Horton

Re: POC: converting Lists into arrays

Andres Freund <andres@anarazel.de>

View thread View on postgr.es
From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@lists.postgresql.org
Date: 2019-07-31T22:57:56Z
Lists: pgsql-hackers 
Hi,

I was just looking at the diff for a fix, which adds a "ListCell *lc;"
to function scope, even though it's only needed in a pretty narrow
scope.

Unfortunately foreach(ListCell *lc, ...) doesn't work with the current
definition. Which I think isn't great, because the large scopes for loop
iteration variables imo makes the code harder to reason about.

I wonder if we could either have a different version of foreach() that
allows that, or find a way to make the above work. For the latter I
don't immediately have a good idea of how to accomplish that. For the
former it's easy enough if we either don't include the typename (thereby
looking more alien), or if we reference the name separately (making it
more complicated to use).


I also wonder if a foreach version that includes the typical
(Type *) var = (Type *) lfirst(lc);
or
(Type *) var = castNode(Type, lfirst(lc));
or
OpExpr	   *hclause = lfirst_node(OpExpr, lc);

would make it nicer to use lists.

foreach_node_in(Type, name, list) could mean something like

foreach(ListCell *name##_cell, list)
{
    Type* name = lfirst_node(Type, name##_cell);
}

(using a hypothetical foreach that supports defining the ListCell in
scope, just for display simplicity's sake).

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.