Re: POC: converting Lists into arrays

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
Cc: Andres Freund <andres(at)anarazel(dot)de>, Robert Haas <robertmhaas(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: POC: converting Lists into arrays
Date: 2019-05-25 00:53:47
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Here's a new version of the Lists-as-arrays patch. It's rebased up to
HEAD, and I also realized that I could fix the problem with multiple
evaluation of the List arguments of foreach etc. by using structure
assignment. So that gets rid of a large chunk of the semantic gotchas
that were in the previous patch. You still have to be careful about
code that deletes list entries within a foreach() over the list ---
but nearly all such code is using list_delete_cell, which means
you'll have to touch it anyway because of the API change for that

Previously, the typical logic for deletion-within-a-loop involved
either advancing or not advancing a "prev" pointer that was used
with list_delete_cell. The way I've recoded that here changes those
loops to use an integer list index that gets incremented or not.

Now, it turns out that the new formulation of foreach() is really
strictly equivalent to

for (int pos = 0; pos < list_length(list); pos++)
whatever-type item = list_nth(list, pos);

which means that it could cope fine with deletion of the current
list element if we were to provide some supported way of not
incrementing the list index counter. That is, instead of
code that looks more or less like this:

for (int pos = 0; pos < list_length(list); pos++)
whatever-type item = list_nth(list, pos);
if (delete_cur)
list = list_delete_nth_cell(list, pos);
pos--; /* keep loop in sync with deletion */

we could write, say:

foreach(lc, list)
whatever-type item = lfirst(lc);
if (delete_cur)
list = list_delete_cell(list, lc);
foreach_backup(lc); /* keep loop in sync with deletion */

which is the same thing under the hood. I'm not quite sure if that way
is better or not. It's more magical than explicitly manipulating a list
index, but it's also shorter and therefore less subject to typos.

regards, tom lane

Attachment Content-Type Size
reimplement-List-as-array-4.patch.gz application/x-gzip 44.4 KB

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