Horton

Re: Hash Indexes

Andres Freund <andres@anarazel.de>

View thread View on postgr.es
From: Andres Freund <andres@anarazel.de>
To: Oskari Saarenmaa <os@ohmu.fi>
Cc: Robert Haas <robertmhaas@gmail.com>, Bruce Momjian <bruce@momjian.us>, Jeff Janes <jeff.janes@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2016-09-21T22:29:54Z
Lists: pgsql-hackers 
On 2016-09-21 19:49:15 +0300, Oskari Saarenmaa wrote:
> 21.09.2016, 15:29, Robert Haas kirjoitti:
> > For PostgreSQL, I expect the benefits of improving hash indexes to be
> > (1) slightly better raw performance for equality comparisons and (2)
> > better concurrency.
> 
> There's a third benefit: with large columns a hash index is a lot smaller on
> disk than a btree index.  This is the biggest reason I've seen people want
> to use hash indexes instead of btrees.  hashtext() btrees are a workaround,
> but they require all queries to be adjusted which is a pain.

Sure. But that can be addressed, with a lot less effort than fixing and
maintaining the hash indexes, by adding the ability to do that
transparently using btree indexes + a recheck internally.  How that
compares efficiency-wise is unclear as of now. But I do think it's
something we should measure before committing the new code.

Andres

Commits

  1. Remove _hash_wrtbuf() in favor of calling MarkBufferDirty().

  2. Fix race introduced by 6d46f4783efe457f74816a75173eb23ed8930020.

  3. Improve hash index bucket split behavior.

  4. Reduce pinning and buffer content locking for btree scans.