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Re: slow self-join query

From: Robert Poor <rdpoor(at)gmail(dot)com>
To: Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: slow self-join query
Date: 2012-03-18 14:37:24
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Lists: pgsql-performance
On Sat, Mar 17, 2012 at 23:07, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>wrote:

> Yeah try setting [work_mem] to something absurd like 500MB and see if the
> plan changes.

Suweet!  Sorting now runs in-memory, and that makes a big difference, even
when groveling over 1M records (under 12 seconds rather than 7 hours).
 Results in

On Sat, Mar 17, 2012 at 23:09, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>

> Also it looks like you're still not using the index on this:
> Subquery Scan u1 (cost=0.00..313.55 rows=50 width=4) (actual
> time=0.030..147.136 rows=10000 loops=1)
>    Filter: ((u1.type)::text = 'User::Twitter'::text)
> Are you sure you're using an indexable condition?

I know that users.type is indexed -- what would keep that from being
honored?  FWIW, I believe that all user.type fields are set to
User::Twitter, but that will change in the future.

On Sat, Mar 17, 2012 at 23:12, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>

> Also also this looks like it's the most expensive operation:
> Seq Scan on followings f2 (cost=0.00..93523.95 rows=5534395 width=8)
> (actual time=0.041..19365.834 rows=5535964 loops=1)
> I'm guessing the f2.follower_id isn't very selective?

Not 100% sure what you mean -- f2.follower_id is very sparse (compared to
f1.follower_id), but that's the point of this particular query.  But since
upping work_mem makes it run really fast, I'm not overly concerned about
this one.  Thanks for your help!

One last thought: I could re-cast this as a subquery / query pair, each
with a single join.  Am I correct in thinking that could make it really
easy on the planner (especially if the tables were properly indexed)?

Thanks again.

- r

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