Horton

Re: Postgres 11 release notes

Bruce Momjian <bruce@momjian.us>

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From: Bruce Momjian <bruce@momjian.us>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2018-05-15T18:47:26Z
Lists: pgsql-hackers 
On Tue, May 15, 2018 at 08:45:07AM +0530, Amit Kapila wrote:
> No, it is not like that.  We divide the scan among workers and each
> worker should perform projection of the rows it scanned (after
> applying filter).  Now, if the expensive functions are part of target
> lists, then we can push the computation of expensive functions (as
> part of target list) in workers which will divide the work.
> 
> >  Really?  Do
> > we run each column in its own worker or do we split the result set into
> > parts and run those in parallel?  How do we know, just the function call
> > costs?
> >
> 
> The function's cost can be determined via pg_proc->procost.  For this
> particular case, you can refer the call graph -
> create_pathtarget->set_pathtarget_cost_width->cost_qual_eval_node->cost_qual_eval_walker->get_func_cost
> 
> >  I can admit I never saw that coming.
> >
> 
> I think the use case becomes interesting with parallel query because
> now you can divide such cost among workers.
> 
> Feel free to ask more questions if above doesn't clarify the usage of
> these features.

OK, I have added the following release note item for both of these:

2017-11-16 [e89a71fb4] Pass InitPlan values to workers via Gather (Merge).
2018-03-29 [3f90ec859] Postpone generate_gather_paths for topmost scan/join rel
2018-03-29 [11cf92f6e] Rewrite the code that applies scan/join targets to paths

        Allow single-evaluation queries, e.g. <literal>FROM</literal>
        clause queries, and functions in the target list to be
        parallelized (Amit Kapila, Robert Haas)

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

+ As you are, so once was I.  As I am, so you will be. +
+                      Ancient Roman grave inscription +

Commits

  1. doc: update PG 11 release notes

  2. Fix misspelled pg_trgm contrib name in PostgreSQL 11 release notes

  3. Doc: clarify release note text about v11's new window function features.

  4. Improve wording of release notes item

  5. Fix typos in release notes

  6. Doc: preliminary list of PG11 major features.

  7. Make numeric power() handle NaNs according to the modern POSIX spec.

  8. Various improvements of skipping index scan during vacuum technics

  9. Revert back-branch changes in power()'s behavior for NaN inputs.

  10. Avoid wrong results for power() with NaN input on more platforms.

  11. Avoid wrong results for power() with NaN input on some platforms.

  12. Skip full index scan during cleanup of B-tree indexes when possible

  13. Rewrite the code that applies scan/join targets to paths.

  14. Postpone generate_gather_paths for topmost scan/join rel.

  15. Add casts from jsonb

  16. Make plpgsql use its DTYPE_REC code paths for composite-type variables.

  17. Don't allow VACUUM VERBOSE ANALYZE VERBOSE.

  18. Pass InitPlan values to workers via Gather (Merge).

  19. Account for the effect of lossy pages when costing bitmap scans.

  20. Allow no-op GiST support functions to be omitted.

  21. Rearm statement_timeout after each executed query.

  22. Push limit through subqueries to underlying sort, where possible.