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Re: : Cost calculation for EXPLAIN output

From: Shaun Thomas <sthomas(at)peak6(dot)com>
To: Venkat Balaji <venkat(dot)balaji(at)verse(dot)in>
Cc: PGSQL Performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: : Cost calculation for EXPLAIN output
Date: 2012-02-23 16:56:22
Message-ID: 4F466FB6.8070609@peak6.com (view raw or flat)
Thread:
Lists: pgsql-performance
On 02/23/2012 06:21 AM, Venkat Balaji wrote:

> The cost is "13.88" to fetch 1 row by scanning an Primary Key
> indexed column.
>
> Isn't the cost for fetching 1 row is too high ?

Not really. The "cost" is really just an estimate to rank alternate 
query plans so the database picks the least expensive plan. The number 
'13.88' is basically meaningless. It doesn't translate to any real-world 
equivalent. What you actually care about is the execution time. If it 
takes 0.25ms or something per row, that's what really matters.

For what it's worth, it looks like you have the right query plan, there. 
Scan the primary key for one row. What's wrong with that? Our systems 
have tables far larger than yours, handling 300M queries per day that 
are far more expensive than a simple primary key index scan. You'll be 
fine. :)

I suggest you set log_min_duration_statement to something like 1000, to 
send any query that takes longer than 1 second to the PG logs. 
Concentrate on those queries, because ones like this are already working 
right.

-- 
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 500 | Chicago IL, 60604
312-444-8534
sthomas(at)peak6(dot)com

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