Re: optimize lookups in snapshot [sub]xip arrays

On Tue, Aug 9, 2022 at 4:37 AM Nathan Bossart <nathandbossart(at)gmail(dot)com> wrote:
>
> On Mon, Aug 08, 2022 at 12:56:28PM +0530, Bharath Rupireddy wrote:
> > 1) pg_lfind32 - why just uint32? If it's not possible to define
> > functions for char, unsigned char, int16, uint16, int32, int64, uint64
> > and so on, can we add a few comments around that? Also, the comments
> > can talk about if the base type or element data type of array or data
> > type of key matters to use pg_lfind32.
>
> I figured that we'd add functions for other types when needed. I
> considered making the new function generic by adding an argument for the
> element size. Then, we could branch to optimized routines based on the
> element size (e.g., pg_lfind() would call pg_lfind32() if the element size
> was 4 bytes). However, that seemed like more complexity than is required,
> and it's probably nice to avoid the extra branching.
>
> > 3) Can pg_lfind32 return the index of the key found, for instance to
> > use it for setting/resetting the found element in the array?
>
> As discussed upthread, only returning whether the element is present in the
> array is slightly faster. If we ever needed a version that returned the
> address of the matching element, we could reevaluate whether this small
> boost was worth creating a separate function or if we should just modify
> pg_lfind32() to be a tad slower. I don't think we need to address that
> now, though.

Isn't it a good idea to capture the above two points as comments in
port/pg_lfind.h just to not lose track of it? I know these are present
in the hackers thread, but having them in the form of comments helps
developers who attempt to change or use the new function.

--
Bharath Rupireddy
RDS Open Source Databases: https://aws.amazon.com/rds/postgresql/