Horton

Re: Parallel bitmap heap scan

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Dilip Kumar <dilipbalaut@gmail.com>
Cc: 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: 2017-02-08T13:44:16Z
Lists: pgsql-hackers 
On Wed, Feb 8, 2017 at 1:59 AM, Dilip Kumar <dilipbalaut@gmail.com> wrote:
> IIUC, tbm_prepare_shared_iterate will be called only by the leader,
> for tbmiterator as well as for the prefetch_iterator. And,
> tbm_attach_shared_iterate will be called by each backend and for both
> the iterators.

That's what I had in mind.

> IMHO, tbm_attach_shared_iterate should return TBMIterator with
> reference to TBMSharedIterator inside it. The reason behind same is
> that we can not keep TBMIterateResult inside TBMSharedIterator
> otherwise, results will also go in shared memory but we want to have
> local result memory for each worker so that other worker doesn't
> disturb it.

No, I don't agree.  I think TBMSharedIterator should be an unshared
structure created by tbm_attach_shared_iterate, which can internally
contain backend-private state like a TBMIterateResult, and which can
also contain a pointer to the shared-memory structure previously
created by tbm_prepare_shared_iterate.  That thing needs to be called
something other than a TBMSharedIterator, like TBMSharedIterationState
or something.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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.