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 Post subject: What does the distinction variable in performList method do
PostPosted: Mon Aug 22, 2016 2:58 am 
Newbie

Joined: Mon Aug 22, 2016 2:36 am
Posts: 1
Why they declared distinction local variable in org.hibernate.engine.query.spi.HQLQueryPlan class.
I am not understand why they provided this condition : if (!distinction.add(result) || ++includedCount < first) continue;

Code:
public List performList(QueryParameters queryParameters, SessionImplementor session) throws HibernateException {
        boolean needsLimit;
        QueryParameters queryParametersToUse;
        if (LOG.isTraceEnabled()) {
            LOG.tracev("Find: {0}", (Object)this.getSourceQuery());
            queryParameters.traceParameters(session.getFactory());
        }
        boolean hasLimit = queryParameters.getRowSelection() != null && queryParameters.getRowSelection().definesLimits();
        boolean bl = needsLimit = hasLimit && this.translators.length > 1;
        if (needsLimit) {
            LOG.needsLimit();
            RowSelection selection = new RowSelection();
            selection.setFetchSize(queryParameters.getRowSelection().getFetchSize());
            selection.setTimeout(queryParameters.getRowSelection().getTimeout());
            queryParametersToUse = queryParameters.createCopyUsing(selection);
        } else {
            queryParametersToUse = queryParameters;
        }
        ArrayList combinedResults = new ArrayList();
        IdentitySet distinction = new IdentitySet();
        int includedCount = -1;
        block0 : for (QueryTranslator translator : this.translators) {
            List tmp = translator.list(session, queryParametersToUse);
            if (needsLimit) {
                int first = queryParameters.getRowSelection().getFirstRow() == null ? 0 : queryParameters.getRowSelection().getFirstRow();
                int max = queryParameters.getRowSelection().getMaxRows() == null ? -1 : queryParameters.getRowSelection().getMaxRows();
                for (Object result : tmp) {
                    if (!distinction.add(result) || ++includedCount < first) continue;
                    combinedResults.add(result);
                    if (max < 0 || includedCount <= max) continue;
                    break block0;
                }
                continue;
            }
            combinedResults.addAll(tmp);
        }
        return combinedResults;
    }


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 Post subject: Re: Why use distinction variable in performList method
PostPosted: Mon Aug 29, 2016 6:42 am 
Hibernate Team
Hibernate Team

Joined: Thu Sep 11, 2014 2:50 am
Posts: 1628
Location: Romania
It might be used for when you use pagination along with a JOIN FETCH, when the pagination cannot be applied at SQL-level so it needs to be performed after the whole List is being fetched. Check out this article for more information on this topic.


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