[teiid-issues] [JBoss JIRA] Commented: (TEIID-1474) Add query hint to limit parallel execution of union members

[Howard Abrams (JIRA)]

    [ https://issues.jboss.org/browse/TEIID-1474?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12583370#comment-12583370 ] 

Howard Abrams commented on TEIID-1474:
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Good questions.

Just for some background info, our current use-case for this is that we have a union made up of over 400 queries; bad things happen within out underlying data-sources when we try to execute an unconstrained query against it. So for now, we're just looking to be able to serialize (or limit the parallelism) of a specific union to a) not overload our system, and b) if we issue a query with a "limit 10" that is fulfilled by the first query in the union don't bother hitting up the other 399 queries.

Presumably, if possible, the hint could effect joins as well, but I haven't given that too much thought.

I would want the hint to only effect the specif union it next to. That way if we had a very complex query with unions within union/joins/etc, we can keep some amount of parallelism.

Assuming there was nothing inherit in the query against the union that would indicate ordering, executing them in the order they appear in the union would seem most natural.




> Add query hint to limit parallel execution of union members
> -----------------------------------------------------------
>
>                 Key: TEIID-1474
>                 URL: https://issues.jboss.org/browse/TEIID-1474
>             Project: Teiid
>          Issue Type: Feature Request
>            Reporter: Howard Abrams
>            Assignee: Steven Hawkins
>
> We would like a new query hint to limit the number of parallel executions of union members. This is useful in two situations:
> 1. When the queries are expensive, but the overall union contains a limit that is most likely to be fulfilled by the first member of the union
> 2. When the queries are backed by a common resource which may be overloaded by multiple queries
> The addition of a hint would make this manageable by the client; perhaps something like "/*+ parallel=2*/"? 

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