Thread

Re: [PATCH] Optionally record Plan IDs to track plan changes for a query

Lukas Fittl <lukas@fittl.com> — 2025-02-05T02:16:07Z

Hi Andrei,

On Fri, Jan 24, 2025 at 1:23 AM Andrei Lepikhov <lepihov@gmail.com> wrote:

> I may not be close to the task monitoring area, but I utilise queryId
> and other tools to differ plan nodes inside extensions. Initially, like
> queryId serves as a class identifier for queries, plan_id identifies a
> class of nodes, not a single node. In the implementation provided here,
> nodes with the same hash can represent different subtrees. For example,
> JOIN(A, JOIN(B,C)) and JOIN(JOIN(B,C),A) may have the same ID.
>

> Moreover, I wonder if this version of plan_id reacts to the join level
> change. It appears that only a change of the join clause alters the
> plan_id hash value, which means you would end up with a single hash for
> very different plan nodes. Is that acceptable? To address this, we
> should consider the hashes of the left and right subtrees and the hashes
> of each subplan (especially in the case of Append).
>

I looked back at this again just to confirm we're not missing anything:

I don't think any of the posted patch versions (including the just shared
v4) have a problem with distinguishing two plans that are very similar but
only differ in JOIN order. Since we descend into the inner/outer plans via
the setrefs.c treewalk, the placement of JOIN nodes vs other nodes should
cause a different plan jumble (and we include both the node tag for the
join/scan nodes, as well as the RT index the scans point to in the jumble).

Do you have a reproducer that shows these two generate the same plan ID?

Thanks,
Lukas

-- 
Lukas Fittl