Horton

Re: Change GUC hashtable to use simplehash?

Jeff Davis <pgsql@j-davis.com>

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From: Jeff Davis <pgsql@j-davis.com>
To: Andres Freund <andres@anarazel.de>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Gurjeet Singh <gurjeet@singh.im>, pgsql-hackers@postgresql.org
Date: 2023-11-18T00:01:31Z
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

On Fri, 2023-11-17 at 15:27 -0800, Andres Freund wrote:
> At
> first I thought that that's largely because you aren't using
> SH_STORE_HASH.

I might want to use that in the search_path cache, then. The lookup
wasn't showing up much in the profile the last I checked, but I'll take
a second look.

> Then I noticed that memory usage was too large when creating many
> GUCs - a bit
> of debugging later, I figured out that that's due to guc_name_hash()
> being
> terrifyingly bad. There's no bit mixing whatsoever!

Wow.

It seems like hash_combine() could be more widely used in other places,
too? Here it seems like a worse problem because strings really need
mixing, and maybe ExecHashGetHashValue doesn't. But it seems easier to
use hash_combine() everywhere so that we don't have to think about
strange cases.

> I think, independent of this patch, it might be worth requiring that
> hash
> table lookups applied the transformation before the lookup. A
> comparison
> function this expensive is not great...

The requested name is already case-folded in most contexts. We can do a
lookup first, and if that fails, case-fold and try again. I'll hack up
a patch -- I believe that would be measurable for the proconfigs.

Regards,
	Jeff Davis