Horton

Re: POC: converting Lists into arrays

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2019-02-25T21:03:44Z
Lists: pgsql-hackers 
Hi,

On 2019-02-25 11:59:06 -0800, Peter Geoghegan wrote:
> On Mon, Feb 25, 2019 at 10:59 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > Because it involves touching ten times more code (and that's a very
> > conservative estimate).  Excluding changes in pg_list.h + list.c,
> > what I posted touches approximately 600 lines of code (520 insertions,
> > 644 deletions to be exact).  For comparison's sake, there are about
> > 1800 uses of foreach in the tree, each of which would require at least
> > 3 changes to replace (the foreach itself, the ListCell variable
> > declaration, and at least one lfirst() reference in the loop body).
> 
> If we knew that the list code was the bottleneck in a handful of
> cases, then I'd come down on Robert's side here. It would then be
> possible to update the relevant bottlenecked code in an isolated
> fashion, while getting the lion's share of the benefit. However, I
> don't think that that's actually possible. The costs of using Lists
> everywhere are real and measurable, but it's also highly distributed.
> At least, that's my recollection from previous discussion from several
> years back. I remember talking about this with Andres in early 2016.

It's distributed, but not *that* distributed. The largest source of
"cost" at execution time used to be all-over expression evaluation, but
that's gone now. That was a lot of places, but it's not outside of reach
of a targeted change. Now it's targetlist handling, which'd have to
change together with plan time, where it's a large issue.


> > So we've already blown past 5000 lines worth of changes if we want to
> > do it another way ... and that's just *one* component of the List API.
> 
> If you want to stop third party code from compiling, you can find a
> way to do that without really changing your approach. Nothing stops
> you from changing some symbol names minimally, and then making
> corresponding mechanical changes to all of the client code within the
> tree. Third party code authors would then follow this example, with
> the expectation that it's probably going to be a totally mechanical
> process.
> 
> I'm not necessarily advocating that approach. I'm simply pointing out
> that a compromise is quite possible.

That breaks extension code using lists unnecessarily though. And given
that there's semantic change, I don't htink it's an entirely mechanical
process.

Greetings,

Andres Freund

Commits

  1. Remove EState.es_range_table_array.

  2. Rationalize use of list_concat + list_copy combinations.

  3. Cosmetic improvements in setup of planner's per-RTE arrays.

  4. Make better use of the new List implementation in a couple of places

  5. Fix sepgsql test results for commit d97b714a2.

  6. Avoid using lcons and list_delete_first where it's easy to do so.

  7. Remove lappend_cell...() family of List functions.

  8. Clean up some ad-hoc code for sorting and de-duplicating Lists.

  9. Redesign the API for list sorting (list_qsort becomes list_sort).

  10. Remove dead code.

  11. Represent Lists as expansible arrays, not chains of cons-cells.

  12. Standardize some more loops that chase down parallel lists.

  13. Reimplement the linked list data structure used throughout the backend.