On Fri, 04 Sep 2026, Ewan Young <kdbase.hack@gmail.com> wrote:
> - Kahn's algorithm implementation looks right.
Not entirely, the implementation assumes that the graph is a DAG with
an assert, which unfortunately isn't the case:
At src/backend/commands/tablecmds.c:17958-17964 (ATExecAddInherit)
This is not completely bulletproof because of race conditions: in
multi-level inheritance trees, someone else could concurrently be
making another inheritance link that closes the loop but does not join
either of the rels we have locked. ... find_all_inheritors() will
cope with circularity anyway, so don't sweat it too much."
This can be reproduced a parallel alter, e.g.:
With r->b, b->c, d->e already in place:
S1: begin; alter table d inherit c; -- AEL(d), SUE(c), AS(d,e)
S2: begin; alter table b inherit e; -- AEL(b), SUE(e), AS(b,c)
S1: commit;
S2: commit;
And then alters can either hit the
Assert(list_length(ordered) == list_length(agenda));
assertion in debug builds or end up with a inconsistent results in
release builds.
