Re: 11g: select with union all from two different table function not parallel

From: Randolf Geist <>
Date: Thu, 23 Feb 2012 00:50:14 -0800 (PST)
Message-ID: <>

On 22 Feb., 03:20, Frank Bergemann <> wrote:

> But what about
> Shouldn't 'union all' allow parallel execution?

And which part of the AskTom thread do you think confirms your theory? Oracle supports Parallel Execution but not in the way you think / hope.

Oracle can perform parts of the execution plan in parallel by multiple slave sets but it is based on producer / consumer model having at most one producer and one consumer active at the same time (within the same so called "Data Flow Operation" (DFOs) - there can be multiple DFOs active at the same time but not in such a way as you have in mind). Hence your idea of two parallel slave sets executing the two branches below the UNION operator concurrently as producers and feeding the results into a third consumer is (currently) not supported.

Even with Parallel Execution enabled the rule outlined is still valid: If the UNION is executed using Parallel Execution, the first child operation will be executed first in parallel by multiple slaves and as soon as this is completed, the second child will be started, and so on.

Remember that the UNION operator (and other operators like FILTER, UPDATE, DELETE etc.)can have an arbitrary number of child operations, how many of them would you like Oracle to consider running concurrently? Therefore Oracle uses a different approach to Parallel Execution that is independent from such considerations.

Hope this helps,
Randolf Received on Thu Feb 23 2012 - 02:50:14 CST

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