From: | Josh Berkus <josh(at)agliodbs(dot)com> |
---|---|
To: | Robert Haas <robertmhaas(at)gmail(dot)com>, Peter Geoghegan <pg(at)heroku(dot)com> |
Cc: | "ktm(at)rice(dot)edu" <ktm(at)rice(dot)edu>, Tan Tran <tankimtran(at)gmail(dot)com>, Greg Stark <stark(at)mit(dot)edu>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: GSoC on WAL-logging hash indexes |
Date: | 2014-04-30 17:58:29 |
Message-ID: | 536139C5.7070601@agliodbs.com |
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Lists: | pgsql-advocacy pgsql-hackers pgsql-students |
All,
(dropping pgsql-advocacy off the cc list)
On 04/30/2014 10:11 AM, Robert Haas wrote:
> On Wed, Apr 30, 2014 at 12:54 PM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:
>> On Wed, Apr 30, 2014 at 5:55 AM, ktm(at)rice(dot)edu <ktm(at)rice(dot)edu> wrote:
>>> I do not think that CPU costs matter as much as the O(1) probe to
>>> get a result value specifically for very large indexes/tables where
>>> even caching the upper levels of a B-tree index would kill your
>>> working set in memory. I know, I know, everyone has so much memory
>>> and can just buy more... but this does matter.
>>
>> Have you actually investigated how little memory it takes to store the
>> inner pages? It's typically less than 1% of the entire index. AFAIK,
>> hash indexes are not used much in any other system. I think MySQL has
>> them, and SQL Server 2014 has special in-memory hash table indexes for
>> in memory tables, but that's all I can find on Google.
Hash indexes are more important for MySQL because they have
index-organized tables.
> I thought the theoretical advantage of hash indexes wasn't that they
> were smaller but that you avoided a central contention point (the
> btree root).
Yes. And being smaller isn't insignificant; think of billion-row tables
with fairly random access over the whole table. Also, *theoretically*,
a hash index could avoid the rebalancing issues which cause our btree
indexes to become bloated and need a REINDEX with certain update patterns.
--
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com
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