Hi,
Is there any way to export data from the database to other databases?
Thanks -Raj
From: basex-talk-bounces@mailman.uni-konstanz.de [mailto:basex-talk-bounces@mailman.uni-konstanz.de] On Behalf Of Klavs Prieditis Sent: Wednesday, March 02, 2016 11:21 AM To: Christian Grün christian.gruen@gmail.com; Dirk Kirsten dk@basex.org Cc: BaseX basex-talk@mailman.uni-konstanz.de Subject: Re: [basex-talk] Parallel execution
Hi,
Thanks Dirk and Christian! My apologies for using such a strong word to describe such a minor issue. It is not my intention to underestimate your work.
My main concern is that `async:fork-join()` is too heavy. I mean, in essence, I am solving the same problem in these two examples.
```xquery for $i in 1 to 10 return fetch:text("http://example.com/magic-story/for-number/" || $i) ```
```xquery async:fork-join( for $i in 1 to 10 return function() {fetch:text("http://example.com/magic-story/for-number/" || $i)} ) ```
If you solve the same problem in Java 8 using Streams API you get the following two solutions.
```java rangeFrom1To10AsList.stream().map(fetchText).collect(...); ```
```java rangeFrom1To10AsList.parallelStream().map(fetchText).collect(...); ```
Considering these two Java 8 solutions, I guess what I actually want is the `async` keyword. So that the second Java example could be rewritten as follows.
```xquery for async $i in 1 to 10 return fetch:text("http://example.com/magic-story/for-number/" || $i) ```
Problem here is that you need to introduce a new keyword to the language. I am just trying to figure out if there could be a solution like this.
```xquery for $i in 1 to 10 return fetch:text-with-implicit-parallel-execution-characteristics("http://example.com/magic-story/for-number/" || $i) ```
Thanks for your input! Currently I lack in-depth knowledge on XQuery, I'll try to get a spare time to study the XQuery specification so that I am able reason about the various consequences.
2016-03-02 11:36 GMT+02:00 Christian Grün <christian.gruen@gmail.commailto:christian.gruen@gmail.com>: Hi Klāvs,
Dirk already gave some good reasons why implicit parallelization is not the default in BaseX, although it may seems to be the most natural and desirable choice. Indeed it’s fairly easy to create FLWOR expressions which are slower when executed in parallel. The XQuery specification, by the way, does not say anything about parallelization, so it’s completely up to the implementation to choose the most promising evaluation strategy.
You asked if it was not possible to introduce sth. like a `can-be-optimized-for-parallel-execution` flag, and you may be interested to hear that we had some first thoughts on an 'async' keyword, which could be used in FLWOR expressions [1]. In the GitHub issue, only the most basic cases have been considered so far, but if we manage to push this further, we would have a syntax that will be more easily accessible to most users. Ideally, some time in future, we could then apply this pattern automatically if it will be absolutely clear that parallelization leads to faster results.
Everyone: If you notice that the async:fork-join function makes your code faster, don’t hesitate and send us your queries.
Hope this helps, Christian
[1] https://github.com/james-jw/xq-promise/issues/15
On Wed, Mar 2, 2016 at 10:05 AM, Dirk Kirsten <dk@basex.orgmailto:dk@basex.org> wrote:
Labrīt Klāvs,
well, I would the word "broken" is a strong word for something, which still works correctly and returns the correct result. It might not be perform optimal, but broken is something different. Also, given that the feature is still not even one day old, you might have to be a bit more patient with us in applying optimization techniques.
The problem here has several dimensions:
- XQuery returns results in order. So in order to not break BaseX we
would have to cache the results of the fetch and return it in the correct order
- This caching might be much more complex, if it is a more complicated
expression
- It might not always be desirable to turn on parallelisation. In this
case you call an external HTTP server 10 times, in this case there might be a speedup. But using fetch:text() could also point to a local URI and to parallelize I/O is not always beneficial.
So given the many different use cases here I don't think an implicit parallelization will work here to the satisfaction of everyone. Also, I invite you to read this closely related question about Haskell on SO and the linked research: https://stackoverflow.com/questions/15005670/why-is-there-no-implicit-parall...
Cheers Dirk
On 03/02/2016 01:41 AM, Klāvs Priedītis wrote:
Hello, BaseX team and community!
Disclosure: I am not yet an expert on these issues and I have not fully studied the specs of XQuery.
Now that Async Module is incorporated, I am wondering about some basic properties of xquery.
Example:
for $i in 1 to 10 return fetch:text("http://example.com/magic-story/for-number/" || $i)It feels to me that the xquery processor is broken if it cannot figure out on its own that the fetch can be done in parallel. Why does this not work out of the box? Can it work out of the box? Wouldn't it be able to do this in parallel if a flag `can-be-optimized-for-parallel-execution` was defined for the `fetch:text`?
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