[jira] [Created] (CALCITE-4515) Do not generate the new join tree from commute/associate rules if it contains "always true" condition

["Vladimir Ozerov (Jira)" <ji...@apache.org>]

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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