Re: pgsql: Add parallel-aware hash joins.

Hi,

On 2017-12-31 02:51:26 +1300, Thomas Munro wrote:
> You mentioned that prairiedog sees the problem about one time in
> thirty.  Would you mind checking if it goes away with this patch
> applied?
> 
> -- 
> Thomas Munro
> http://www.enterprisedb.com

> From cbed027275039cc5debf8db89342a133a831c9ca Mon Sep 17 00:00:00 2001
> From: Thomas Munro <thomas.munro@enterprisedb.com>
> Date: Sun, 31 Dec 2017 02:03:07 +1300
> Subject: [PATCH] Fix EXPLAIN ANALYZE output for Parallel Hash.
> 
> In a race case, EXPLAIN ANALYZE could fail to display correct nbatch and size
> information.  Refactor so that participants report only on batches they worked
> on rather than trying to report on all of them, and teach explain.c to
> consider the HashInstrumentation object from all participants instead of
> picking the first one it can find.  This should fix an occasional build farm
> failure in the "join" regression test.

This seems buggy independent of whether it fixes the issue on prairedog,
right? So I'm inclined to go ahead and just fix it...


> +    /*
> +     * Merge results from workers.  In the parallel-oblivious case, the
> +     * results from all participants should be identical, except where
> +     * participants didn't run the join at all so have no data.  In the
> +     * parallel-aware case, we need to aggregate the results.  Each worker may
> +     * have seen a different subset of batches and we want to report the peak
> +     * memory usage across all batches.
> +     */

It's not necessarily the peak though, right? The largest batches might
not be read in at the same time. I'm fine with approximating it as such,
just want to make sure I understand.


> +    if (hashstate->shared_info)
>      {
>          SharedHashInfo *shared_info = hashstate->shared_info;
>          int        i;
>  
> -        /* Find the first worker that built a hash table. */
>          for (i = 0; i < shared_info->num_workers; ++i)
>          {
> -            if (shared_info->hinstrument[i].nbatch > 0)
> +            HashInstrumentation *worker_hi = &shared_info->hinstrument[i];
> +
> +            if (worker_hi->nbatch > 0)
>              {
> -                hinstrument = &shared_info->hinstrument[i];
> -                break;
> +                /*
> +                 * Every participant should agree on the buckets, so to be
> +                 * sure we have a value we'll just overwrite each time.
> +                 */
> +                hinstrument.nbuckets = worker_hi->nbuckets;
> +                hinstrument.nbuckets_original = worker_hi->nbuckets_original;
> +                /*
> +                 * Normally every participant should agree on the number of
> +                 * batches too, but it's possible for a backend that started
> +                 * late and missed the whole join not to have the final nbatch
> +                 * number.  So we'll take the largest number.
> +                 */
> +                hinstrument.nbatch = Max(hinstrument.nbatch, worker_hi->nbatch);
> +                hinstrument.nbatch_original = worker_hi->nbatch_original;
> +                /*
> +                 * In a parallel-aware hash join, for now we report the
> +                 * maximum peak memory reported by any worker.
> +                 */
> +                hinstrument.space_peak =
> +                    Max(hinstrument.space_peak, worker_hi->space_peak);

I bet pgindent will not like this layout.

Ho hum. Is this really, as you say above, an "aggregate the results"?


Greetings,

Andres Freund