Thread

Re: Small optimization with expanding dynamic hash table

Rahila Syed <rahilasyed90@gmail.com> — 2025-07-08T06:30:14Z

Hi,


> If I understand correctly, we only need to check the specific bit
> to determine whether a hash element needs relocation:
>

> diff --git a/src/backend/utils/hash/dynahash.c
> b/src/backend/utils/hash/dynahash.c
> index 1ad155d446e..32fbae71995 100644
> --- a/src/backend/utils/hash/dynahash.c
> +++ b/src/backend/utils/hash/dynahash.c
> @@ -1626,7 +1626,7 @@ expand_table(HTAB *hashp)
>                  currElement = nextElement)
>         {
>                 nextElement = currElement->link;
> -               if ((long) calc_bucket(hctl, currElement->hashvalue) ==
> old_bucket)
> +               if (!(currElement->hashvalue & (hctl->low_mask + 1)))
>                 {
>                         *oldlink = currElement;
>                         oldlink = &currElement->link;
>
>
Will this still work if new_bucket is not equal to hctl->low_mask + 1?

The above situation seems possible because we increment new_bucket every
time expand_table is called,
but we only update low_mask when new_bucket exceeds high_mask.

This change has successfully passed the tests on Github CI. According to
[1], the code has decent test coverage,
but I'm not certain if the specific case mentioned above is included in the
tests.

[1] LCOV - PostgreSQL 19devel - src/backend/utils/hash/dynahash.c
<https://coverage.postgresql.org/src/backend/utils/hash/dynahash.c.gcov.html>

Thank you,
Rahila Syed