e03] CoreData: annotation: sql connection fetch time: 0.0061s
2012-06-22 19:39:59.179 myapp[81825:15e03] CoreData: annotation: total fetch execution time: 0.0067s for 2052 rows.
2012-06-22 19:39:59.179 myapp[81825:15e03] CoreData: sql: SELECT 0, t0.Z_PK, t0.Z_OPT, t0.ZAUTHOREMAIL, t0.ZAUTHORNAME, t0.ZCREATEDATE, t0.ZISGLOBAL, t0.ZISLOCKED, t0.ZISNEW, t0.ZISPENDINGDELETE, t0.ZISPENDINGLIKE, t0.ZISPENDINGREAD, t0.ZISPENDINGSYNC, t0.ZLASTUPDATED, t0.ZLOCALLYMODIFIEDDATE, t0.ZMAINIDEA, t0.ZMETALASTUPDATED, t0.ZNOTEID, t0.ZNUMBEROFCHILDREN, t0.ZPARENTAUTHOREMAIL, t0.ZPARENTNOTEID, t0.ZROOTAUTHOREMAIL, t0.ZROOTNOTEID, t0.Z4PENDINGADDNOTES, t0.Z4PENDINGREMOVENOTES FROM ZMBNOTEOBJECT t0 WHERE t0.Z_PK IN (?,?,?,?,?,?,?,?,?,?,?,?,?,?,?) ORDER BY t0.ZCREATEDATE DESC LIMIT 15
2012-06-22 19:39:59.180 myapp[81825:15e03] CoreData: annotation: sql connection fetch time: 0.0008s
2012-06-22 19:39:59.181 myapp[81825:15e03] CoreData: annotation: total fetch execution time: 0.0018s for 15 rows.
2012-06-22 19:39:59.182 myapp[81825:15e03] CoreData: sql: SELECT 0, t0.Z_PK, t0.Z_OPT, t0.ZAUTHOREMAIL, t0.ZAUTHORNAME, t0.ZCREATEDATE, t0.ZISGLOBAL, t0.ZISLOCKED, t0.ZISNEW, t0.ZISPENDINGDELETE, t0.ZISPENDINGLIKE, t0.ZISPENDINGREAD, t0.ZISPENDINGSYNC, t0.ZLASTUPDATED, t0.ZLOCALLYMODIFIEDDATE, t0.ZMAINIDEA, t0.ZMETALASTUPDATED, t0.ZNOTEID, t0.ZNUMBEROFCHILDREN, t0.ZPARENTAUTHOREMAIL, t0.ZPARENTNOTEID, t0.ZROOTAUTHOREMAIL, t0.ZROOTNOTEID, t0.Z4PENDINGADDNOTES, t0.Z4PENDINGREMOVENOTES FROM ZMBNOTEOBJECT t0 WHERE t0.Z_PK IN (?,?,?,?,?,?,?,?,?,?,?,?,?,?,?) ORDER BY t0.ZCREATEDATE DESC LIMIT 15
2012-06-22 19:39:59.186 myapp[81825:15e03] CoreData: annotation: sql connection fetch time: 0.0042s
2012-06-22 19:39:59.187 myapp[81825:15e03] CoreData: annotation: total fetch execution time: 0.0049s for 15 rows.
2012-06-22 19:39:59.187 myapp[81825:15e03] CoreData: sql: SELECT 0, t0.Z_PK, t0.Z_OPT, t0.ZAUTHOREMAIL, t0.ZAUTHORNAME, t0.ZCREATEDATE, t0.ZISGLOBAL, t0.ZISLOCKED, t0.ZISNEW, t0.ZISPENDINGDELETE, t0.ZISPENDINGLIKE, t0.ZISPENDINGREAD, t0.ZISPENDINGSYNC, t0.ZLASTUPDATED, t0.ZLOCALLYMODIFIEDDATE, t0.ZMAINIDEA, t0.ZMETALASTUPDATED, t0.ZNOTEID, t0.ZNUMBEROFCHILDREN, t0.ZPARENTAUTHOREMAIL, t0.ZPARENTNOTEID, t0.ZROOTAUTHOREMAIL, t0.ZROOTNOTEID, t0.Z4PENDINGADDNOTES, t0.Z4PENDINGREMOVENOTES FROM ZMBNOTEOBJECT t0 WHERE t0.Z_PK IN (?,?,?,?,?,?,?,?,?,?,?,?,?,?,?) ORDER BY t0.ZCREATEDATE DESC LIMIT 15
2012-06-22 19:39:59.188 myapp[81825:15e03] CoreData: annotation: sql connection fetch time: 0.0008s
2012-06-22 19:39:59.189 myapp[81825:15e03] CoreData: annotation: total fetch execution time: 0.0014s for 15 rows.
... (thousands of more lines similar to above)
我对阅读本文不是很熟悉，但似乎它在 .0067 秒内获取 2052 行。那么为什么在那之后它继续做更多的事情呢？如果请求完成获取行，它不应该完成吗？是数据出错还是什么？
另外，我已经删除了setFetchBatchSize - 它摆脱了数千行，但获取请求仍然需要很长时间。这是我得到的输出：
2012-06-22 20:07:25.316 myapp[8927:707] about to execute fetch
2012-06-22 20:07:25.322 myapp[8927:707] CoreData: sql: SELECT 0, t0.Z_PK, t0.Z_OPT, t0.ZAUTHOREMAIL, t0.ZAUTHORNAME, t0.ZCREATEDATE,t0.ZISLOCKED, t0.ZISNEW, t0.ZISPENDINGDELETE, t0.ZISPENDINGSYNC, t0.ZLASTUPDATED, t0.ZLOCALLYMODIFIEDDATE, t0.ZMAINIDEA, t0.ZMETALASTUPDATED, t0.ZNOTEID, t0.ZNUMBEROFCHILDREN, t0.ZPARENTAUTHOREMAIL, t0.ZPARENTNOTEID, t0.ZROOTAUTHOREMAIL, t0.ZROOTNOTEID, t0.Z4PENDINGADDNOTES, t0.Z4PENDINGREMOVENOTES FROM ZMBNOTEOBJECT t0 ORDER BY t0.ZCREATEDATE DESC
2012-06-22 20:07:26.758 myapp[8927:707] CoreData: annotation: sql connection fetch time: 1.0891s
2012-06-22 20:07:26.763 myapp[8927:707] CoreData: annotation: total fetch execution time: 1.4407s for 4000 rows.
2012-06-22 20:07:35.967 myapp[8927:707] finished fetching
07：26.763 它显然说 1.4407 行花了 4000 秒，但又没有再过 9 秒，我得到的输出是"完成获取"（这是在 [[self fetchedResultsController] performFetch:&error] 之后出现的 NSLog 语句）这是怎么回事？
删除 setFetchBatchSize。
如果您的目的是一次加载所有内容，只需将其删除即可。
此外，如果您需要加载所有属性，请添加：
fetchRequest.returnsObjectsAsFaults = NO;
它将加载每个实体并填充属性。
您可能只想加载某些属性，因此请使用此选项来选择所需的属性：
fetchRequest.propertiesToFetch = ...
大功告成。