Contoh subquery yang berkorelasi SQL Mari kita lihat beberapa contoh untuk memahami gagasan subkueri yang berkorelasi. Subkueri yang berkorelasi juga dikenal sebagai subkueri ulang atau subkueri yang disinkronisasi. Selain itu, subkueri yang berkorelasi mengeksekusi satu kali untuk setiap baris yang dipilih dari kueri luar. Perbedaan utama antara subkueri yang berkorelasi dan subkueri yang tidak berkorelasi adalah bahwa Anda tidak dapat mengeksekusi subkueri yang berkorelasi sendiri seperti subkueri yang tidak berkorelasi. Ini berarti klausa WHERE dari subkueri yang berkorelasi menggunakan data kueri luar. Pengantar subquery berkorelasi SQL Subquery yang berkorelasi adalah subkueri yang bergantung pada kueri luar. Check it out the subquery tutorial sebelum bergerak maju dengan tutorial ini. Jika Anda tidak tahu apa-apa tentang subkueri itu. Tutorial ini membutuhkan pengetahuan yang baik tentang subkueri. Yang merupakan subkueri yang bergantung pada kueri luar. Dalam tutorial ini, Anda akan belajar tentang subkueri yang berhubungan dengan SQL. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.Ikhtisar Subquery yang Berhubungan dengan SQL. Have something to add to the explanation? Sound off in the the comments. To find out what the cluster size is on an existing disk: For a windows boot partition I use the Windows default (which is 4K for any NTFS drive smaller than 16TB). 16x the block size means 1/16th the number of blocks to keep track of.įor a media disk where you photos, music and videos are stored, every file is at least 1MB I use the biggest AUS. With a 64K AUS there are a lot fewer blocks to keep track of and less fragmentation. With a 4K AUS the data needs to be split over two blocks – and they may not be together so you get fragmentation. Let’s say you have a 3K file which needs to grow 2K. Instead think about how the OS uses space. However, as Jonathon points out, modern drives are massive and a little wasted space is not worth fussing over and this shouldn’t be a determining factor (unless you are on a small SSD).Ĭompare 4K vs 64K average case waste (32K-2K = 30K), for 10,000 files that only comes out to 300,000KB or around 300MB. So 4K wastes 2K per file and 64K wastes 32K. The average space wasted per file will be half the chosen AUS. In terms of space efficiency, smaller allocation unit sizes perform better. Suggest you just keep the default.Īlso keep in mind that the majority file are relatively small, larger files are large in size but small in units.Īndrew expands upon Jonathan’s answer with: If you have lots of large files, keeping it higher will increase the system performance by having less blocks to seek.īut again, nowadays hard drive capacity is getting higher and higher it makes small difference by choosing the right allocation size. If you have lots of small files, then it’s a good idea to keep the allocation size small so your harddrive space won’t be wasted. Basically, the allocation unit size is the block size on your hard drive when it formats NTFS. If you are a “Standard User” by Microsoft’s definition, you should keep the default 4096 bytes. SuperUser contributors Jonathan and Andrew offer some insight. While the default setting is usually the best choice for most users, let’s dig a little deeper.
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