Horton

Re: Parallel bitmap heap scan

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Dilip Kumar <dilipbalaut@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2016-10-19T07:09:30Z
Lists: pgsql-hackers 
On 2016-10-19 09:43:10 +0530, Dilip Kumar wrote:
> On Tue, Oct 18, 2016 at 1:42 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> > But what's the impact on performance?  Presumably parallel bitmap heap
> > scan was already slower than the non-parallel version, and that commit
> > presumably widens the gap.  Seems like something to worry about...
> 
> I have checked the performance in my local machine and there is no
> impact on the gap.

Try measuring with something more heavy on bitmap scan time
itself. E.g.
SELECT SUM(l_extendedprice) FROM lineitem WHERE (l_shipdate >= '1995-01-01'::date) AND (l_shipdate <= '1996-12-31'::date);
or similar.  The tpch queries don't actually spend that much time in the
bitmapscan itself - the parallization of the rest of the query is what
matters...

Greetings,

Andres Freund

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.