Vladimir Ozerov created CALCITE-4515:
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Summary: Do not generate the new join tree from commute/associate
rules if it contains "always true" condition
Key: CALCITE-4515
URL: https://issues.apache.org/jira/browse/CALCITE-4515
Project: Calcite
Issue Type: Improvement
Components: core
Affects Versions: 1.27.0
Reporter: Vladimir Ozerov
Assignee: Vladimir Ozerov
{{JoinCommuteRule}} and {{JoinAssociateRule}} are the complete ruleset to
generate all bushy trees (plus join-project transposes to counter new projects
created during commute).
These rules do not check whether the new join trees contain an always {{TRUE}}
condition which is a cross-product. Such join trees are not optimal in the
majority of cases, and practical optimizers might want to suppress them.
In this ticket, I propose to add a configuration property to
{{JoinCommuteRule}} and {{JoinAssociateRule}} to disable the creation of trees
with always {{TRUE}} condition:
# The property should be disabled by default because sometimes cross-products
might be present in the query on purpose (e.g., see
[https://dl.acm.org/doi/10.5555/645916.671976], page 315)
# We consider the "always {{TRUE}}" condition only, even though they represent
only part of cross-products. The reason is that it might be difficult and
expensive to understand whether the condition is cross-join or not (e.g., for
hypergraphs). On the other hand, the check for "always true" is cheap and
robust. We assume that the user might need to do some pre-processing (e.g.,
filter push-down) to take advantage of the new property.
# We do not extend the {{JoinPushThroughJoinRule}} for now, since the [commute
+ associate] ruleset already explores all bushy trees and testing of the new
property for {{JoinPushThroughJoinRule}} is difficult. We may extend this rule
in a separate ticket if needed.
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