Horton

Re: Change GUC hashtable to use simplehash?

Andres Freund <andres@anarazel.de>

View thread
From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: John Naylor <johncnaylorls@gmail.com>, Jeff Davis <pgsql@j-davis.com>, Gurjeet Singh <gurjeet@singh.im>, pgsql-hackers@postgresql.org
Date: 2023-11-22T21:22:21Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Silence warning in older versions of Valgrind

  2. Revert "Speed up tail processing when hashing aligned C strings, take two"

  3. Speed up tail processing when hashing aligned C strings, take two

  4. Teach fasthash_accum to use platform endianness for bytewise loads

  5. Add macro to disable address safety instrumentation

  6. Convert uses of hash_string_pointer to fasthash equivalent

  7. Speed up tail processing when hashing aligned C strings

  8. Add helper functions for dshash tables with string keys.

  9. Fix warnings in cpluspluscheck

  10. Further cosmetic review of hashfn_unstable.h

  11. Simplify initialization of incremental hash state

  12. Add optimized C string hashing

  13. Add inline incremental hash functions for in-memory use

  14. Make all Perl warnings fatal

Hi,

On 2023-11-22 15:56:21 -0500, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > On 2023-11-21 16:42:55 +0700, John Naylor wrote:
> >> The strlen call required for hashbytes() is not free. The lack of
> >> mixing in the (probably inlined after 0001) previous hash function can
> >> remedied directly, as in the attached:
>
> > I doubt this is a good hashfunction. For short strings, sure, but after
> > that...  I don't think it makes sense to reduce the internal state of a hash
> > function to something this small.
>
> GUC names are just about always short, though, so I'm not sure you've
> made your point?

With short I meant <= 6 characters (32 / 5 = 6.x). After that you're
overwriting bits that you previously set, without dispersing the "overwritten"
bits throughout the hash state.

It's pretty easy to create conflicts this way, even just on paper. E.g. I
think abcdefgg and cbcdefgw would have the same hash, because the accumulated
value passed to murmurhash32 is the same.

The fact that this happens when a large part of the string is the same
is bad, because it makes it more likely that prefixed strings trigger such
conflicts, and they're obviously common with GUC strings.

Greetings,

Andres Freund