Re: speeding up planning with partitions

On Sat, 12 Jan 2019 at 21:49, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> wrote:
> > Can you say what you think is wrong with this way of making a copy of the ECs?
>
> If you shallow copy with memcpy(new_ec, ec,
> sizeof(EquivalenceClass));, then fields such as ec_relids will just
> point to the same memory as the parent PlannerInfo's
> EquivalenceClasses, so when you do your adjustment (as above) on the
> child eclasses, you'll modify memory belonging to the parent. I'd
> assume you'd not want to do that since you need to keep the parent
> intact at least to make copies for other child rels. You'd have
> gotten away with it before since you performed a list_copy() and only
> were deleting the parent's EMs with list_delete_cell() which was
> modifying the copy, not the original list. Since you were missing the
> alteration to ec_relids, then you might not have seen issues with the
> shallow copy, but now that you are changing that field, are you not
> modifying the ec_relids field that still set in the parent's
> PlannerInfo? In this instance, you've sidestepped that issue by using
> bms_difference() which creates a copy of the Bitmapset and modifies
> that. I think you'd probably see issues if you tried to use
> bms_del_members(). I think not doing the deep copy is going to give
> other people making changes in this are a hard time in the future.

Setting to waiting on author pending clarification about shallow vs
deep copying of ECs.

--
David Rowley http://www.2ndQuadrant.com/
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