Horton

Re: Parallel bitmap heap scan

Dilip Kumar <dilipbalaut@gmail.com>

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From: Dilip Kumar <dilipbalaut@gmail.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>, Haribabu Kommi <kommi.haribabu@gmail.com>, Rafia Sabih <rafia.sabih@enterprisedb.com>, tushar <tushar.ahuja@enterprisedb.com>, Amit Khandekar <amitdkhan.pg@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2017-03-08T14:10:24Z
Lists: pgsql-hackers 
On Wed, Mar 8, 2017 at 6:42 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> I don't think I understand exactly why this system might be prone to a
> little bit of extra prefetching - can you explain further?
Let me explain with an example, suppose there are 2 workers
prefetching jointly, lets assume
prefetch_target is 3, so for example when both the workers prefetch 1
page each then prefetch_page count will be 2, so both the workers will
go for next prefetch because prefetch_page is still less than prefetch
target, so again both the workers will do prefetch and totally they
will prefetch 4 pages.

> I don't think it will hurt anything as long as we are talking about a small
> number of extra prefetches here and there.

I completely agree with this point and I mentioned in the mail so that
it don't go unnoticed.

And, whatever extra prefetch we have done we will anyway use it unless
we stop the execution because of some limit clause.

>
>> Fixed
>
> Committed 0001.

Thanks


-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com

Commits

  1. Pass DSA_ALLOC_HUGE when allocating a shared TIDBitmap.

  2. Support parallel bitmap heap scans.

  3. tidbitmap: Support shared iteration.

  4. Shut down Gather's children before shutting down Gather itself.

  5. Revise the way the element allocator for a simplehash is specified.

  6. Refactor bitmap heap scan estimation of heap pages fetched.

  7. Use more efficient hashtable for tidbitmap.c to speed up bitmap scans.