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https://issues.apache.org/jira/browse/OPENJPA-134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12476232
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John Stecher commented on OPENJPA-134:
--------------------------------------
I think the links to the mailing list post that Kevin has shows a good example
of the code causing the problem. In reality any one to many relationship with
a back pointer generates two joins when only one is necessary to pull in all
the information needed for the query.
Dave's example in the mailing list should be something you can run pretty quick
and see the problem. Catalina's original JIRA entry contains a pretty good
example of the SQL.
> Extra unneeded SQL joins for OneToMany relationship with fetch type EAGER
> -------------------------------------------------------------------------
>
> Key: OPENJPA-134
> URL: https://issues.apache.org/jira/browse/OPENJPA-134
> Project: OpenJPA
> Issue Type: Bug
> Components: sql
> Reporter: Catalina Wei
>
> Running JPAConfiguration default setting for EagerFetchMode
> (FetchModeValue.EAGER_PARALLEL),
> the SQL generated is sub-optimal.
> Consider the following entities:
> lineitems( OneToMany )
> Order <===========================> OrderItem
> order ( ManyToOne )
> Case 1: why not combining 2 SQL to 1 SQL ?
> =================================================================================================
> Order.lineitmes(EAGER):
> OrderItem.order(LAZY):
> Executing query: select o from Order o
> 2173 demo TRACE [main] openjpa.jdbc.SQL - <t 1094730048, conn 1318342292>
> executing prepstmnt 1299336562
> SELECT t0.oid, t0.version, t0.amount, t0.customer_countryCode,
> t0.customer_id, t0.delivered, t0.shipaddr FROM Order t0
> 2213 demo TRACE [main] openjpa.jdbc.SQL - <t 1094730048, conn 1318342292>
> [40 ms] spent
> 2223 demo TRACE [main] openjpa.jdbc.SQL - <t 1094730048, conn 1318342292>
> executing prepstmnt 1406424020
> SELECT t0.oid, t1.lid, t1.version, t1.cost, t1.order_oid, t1.part_partno,
> t1.quantity FROM Order t0 INNER JOIN OrderItem t1 ON t0.oid = t1.order_oid
> ORDER BY t0.oid ASC
> Case 2: extra unneeded LEFT OUTER JOIN, if eliminated, the selection aliase
> t2 should change to t1:
> =============================================================================================
> Order.lineitmes(EAGER):
> OrderItem.order(LAZY):
> Executing query: select o from Order o left join fetch o.lineitems
> 2403 demo TRACE [main] openjpa.jdbc.SQL - <t 1094730048, conn 1314410072>
> executing prepstmnt 1500797300
> SELECT t0.oid, t0.version, t0.amount, t0.customer_countryCode,
> t0.customer_id, t0.delivered, t0.shipaddr, t2.order_oid, t2.lid, t2.version,
> t2.cost, t2.part_partno, t2.quantity FROM Order t0 LEFT OUTER JOIN OrderItem
> t1 ON t0.oid = t1.order_oid LEFT OUTER JOIN OrderItem t2 ON t0.oid =
> t2.order_oid ORDER BY t2.order_oid ASC
> Case 3: why not generating 1 SQL ?
> ==================================================================================================
> Order.lineitmes(EAGER):
> OrderItem.order(EAGER):
> Executing query: select o from Order o
> 2343 demo TRACE [main] openjpa.jdbc.SQL - <t 1094730048, conn 1318342292>
> executing prepstmnt 384833264 SELECT t0.oid, t0.version, t0.amount,
> t0.customer_countryCode, t0.customer_id, t0.delivered, t0.shipaddr FROM Order
> t0
> 2383 demo TRACE [main] openjpa.jdbc.SQL - <t 1094730048, conn 1318342292>
> [40 ms] spent
> 2393 demo TRACE [main] openjpa.jdbc.SQL - <t 1094730048, conn 1318342292>
> executing prepstmnt 1722705582
> SELECT t0.oid, t1.lid, t1.version, t1.cost, t2.oid, t2.version, t2.amount,
> t2.customer_countryCode, t2.customer_id, t2.delivered, t2.shipaddr,
> t1.part_partno, t1.quantity FROM Order t0 INNER JOIN OrderItem t1 ON t0.oid =
> t1.order_oid LEFT OUTER JOIN Order t2 ON t1.order_oid = t2.oid ORDER BY
> t0.oid ASC
> 2393 demo TRACE [main] openjpa.jdbc.SQL - <t 1094730048, conn 1318342292>
> [0 ms] spent
> 3134 demo TRACE [main] openjpa.jdbc.SQL - <t 1094730048, conn 1318342292>
> executing prepstmnt 950548648
> SELECT t0.lid, t0.version, t0.cost, t1.oid, t1.version, t1.amount,
> t1.customer_countryCode, t1.customer_id, t1.delivered, t1.shipaddr,
> t0.part_partno, t0.quantity FROM OrderItem t0 LEFT OUTER JOIN Order t1 ON
> t0.order_oid = t1.oid WHERE t0.order_oid = ? [params=(int) 88]
> 3134 demo TRACE [main] openjpa.jdbc.SQL - <t 1094730048, conn 1318342292>
> [0 ms] spent
> Case 4: duplicate selections and redundant joins
> ==================================================================================================
> Order.lineitmes(EAGER):
> OrderItem.order(EAGER):
> Executing query: select o from Order o left join fetch o.lineitems
> 2273 demo TRACE [main] openjpa.jdbc.SQL - <t 1094730048, conn 1307463150>
> executing prepstmnt 1565154634
> SELECT t0.oid, t0.version, t0.amount, t0.customer_countryCode,
> t0.customer_id, t0.delivered, t0.shipaddr, t2.order_oid, t2.lid, t2.version,
> t2.cost, t3.oid, t3.version, t3.amount, t3.customer_countryCode,
> t3.customer_id, t3.delivered, t3.shipaddr, t2.part_partno, t2.quantity FROM
> Order t0 LEFT OUTER JOIN OrderItem t1 ON t0.oid = t1.order_oid LEFT OUTER
> JOIN OrderItem t2 ON t0.oid = t2.order_oid LEFT OUTER JOIN Order t3 ON
> t2.order_oid = t3.oid ORDER BY t2.order_oid ASC
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