Horton

Re: why partition pruning doesn't work?

Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>

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From: Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: David Rowley <david.rowley@2ndquadrant.com>, Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>, Dmitry Dolgov <9erthalion6@gmail.com>, Jeff Janes <jeff.janes@gmail.com>, Pavel Stehule <pavel.stehule@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2018-06-08T05:22:26Z
Lists: pgsql-hackers 
On Fri, Jun 8, 2018 at 8:52 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> David Rowley <david.rowley@2ndquadrant.com> writes:
>> On 8 June 2018 at 03:43, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> Maybe there's something I'm missing here, but I sort of hoped that this
>>> patch would nuke all the special-case code for Params in this area.
>>> Why is there any need to distinguish them from other stable expressions?
>
>> We need to know which Params exist in the Expr as if there are no
>> Params, or only external Params, then we can run-time prune during
>> startup of the executor.
>
> This does not refute my question.  Why doesn't the same logic apply
> to any stable expression?  That is, ISTM a Param is a special case
> of that.
>

+1.

I don't think we need to perform pruning at the start of execution,
but we could fold all the stable expressions to constants at that
time. The PARAM_EXECs can not be folded into constant at execution
start since those not assigned any values yet. AFAIU expressions,
within a given node, with those parameters can be folded into
constants (if possible) during ExecRewind() on that node. We have to
perform pruning just before the (Merge)Append node scan starts.

-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company

Commits

  1. Fix up run-time partition pruning's use of relcache's partition data.

  2. Fix access to just-closed relcache entry.

  3. Improve ExecFindInitialMatchingSubPlans's subplan renumbering logic.

  4. Improve commentary about run-time partition pruning data structures.

  5. Fix run-time partition pruning code to handle NULL values properly.

  6. Assorted cosmetic cleanup of run-time-partition-pruning code.

  7. Relocate partition pruning structs to a saner place.

  8. Improve run-time partition pruning to handle any stable expression.

  9. Support partition pruning at execution time