Horton

Re: Parallel bitmap heap scan

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>, Dilip Kumar <dilipbalaut@gmail.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: 2017-02-20T19:04:50Z
Lists: pgsql-hackers 
Robert Haas <robertmhaas@gmail.com> writes:
> On Mon, Feb 20, 2017 at 6:19 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> The thing that you really have to worry about for this kind of proposal
>> is "what if the query errors out and we never get to ExecEndNode"?
>> It's particularly nasty if you're talking about parallel queries where
>> maybe only one or some of the processes involved detect an error.

> I think that's not actually a problem, because we've already got code
> to make sure that all DSM resources associated with the query get
> blown away in that case.  Of course, that code might have bugs, but if
> it does, I think it's better to try to fix those bugs than to insert
> some belt-and-suspenders mechanism for reclaiming every possible chunk
> of memory in retail fashion, just like we blow up es_query_cxt rather
> than trying to pfree allocations individually.

Actually, I think we're saying the same thing: rely on the general DSM
cleanup mechanism, don't insert extra stuff that you expect will get
done by executor shutdown.

			regards, tom lane

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.