Re: [HACKERS] why not parallel seq scan for slow functions

On Thu, Feb 15, 2018 at 7:47 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Thu, Feb 15, 2018 at 4:18 PM, Ashutosh Bapat
> <ashutosh(dot)bapat(at)enterprisedb(dot)com> wrote:
>> I happened to look at the patch for something else. But here are some
>> comments. If any of those have been already discussed, please feel
>> free to ignore. I have gone through the thread cursorily, so I might
>> have missed something.
>>
>> In grouping_planner() we call query_planner() first which builds the
>> join tree and creates paths, then calculate the target for scan/join
>> rel which is applied on the topmost scan join rel. I am wondering
>> whether we can reverse this order to calculate the target list of
>> scan/join relation and pass it to standard_join_search() (or the hook)
>> through query_planner().
>>
>
> I think the reason for doing in that order is that we can't compute
> target's width till after query_planner(). See the below note in
> code:
>
> /*
> * Convert the query's result tlist into PathTarget format.
> *
> * Note: it's desirable to not do this till after query_planner(),
> * because the target width estimates can use per-Var width numbers
> * that were obtained within query_planner().
> */
> final_target = create_pathtarget(root, tlist);
>
> Now, I think we can try to juggle the code in a way that the width can
> be computed later, but the rest can be done earlier.

/* Convenience macro to get a PathTarget with valid cost/width fields */
#define create_pathtarget(root, tlist) \
set_pathtarget_cost_width(root, make_pathtarget_from_tlist(tlist))
create_pathtarget already works that way. We will need to split it.

Create the Pathtarget without widths. Apply width estimates once we
know the width of Vars somewhere here in query_planner()
/*
* We should now have size estimates for every actual table involved in
* the query, and we also know which if any have been deleted from the
* query by join removal; so we can compute total_table_pages.
*
* Note that appendrels are not double-counted here, even though we don't
* bother to distinguish RelOptInfos for appendrel parents, because the
* parents will still have size zero.
*
The next step is building the join tree. Set the pathtarget before that.

> However, I think
> that will be somewhat major change

I agree.

> still won't address all kind of
> cases (like for ordered paths) unless we can try to get all kind of
> targets pushed down in the call stack.

I didn't understand that.

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