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I understand that .collect(pf), where pf is a partial function, is the equivalent to .filter(pf.isDefinedAt _).map(pf). What I don't understand is what just .collect() does. Can anyone explain this?
collect without parameters fetches all data stored in a RDD to the driver.
Return an array that contains all of the elements in this RDD.
Note
This method should only be used if the resulting array is expected to be small, as all the data is loaded into the driver's memory.
There is no connection to the version with PartialFunction whatsoever. Both are used for completely different things.
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Closed 3 years ago.
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I have a problem with a stored procedure.
The procedure gets as an argument the number of rows needed, but the following does not work in HANA:
SELECT TOP :NUM_OF_ROWS * FROM TABLE_NAME
I read that TOP in HANA only receives a number, not an expression. Is there another way to do this? My solution for the moment is to select everything and delete the unneeded records on the service, but it's not very efficient.
Instead of TOP n you can use the LIMIT n option.
That one can bind variables.
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Closed 9 years ago.
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In Postgresql, the hstore and json datatypes seem to have very similar use cases. When would you choose to use one vs. the other? Initial thoughts:
You can nest with json; you can't with hstore
Functions for parsing json won't be available until 9.3
The json type is just a string. There are no built in functions to parse it. The only thing to be gained when using it is the validity checking.
Edit for those downvoting: This was written when 9.3 still didn't exist.It is correct for 9.2. Also the question was different. Check the edit history.
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Scala uses a type-system based on System F ω, which is normally said to be strongly normalizing. Strongly normalizing implies non-Turing completeness.
Nevertheless, Scala's type-system is Turing-complete.
Which changes/additions/modifications make Scala's type-system Turing-complete compared to the formal algorithms and systems?
It's not a comprehensive answer but the reason is that you can define recursive types.
I've asked similar questions before (about what a non-Turing complete language might look like). The answers were of the form: a Turing complete language must support either arbitrary looping or recursion. Scala's type system supports the latter
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Closed 11 years ago.
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What speed optimization techniques do antiviruses use today to scan a file, provided they have to check for all the signatures + the behavioral scan?
I'm not a antivirus programmer, but I think the scan engine scans through a file searching for known pattern inside. The greater number of patterns it can identify, the longer it will take to scan.
Optimization maybe similar to database optimization, with patterns indexing.
Identification Methods
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I want to use a persistent HashTable to provide data storage for my application. Is this possible. A large well supported open-source project would be ideal.
You have 2 options :
a) Seralize your Hashtable to file -- after all the Hashtable class implements Serializable.
b) BerkeleyDB Java Edition -- you can download this for free from Oracle. It is open source. Berkeley DB database is a b-tree. It is fairly straight forward to convert your code from HashTable to Berkeley DB .
Note that if you use simple Hashtable for storing your objects, you will run out out memory when the number of obects in Hashtable increases beyond a certain number. With Berkeley DB, there is no such limitation.
Chronicle Map is an off-heap key-value store for Java, providing ConcurrentMap interface and (optionally) persists data to disk. Under the hood, it's implemented via memory-mapped files.