Ruben Q L created CALCITE-3671:
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Summary: Join cost computation should consider join condition
(equi vs non-equi)
Key: CALCITE-3671
URL: https://issues.apache.org/jira/browse/CALCITE-3671
Project: Calcite
Issue Type: Improvement
Affects Versions: 1.21.0
Reporter: Ruben Q L
In some Join algorithms, the actual cost of performing the join would depend on
whether or not the join conditions is an equi-join or not, therefore
computeSelfCost should reflect that.
This would be the case for example of HashJoin (which now supports all type of
join condition, see CALCITE-2973) or MergeJoin (idem, CALCITE-3285).
To sump up, we can have three different scenarios:
a) The condition is a "complete equi-join condition"; this is the best case
scenario, the join is performed purely on a hash/merge based algorithm and no
extra predicate is required.
b) The condition is a "partial equi-join conditiom", i.e. the condition
contains some equi-join items, but also some non-equi-join items; in this case
the join is performed on a hash/merge based algorithm (for the equi-join items)
+ an extra predicate (for the non-equi-join ones).
c) The join condition is a "complete non-equi-join-condition", i.e. there are
no equi-join elements to build a hash/merge based solution, so the algorithm is
performed based on a predicate which evaluates the whole condition. This is the
worst-case scenario, since the Hash/Merge Join actually behaves as a kind of
de-facto nested loop join.
Currently, since the condition nature is not evaluated in the computeSelfCost,
cases a-b-c would have an equivalent cost; we should reflect somehow that: cost
a < cost b < cost c
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