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When and why are database joins expensive?
...e the rows are materialised.
Materialising the result involves bulk disk reads which are the most expensive aspect of the exercise by an order of magnitude. Performing a join, by contrast, logically requires retrieval of only the keys. In practice, not even the key values are fetched: the key hash...
Split string into an array in Bash
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IFS=', ' read -r -a array <<< "$string"
Note that the characters in $IFS are treated individually as separators so that in this case fields may be separated by either a comma or a space rather than the sequence of the two c...
How to specify a multi-line shell variable?
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Use read with a heredoc as shown below:
read -d '' sql << EOF
select c1, c2 from foo
where c1='something'
EOF
echo "$sql"
share
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Import and Export Excel - What is the best library? [closed]
...use you've got the greatest control over the code. Just make sure that you read in the rows and process them one at a time (reading the document to the end and splitting will eat up all of your memory - same with writing, stream it out).
Yes, the user will have to save-as CSV in excel before you ca...
Getting started with Haskell
...d around the functional programming paradigm in Haskell. I've done this by reading tutorials and watching screencasts, but nothing really seems to stick.
Now, in learning various imperative/OO languages (like C, Java, PHP), exercises have been a good way for me to go. But since I don't really know w...
How do I convert a IPython Notebook into a Python file via commandline?
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This answer opens the possibility to read tags and extract only those cells that have it. Harder to do via the command line alone.
– Pablo Adames
Aug 9 at 23:57
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Command substitution: backticks or dollar sign / paren enclosed? [duplicate]
... reason given for preferring the first (POSIX) form is that it's easier to read, especially when command substitutions are nested. Plus, with the backtick form, the backtick characters have to be backslash-escaped in the nested (inner) command substitutions.
With the POSIX form, you don't need to d...
Reading a string with scanf
...nfused about something. I was under the impression that the correct way of reading a C string with scanf() went along the lines of
...
ConcurrentHashMap vs Synchronized HashMap
...ock.
Locking the entire collection is a performance overhead. While one thread holds on to the lock, no other thread can use the collection.
ConcurrentHashMap was introduced in JDK 5.
There is no locking at the object level,The locking is at a much finer granularity. For a ConcurrentHashMap, the...
Proper Repository Pattern Design in PHP?
...tting my persistent storage (database) interaction into two categories: R (Read) and CUD (Create, Update, Delete). My experience has been that reads are really what causes an application to slow down. And while data manipulation (CUD) is actually slower, it happens much less frequently, and is there...