BulkFetchJoinHandler.java

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Copyright (c) 2017 Andy Jefferson and others. All rights reserved.
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
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    ...
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package org.datanucleus.store.rdbms.query;

import java.util.Map;
import java.util.Set;

import org.datanucleus.ClassLoaderResolver;
import org.datanucleus.ExecutionContext;
import org.datanucleus.exceptions.NucleusException;
import org.datanucleus.metadata.AbstractClassMetaData;
import org.datanucleus.metadata.AbstractMemberMetaData;
import org.datanucleus.store.query.Query;
import org.datanucleus.store.rdbms.RDBMSStoreManager;
import org.datanucleus.store.rdbms.scostore.FKArrayStore;
import org.datanucleus.store.rdbms.scostore.FKListStore;
import org.datanucleus.store.rdbms.scostore.FKSetStore;
import org.datanucleus.store.rdbms.scostore.IteratorStatement;
import org.datanucleus.store.rdbms.scostore.JoinArrayStore;
import org.datanucleus.store.rdbms.scostore.JoinListStore;
import org.datanucleus.store.rdbms.scostore.JoinSetStore;
import org.datanucleus.store.types.scostore.Store;

/**
 * Helper class to generate the necessary statement for multi-valued field bulk-fetch, using JOIN semantics.
 * <p>
 * In simple terms if we have a query with resultant SQL like
 * <pre>SELECT COL1, COL2, COL3, ... FROM CANDIDATE_TBL T1 WHERE T1.COL2 = value</pre>
 * 
 * then to retrieve a multi-valued collection field of the candidate class it generates an SQL like
 * <pre>SELECT ELEM.COL1, ELEM.COL2, ... FROM CANDIDATE_TBL T1, ELEMENT_TBL ELEM WHERE ELEM.CAND_ID = T1.ID AND T1.COL2 = value</pre>
 * 
 * Obviously there are differences when using a join-table, or when the elements are embedded into the join-table, but the basic idea is 
 * we generate an iterator statement for the elements (just like the backing store normally would) except instead of restricting the statement 
 * to just a particular owner, it adds a JOIN, restricting to the candidates implied by the query
 * <b>This is work-in-progress</b>
 */
public class BulkFetchJoinHandler implements BulkFetchHandler
{
    /**
     * Convenience method to generate a bulk-fetch statement for the specified multi-valued field of the owning query.
     * @param candidateCmd Metadata for the candidate
     * @param mmd Metadata for the member we are bulk-fetching the value(s) for
     * @param query The query
     * @param parameters Parameters for the query
     * @param datastoreCompilation The datastore compilation of the query
     * @param mapperOptions Any mapper options for query generation
     * @return The statement to use for bulk fetching, together with mappings for extracting the results of the elements
     */
    public IteratorStatement getStatementToBulkFetchField(AbstractClassMetaData candidateCmd, AbstractMemberMetaData mmd,
            Query query, Map parameters, RDBMSQueryCompilation datastoreCompilation, Set<String> mapperOptions)
    {
        ExecutionContext ec = query.getExecutionContext();
        ClassLoaderResolver clr = ec.getClassLoaderResolver();
        RDBMSStoreManager storeMgr = (RDBMSStoreManager) query.getStoreManager();
        Store backingStore = storeMgr.getBackingStoreForField(clr, mmd, null);

        if (backingStore instanceof JoinSetStore || backingStore instanceof JoinListStore || backingStore instanceof JoinArrayStore)
        {
            // Set/List/array using join-table : Generate an iterator query of the form
            // SELECT ELEM_TBL.COL1, ELEM_TBL.COL2, ... FROM CANDIDATE_TBL T1 INNER JOIN JOIN_TBL T2 ON T2.OWNER_ID = T1.ID INNER_JOIN ELEM_TBL T3 ON T3.ID = T2.ELEM_ID
            // WHERE (queryWhereClause)

            // TODO Start from the original query, and remove any grouping, having, ordering etc, and join to join table + element table.
        }
        else if (backingStore instanceof FKSetStore || backingStore instanceof FKListStore || backingStore instanceof FKArrayStore)
        {
            // Set/List/array using foreign-key : Generate an iterator query of the form
            // SELECT ELEM_TBL.COL1, ELEM_TBL.COL2, ... FROM ELEM_TBL
            // WHERE EXISTS (SELECT OWNER_TBL.ID FROM OWNER_TBL WHERE (queryWhereClause) AND ELEM_TBL.OWNER_ID = OWNER_TBL.ID)

            // TODO Start from the original query, and remove any grouping, having, ordering etc, and join to element table.
        }
        throw new NucleusException("BulkFetch via JOIN is not yet implemented");
//        return iterStmt;
    }
}