[jira] Commented: (WICKET-2618) Don't call IDataProvider.size method when IDataProvider.iterator(first, count) returned the number of elements less than "count"

["Michael Mikhulya (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/WICKET-2618?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12801771#action_12801771 ] 

Michael Mikhulya commented on WICKET-2618:
------------------------------------------

There is one problem with my patch:

	@Override
	public Iterator<? extends T> iterator(int first, int count) {
		ArrayList<T> result = new ArrayList<T>(count);

Here we shouldn't preallocate space in ArrayList, since in some views (w/o paginators) the count is equals to Integer.MAX_VALUE.

So the fixed code is:

> Don't call IDataProvider.size method when IDataProvider.iterator(first, count) returned the number of elements less than "count"
> --------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: WICKET-2618
>                 URL: https://issues.apache.org/jira/browse/WICKET-2618
>             Project: Wicket
>          Issue Type: Improvement
>          Components: wicket
>    Affects Versions: 1.4.4
>            Reporter: Michael Mikhulya
>         Attachments: AbstractDataProviderDecorator.java, OptimizedDataProviderDecorator.java
>
>
> IDataProvider has two mehtods:
> 	Iterator<? extends T> iterator(int first, int count);
> 	int size()
> which in many implementations do access to database.
> Normally iterator do SQL query with LIMIT and OFFSET, and size do SQL COUNT query.
> SQL COUNT is O(N) for most transactional databases (e.g. for postgresql) and so it is rather heavy.
> In some cases it is possible to avoid call to IDataProvider.size method.
> For example when "iterator" returned the number of elements less than acquired "count".
> This case appears in 90% in my application (whenever search is used normally number of found elements is less than number of elements on page).
> Also pgFouine shows me these queries as the most slow one. So I suggest to implement this optimization.
> First I though that appropriate place to implement it is AbstractPageableView class, but unfortunatelly not all IPageable classes extends this abstract class (for example DataTable doesn't do so).
> So I suggest to implement Decorator of IDataProvider and reuse it in DataViewBase, DataTable, ...
> If nobody see any problems with this suggestion I would like to implement it on my own.

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