[jira] [Created] (CALCITE-5631) Optimization to merge redundant joins

["Ian Bertolacci (Jira)" <ji...@apache.org>]

Ian Bertolacci created CALCITE-5631:
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             Summary: 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


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 T895.id = agg.C5633_586
{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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