From: Björn Wittich <Bjoern_Wittich@gmx.de>
To: pgsql-performance@postgresql.org
Subject: Re: extremly bad select performance on huge table
Date: Tue, 21 Oct 2014 21:32:03 +0200
Message-ID: <5446B4B3.4030802@gmx.de> (raw)
In-Reply-To: <9706.1413918462@sss.pgh.pa.us>
References: <5446972B.9040409@gmx.de>
<9706.1413918462@sss.pgh.pa.us>
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Hi Tom and Igor,
thanks for your help. With the reindex the select query running time was
reduced from 5200 sec to 130 sec. Impressive!
Even a join on this table is now fast.
Unfortunately, there is now another problem: The table in my example has
500 columns which I want to retrieve with my join command.
Example which is fast "select value from smallertable inner join
myhugetable ON smallertable.mycolumn = myhugetable.mycolumn"
Example which is slow "select value,c1,c2,c3,...,c10 from smallertable
inner join myhugetable ON smallertable.mycolumn = myhugetable.mycolumn"
Which is the number of columns to fetch so bad ? Which action is done in
the db system when querying this via pgadmin? I think that there is no
real retrieval included, why is the number of additional columns so bad
for the join performance?
> =?ISO-8859-15?Q?Bj=F6rn_Wittich?= <Bjoern_Wittich@gmx.de> writes:
>> Here is the explain (analyze,buffers) select mycolumn from myhugetable
>> "Index Only Scan using myprimkey on myhugetable (cost=0.00..8224444.82
>> rows=71768080 width=33) (actual time=16.722..2456300.778 rows=71825999
>> loops=1)"
>> " Heap Fetches: 356861"
>> " Buffers: shared hit=71799472 read=613813"
>> "Total runtime: 2503009.611 ms"
> So that works out to about 4 msec per page fetched considering only I/O
> costs, which is about as good as you're likely to get if the data is
> sitting on spinning rust.
>
> You could potentially make it faster with a VACUUM (to mark all pages
> all-visible and eliminate the "heap fetches" costs), or a REINDEX
> (so that the index scan becomes more nearly sequential instead of random
> access). However, unless the data is nearly static those will just be
> temporary fixes: the time will degrade again as you update the table.
>
>> Note: This select is just for testing. My final statement will be a join
>> on this table via the "mycolumn" column.
> In that case it's probably a waste of time to worry about the performance
> of this query as such. In the first place, a join is not likely to use
> the index at all unless it's fetching a relatively small number of rows,
> and in the second place it seems unlikely that the join query can use
> an IndexOnlyScan on this index --- I imagine that the purpose of the join
> will require fetching additional columns.
>
> regards, tom lane
>
>
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Subject: Re: extremly bad select performance on huge table
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