[jira] [Commented] (IGNITE-7526) SQL: Introduce memory region for reducer merge results with disk offload

["Denis Magda (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/IGNITE-7526?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16416350#comment-16416350 ] 

Denis Magda commented on IGNITE-7526:
-------------------------------------

[~vozerov], according to the summary the optimizations will be applied to the client applications (who are reducers in 95% of the time). However, don't we need to do the same on the mappers side? I do remember the reportings when servers went down just because someone executed "SELECT * FROM table;" query. 

> SQL: Introduce memory region for reducer merge results with disk offload
> ------------------------------------------------------------------------
>
>                 Key: IGNITE-7526
>                 URL: https://issues.apache.org/jira/browse/IGNITE-7526
>             Project: Ignite
>          Issue Type: Task
>          Components: sql
>            Reporter: Vladimir Ozerov
>            Priority: Major
>
> Currently all results received from map nodes are stored inside reducer's heap memory. What is worse, in case of complex queries, such as having sorts or groupings, need to collect all results from mappers first before final processing could be applied. In case of big results set (or intermediate results) this could easily lead to OOME on reducer. 
> To mitigate this we should introduce special memory area where intermediate results could be stored. All final processing should be stored in the same area as well. This area should be of limited size and should be able to offload results to disk in case of overflow.
> We could start with our B+Tree and free list and store results in some K-V form. 



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