[jira] [Assigned] (CAY-2917) Add Qualifier Joins Before Dependent Table Joins

["Nikita Timofeev (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/CAY-2917?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Nikita Timofeev reassigned CAY-2917:
------------------------------------

    Assignee: Nikita Timofeev

> Add Qualifier Joins Before Dependent Table Joins
> ------------------------------------------------
>
>                 Key: CAY-2917
>                 URL: https://issues.apache.org/jira/browse/CAY-2917
>             Project: Cayenne
>          Issue Type: Improvement
>            Reporter: Jadon Hansell
>            Assignee: Nikita Timofeev
>            Priority: Major
>             Fix For: 5.0-M2
>
>          Time Spent: 20m
>  Remaining Estimate: 0h
>
> When a query is executed on the parent model in a vertical inheritance scenario, the ColumnExtractorStage adds left joins to all dependent tables to gather the necessary columns for the select. After that, the rest of the joins for the query are added in the PrefetchNodeStage, QualifierTranslationStage, etc. This can have a large performance impact when there are several dependent tables.
> For example, PostgreSQL's query planner will only attempt to reorder the first 8 joins (by default) to try to determine an optimal join order for the query. With a small number of dependent tables, it will usually execute the joins added by the QualifierTranslationStage first to narrow down the result set before executing the dependent tables' left joins for the rest of the data. However, if you have 8 or more dependent tables, a very inefficient query plan is created. All the dependent tables' left joins are executed first, ballooning the number of rows the query has to process until it gets to the qualifier joins at the end. This can significantly reduce the speed of some queries.



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