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When and why are database joins expensive?
...ires retrieval of only the keys. In practice, not even the key values are fetched: the key hash values are used for join comparisons, mitigating the cost of multi-column joins and radically reducing the cost of joins involving string comparisons. Not only will vastly more fit in cache, there's a lot...
Why does JPA have a @Transient annotation?
...ystem uses tools to generate entities from database. (Hibernate has those, etc...)
Now, suppose that by your business logic you need a particular field NOT to be persisted. You have to "configure" your entity in a particular way.
While Transient keyword works on an object - as it behaves within a j...
Should I use Vagrant or Docker for creating an isolated environment? [closed]
...ctivity benefits:
Vagrant can launch VirtualBox, VMware, AWS, OpenStack, etc. machines. It doesn't matter what you need, Vagrant can launch it. If you are using Docker, Vagrant can install Docker on any of these so you can use them for that purpose.
Vagrant is a single workflow for all your projec...
C# DateTime.Now precision
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Why would DateTime.Now be made less precise than what most CPU clocks could handle?
A good clock should be both precise and accurate; those are different. As the old joke goes, a stopped clock is exactly accurate twice a day, a clock a minute slow is never accurate at any time. Bu...
Increasing the maximum number of TCP/IP connections in Linux
... eth0 txqueuelen 5000
echo "/sbin/ifconfig eth0 txqueuelen 5000" >> /etc/rc.local
Similarly bump up the values for net.core.netdev_max_backlog and net.ipv4.tcp_max_syn_backlog. Their default values are 1000 and 1024 respectively.
sysctl net.core.netdev_max_backlog=2000
sysctl net.ipv4.tcp_m...
Debug vs. Release performance
...oven to be invariants
code written under #debug directive is not included, etc.
The rest is up to the JIT.
Full list of optimizations here courtesy of Eric Lippert.
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使用App Inventor 2 控制物联网设备/低功耗蓝牙设备(BLE) · App Inventor 2 中文网
...接。 因此，我们开发蓝牙 LE 组件是为了解决物联网连接问题。
三． 技术方法
BluetoothLE 组件是 MIT App Inventor 的新颖补充。 如图 4 所示，在 MIT App Inventor 平台上创建时，BLE 组件可以作为移动应用程序的一部分实现。 然后，该...
How to check if a file is a valid image file?
...more about it than just the format, like bitmap dimensions, format version etc.. So you might see this as a superficial test for "validity".
For other definitions of "valid" you might have to write your own tests.
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What integer hash function are good that accepts an integer hash key?
...tegers that are divisible by a common factor (word-aligned memory adresses etc.). Now if your hash table happens to be divisible by the same factor, you end up with only half (or 1/4, 1/8, etc.) buckets used.
– Rafał Dowgird
Mar 20 '09 at 16:56
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Which is faster in Python: x**.5 or math.sqrt(x)?
... Took 0.157436 seconds Took 0.093905 seconds Target system: Ubuntu Linux CPU: Intel(R) Core(TM)2 Duo CPU T9600 @ 2.80GHz As you can see I got different results. According to this your answer is not generic.
– zoli2k
Apr 23 '10 at 3:37
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