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Posted to issues@calcite.apache.org by "Julian Hyde (Jira)" <ji...@apache.org> on 2023/04/10 18:19:00 UTC
[jira] [Commented] (CALCITE-5631) Optimization to merge redundant joins
[ https://issues.apache.org/jira/browse/CALCITE-5631?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17710260#comment-17710260 ]
Julian Hyde commented on CALCITE-5631:
--------------------------------------
I agree that this would be extremely useful. My work in measures (see CALCITE-4496) is turning up a lot of self-join queries.
> Optimization to merge redundant joins
> -------------------------------------
>
> Key: CALCITE-5631
> URL: https://issues.apache.org/jira/browse/CALCITE-5631
> Project: Calcite
> Issue Type: Wish
> Reporter: Ian Bertolacci
> Priority: Major
>
> There are situations where multiple joins can be merged into one.
> For example:
> {code:sql}
> select
> (select numarrayagg(C5633_203) from T893 where C5633_586 = T895.id),
> (select numarrayagg(C5633_170) from T893 where C5633_586 = T895.id)
> from T895
> {code}
> having this RelNode tree (after decorrelation):
> {code:java}
> 01:LogicalProject(EXPR$0=[$70], EXPR$1=[$72])
> └─ 02:LogicalJoin(condition=[=($0, $71)], joinType=[left])
> ├─ 03:LogicalProject(...)
> | └─ 04:LogicalJoin(condition=[=($0, $70)], joinType=[left])
> | ├─ 05:QueryTableScan(table=[[QUERY, T895]])
> | └─ 06:LogicalAggregate(group=[{0}], EXPR$0=[ARRAY_AGG($1)])
> | └─ 07:LogicalProject(C5633_586=[$85], C5633_203=[$45])
> | └─ 08:LogicalFilter(condition=[=($85, $85)])
> | └─ 09:QueryTableScan(table=[[QUERY, T893]])
> └─ 10:LogicalAggregate(group=[{0}], EXPR$0=[ARRAY_AGG($1)])
> └─ 12:LogicalProject(C5633_586=[$85], C5633_170=[$12])
> └─ 12:LogicalFilter(condition=[=($85, $85)])
> └─ 13:QueryTableScan(table=[[QUERY, T893]])
> {code}
> can be simplified as this SQL:
> {code:sql}
> select
> agg.agg1,
> agg.agg2
> from
> T895 left join
> (select
> C5633_586,
> numarrayagg(C5633_203) as agg1,
> numarrayagg(C5633_170) as agg2
> from T893 group by C5633_586
> where C5633_586 is not null
> ) as agg
> on agg.C5633_586 = T895.id
> {code}
> with this RelNode tree:
> {code:java}
> 20:LogicalProject(agg1=[$71], agg2=[$72])
> ├─ 21:LogicalJoin(condition=[=($0, $70)], joinType=[left])
> └─ 23:QueryTableScan(table=[[QUERY, T895]])
> └─ 24:LogicalAggregate(group=[{0}], agg1=[ARRAY_AGG($1)], agg2=[ARRAY_AGG($2)])
> └─ 25:LogicalProject(C5633_586=[$85], C5633_203=[$45], C5633_170=[$12])
> └─ 26:LogicalFilter(condition=[IS_NOT_NULL($85)])
> └─ 27:QueryTableScan(table=[[QUERY, T893]])
> {code}
> This can be done by identifying joins on the same underlying columns with "mergeable" strings on of nodes on the same sides of the target joins
> (I wish I had a precise definition of "mergable", but I don't. but it is easy to see that the two RHSs in the first example can be combined (merged) into the RHS in the second example without changing the result.)
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