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

hi
i just made tests which show following results:

 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

what exactly it is?
is shows that for a single-field table (field of type "data type"), you
get index-scan faster then seq-scan, only when you reach "rec#" records
in table.
for example:
for table:
create table test (field int4); and values inserted sequentially, you
will benefit from creating index only after you'll reach 97 records.
before this, index scan will be slower then seq-scan.

hope this helps some people.
feel free to ask questions.

depesz

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