Re: Why we lost Uber as a user

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Josh Berkus <josh(at)agliodbs(dot)com>
Cc: "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Why we lost Uber as a user
Date: 2016-07-26 21:56:12
Message-ID: 20160726215612.GC27321@momjian.us
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On Tue, Jul 26, 2016 at 02:26:57PM -0700, Josh Berkus wrote:
> On 07/26/2016 01:53 PM, Josh Berkus wrote:
> > The write amplification issue, and its correllary in VACUUM, certainly
> > continues to plague some users, and doesn't have any easy solutions.
>
> To explain this in concrete terms, which the blog post does not:
>
> 1. Create a small table, but one with enough rows that indexes make
> sense (say 50,000 rows).
>
> 2. Make this table used in JOINs all over your database.
>
> 3. To support these JOINs, index most of the columns in the small table.
>
> 4. Now, update that small table 500 times per second.
>
> That's a recipe for runaway table bloat; VACUUM can't do much because
> there's always some minutes-old transaction hanging around (and SNAPSHOT
> TOO OLD doesn't really help, we're talking about minutes here), and
> because of all of the indexes HOT isn't effective. Removing the indexes
> is equally painful because it means less efficient JOINs.
>
> The Uber guy is right that InnoDB handles this better as long as you
> don't touch the primary key (primary key updates in InnoDB are really bad).
>
> This is a common problem case we don't have an answer for yet.

Or, basically, we don't have an answer to without making something else
worse.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ As you are, so once was I. As I am, so you will be. +
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