Hi Shawn and Rick,
Thank you for your response.
My question was whether a client thread can access more than one table at a time in the
course of join execution (parallel access of multiple tables) to speed up the query if an
extra slave thread is available.
I do not know if there is anything in InnoDB (or storage engine in general) which
serializes access to tables preventing parallel access.
From: Shawn Green (MySQL) <shawn.l.green@stripped>
To: Rick James <rjames@stripped>
Sent: Friday, November 18, 2011 5:16 PM
Subject: Re: Concurrent table access from InnoDB
On 11/18/2011 16:29, Rick James wrote:
> Is this what keeps there from being parallelism in PARTITIONed tables?
No. Those partitions are accessed internally. Multiple threads can be spawned in the
context of resolving a single query. Any lack of parallelism in resolving PARTITIONed
table queries is simply a matter of design time and effort.
Now, in the context of accessing internal resources any query that requires a JOIN will
need to touch at least two tables under the same user context. Perhaps I
misunderstood the original question?
> Is it possible for a client connection thread to access 2 tables concurrently from
InnoDB, say using a slave thread ? Or is access limited to one table at a time per
client connection thread ?
Perhaps he was asking about the optimizer processing a command that needed to touch two or
more tables? If that's the purpose of the question then of course more than one
table can be accessed in the context of a single connection. This happens very
frequently. I cannot say with absolute certainty if those tables are processed in
parallel but I would be concerned if they were not.
I was reading his question more like this: Can the same connection be used to open
multiple queries where each query is pointing to a separate table? In that context, each
connection can only have one active command.
-- Shawn Green
MySQL Principal Technical Support Engineer
Oracle USA, Inc. - Hardware and Software, Engineered to Work Together.
Office: Blountville, TN
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