Re: memory problems and crash of db when deleting data from table with thousands of partitions

On Wed, Oct 30, 2019 at 03:35:24PM +0100, Josef Machytka wrote:
>Here are scripts which you can use to simulate problem:
>
>- create_tables.sql - creates all partitions
>- generate_data.sql - generates some data (technically you need only a few
>records, delete command will fail anyway)
>
>and try command:
>DELETE FROM bi.test_multilevel WHERE period_name = '....';
>
>PostgreSQL 12 will start to use more and more memory and will stop
>operation with "out of memory" (PostgreSQL 11 would crash)
>

Thanks for the scripts, I'm able to reproduce the issue. It does seem
most of the memory is allocated in inheritance_planner, where we do this
(around line 1500)

foreach(lc, child_appinfos)
{
...

/*
* Generate modified query with this rel as target. We first apply
* adjust_appendrel_attrs, which copies the Query and changes
* references to the parent RTE to refer to the current child RTE,
* then fool around with subquery RTEs.
*/
subroot->parse = (Query *)
adjust_appendrel_attrs(subroot,
(Node *) parent_parse,
1, &appinfo);

...
}

This unfortunately allocates a Query struct that is about ~4.3MB *per
partition*, and you have ~10000 of them, so 43GB in total.

Unfortunately, this does not seem like a memory leak - we actually do
need the structure, and it happens to be pretty large :-( So IMHO it's
working as designed, it just wasn't optimized for cases with many
partitions yet :-(

Chances are we'll improve this in future releases (13+), but I very much
doubt we can do much in existing releases - we tend not to make big
changes there, and I don't see a simple change addressing this.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services