Hi, David Rowley <dgrowle...@gmail.com> writes:
> On Thu, 18 Jan 2024 at 14:58, Andy Fan <zhihuifan1...@163.com> wrote: >> I want to know if "user just want to zero out the flags in bitmapset >> but keeping the memory allocation" is a valid requirement. Commit >> 00b41463c makes it is hard IIUC. > > Looking at your patch, I see: > > +/* > + * does this break commit 00b41463c21615f9bf3927f207e37f9e215d32e6? > + * but I just found alloc memory and free the memory is too bad > + * for this current feature. So let see ...; > + */ > +void > +bms_zero(Bitmapset *a) > > I understand the requirement here, but, to answer the question in the > comment -- Yes, that does violate the requirements for how an empty > set is represented and as of c7e5e994b and possibly earlier, any > subsequent Bitmapset operations will cause an Assert failure due to > the trailing zero word(s) left by bms_zero(). Thanks for the understanding and confirmation. >> Looks like this is another user case of "user just wants to zero out the >> flags in bitmapset but keeping the memory allocation". > > I don't really see a way to have it work the way you want without > violating the new representation of an empty Bitmapset. Per [1], > there's quite a performance advantage to 00b41463c plus the additional > don't allow trailing empty words rule done in a8c09daa8. So I don't > wish the rules were more lax. I agree with this. > > Maybe Bitmapsets aren't the best fit for your need. Maybe it would be > better to have a more simple fixed size bitset that you could allocate > in the same allocation as the TupleTableSlot's tts_null and tts_values > arrays. The slot's TupleDesc should know how many bits are needed. I think this is the direction we have to go. There is no bitset struct yet, so I prefer to design it as below, The APIs are pretty similar with Bitmapset. typdef char Bitset; Bitset *bitset_init(int size); void bitset_add_member(Bitset *bitset, int x); void bitset_del_member(Bitset *bitset, int x); Bitset *bitset_add_members(Bitset *bitset1, Bitset *bitset2); bool bitset_is_member(int x); int bitset_next_member(Bitset *bitset, int i); Bitset *bitset_clear(); After this, we may use it for DiscreteKnapsack as well since the num_items is fixed as well and this user case wants the memory allocation is done only once. -- Best Regards Andy Fan
