Re: parallelism and sorting

On 11/23/15 5:47 PM, Robert Haas wrote:
> 2. In Parallel Seq Scan, the determination of what page to scan next
> isn't dependent on the contents of any page previously scanned. In
> Parallel Index Scan, it is. Therefore, the amount of effective
> parallelism is likely to be less. This doesn't mean that trying to
> parallelize things here is worthless: one backend can be fetching the
> next index page while some other backend is processing the tuples from
> a page previously read.

Presumably we could simulate that today by asking the kernel for the
next page in advance, like we do for seqscans when
effective_io_concurrency > 1. My guess is a parallel worker won't help
there.

Where a parallel worker might provide a lot of benefit is separating
index scanning from heap scanning (to check visibility or satisfy a
filter). It wouldn't surprise me if a single worker reading an index
could keep a number of children busy retrieving heap tuples and
processing them. It might be nice if an index scan node just fired up
it's own workers and talked to them directly.
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com