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