Re: when to use index, and when not to us index - *the* answer :)

On Tue, Aug 13, 2002 at 10:31:03AM +1000, Martijn van Oosterhout wrote:
> Very interesting, but how large was the table? And is that time in seconds?
> Also, the results would be much more interesting on a table whose size was
> larger than the available memory (thus would not be completely stored within
> the cache).

number of records is shown in "rec#" column. showing results for *big*
tables doesn't make any sense, because there index-scan will always be
faster than seq-scan. my point was just to show *when* index scan
becomes faster than seq-scan.

And yes, i forgot to say. times are in seconds for 100 requests, ie.
"0.984096" means that 100 select took time (together) 0.984096 seconds.

> > data type: | rec# | idx-time | seq-time | diff.
> > --------------+------+----------+----------+----------
> > int4.rnd | 91 | 0.941282 | 0.984096 | 0.042814
> > int4.seq | 97 | 0.937176 | 0.988626 | 0.051450
> > int8.rnd | 135 | 0.946168 | 1.036118 | 0.089950
> > int8.seq | 65 | 0.941527 | 0.963275 | 0.021748
> > char4.rnd | 32 | 0.945356 | 0.965213 | 0.019857
> > char4.seq | 39 | 0.949922 | 0.997675 | 0.047753
> > char8.rnd | 61 | 0.963562 | 1.030349 | 0.066787
> > char8.seq | 35 | 0.991704 | 1.002345 | 0.010641
> > char16.rnd | 37 | 0.978170 | 0.990919 | 0.012749
> > char16.seq | 17 | 0.975246 | 0.978144 | 0.002898
> > char32.rnd | 34 | 0.992438 | 0.998808 | 0.006370
> > char32.seq | 18 | 1.005228 | 1.020824 | 0.015596
> > char64.rnd | 66 | 1.042605 | 1.139358 | 0.096753
> > char64.seq | 20 | 1.066124 | 1.128776 | 0.062652
> > char128.rnd | 55 | 1.146153 | 1.187139 | 0.040986
> > char128.seq | 12 | 1.175579 | 1.215656 | 0.040077

depesz

--
hubert depesz lubaczewski http://www.depesz.pl/
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Mój Boże, spraw abym milczał, dopóki się nie upewnię, że naprawdę mam
coś do powiedzenia. (c) 1998 depesz