Re: [GENERAL] huge RAM use in multi-command ALTER of table heirarchy

On 19 July 2017 at 07:26, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Justin Pryzby <pryzby(at)telsasoft(dot)com> writes:
> > I've seen this before while doing SET STATISTICS on a larger number of
> columns
> > using xargs, but just came up while doing ADD of a large number of
> columns.
> > Seems to be roughly linear in number of children but superlinear WRT
> columns.
> > I think having to do with catalog update / cache invalidation with many
> > ALTERs*children*columns?
>
> I poked into this a bit. The operation is necessarily roughly O(N^2) in
> the number of columns, because we rebuild the affected table's relcache
> entry after each elementary ADD COLUMN operation, and one of the principal
> components of that cost is reading all the pg_attribute entries. However,
> that should only be a time cost not a space cost. Eventually I traced the
> O(N^2) space consumption to RememberToFreeTupleDescAtEOX, which seems to
> have been introduced in Simon's commit e5550d5fe, and which strikes me as
> a kluge of the first magnitude. Unless I am missing something, that
> function's design concept can fairly be characterized as "let's leak
> memory like there's no tomorrow, on the off chance that somebody somewhere
> is ignoring basic coding rules".
>
> I tried ripping that out, and the peak space consumption of your example
> (with 20 child tables and 1600 columns) decreased from ~3GB to ~200MB.
> Moreover, the system still passes make check-world, so it's not clear
> to me what excuse this code has to live.
>
> It's probably a bit late in the v10 cycle to be taking any risks in
> this area, but I'd vote for ripping out RememberToFreeTupleDescAtEOX
> as soon as the v11 cycle opens, unless someone can show an example
> of non-broken coding that requires it. (And if so, there ought to
> be a regression test incorporating that.)

Just FYI, I believe Simon's currently on holiday, so may not notice this
discussion as promptly as usual.

--
Craig Ringer http://www.2ndQuadrant.com/
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