Re: Limit & offset effect on query plans - Mailing list pgsql-performance

From Kevin Grittner
Subject Re: Limit & offset effect on query plans
Date
Msg-id 20121213170700.80080@gmx.com
Whole thread Raw
In response to Limit & offset effect on query plans  (Amitabh Kant <amitabhkant@gmail.com>)
Responses Re: Limit & offset effect on query plans  (Tom Lane <tgl@sss.pgh.pa.us>)
List pgsql-performance
Tom Lane wrote:
> "Kevin Grittner" <kgrittn@mail.com> writes:

>> I ran some quick tests on my i7 under Linux. Plan time was
>> increased by about 40 microseconds (based on EXPLAIN runtime)
>> and added a limit node to the plan. Execution time on a SELECT *
>> FROM tenk1 in the regression database went up by 1.35 ms on
>> fully cached runs.
>
> 1.35ms out of what?

Without the limit node the runtimes (after "priming" the cache)
were:

1.805, 2.533
1.805, 2.495
1.800, 2.446
1.818, 2.470
1.804, 2.502

The first time for each run is "Total runtime" reported by EXPLAIN,
the second is what psql reported from having \timing on.

With the limit node:

3.237, 3.914
3.243, 3.918
3.263, 4.010
3.265, 3.943
3.272, 3.953

I eyeballed that in the console window and said 1.35 based on rough
in-my-head calculations, although with it laid out in a nicer
format, I think I was a little low.

-Kevin


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