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Re: : Cost calculation for EXPLAIN output - Mailing list pgsql-performance

From Venkat Balaji
Subject Re: : Cost calculation for EXPLAIN output
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Msg-id CAFrxt0jaySGjjYg5VrVjKHR=8+rad=Yf9-okEM-r9NKXDwz47Q@mail.gmail.com
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In response to Re: : Cost calculation for EXPLAIN output  (Shaun Thomas <sthomas@peak6.com>)
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Thanks for your valuable inputs !

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. :)

Execution time is 0.025 ms per row. I am quite happy with the execution time, even the plan is going the correct way. The total execution time reduced from 2.5 hrs to 5.5 seconds. I am looking for a room for further improvement -- probably quite ambitious :-) ..

But, when i reduced random_page_cost to 2, the cost reduced to 7.03 and execution also have reduced by almost 50%. Is this an gain ?

Please comment !

Thanks,
VB

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