charting performance measures with number or records in table

We had a discussion with my friend about whether to use an array or an
attached table and I was in favor of the attached table while he was
concerned about the the performance of the select/insert as the number
or records in the attached table grew and so favored to use an array in
the parent table.

To persuade him I wanted to see how the time required to select or
insert records increased as the number of rows in the table grew. I was
less interested in the actual time as it is very hardware dependent more
interested in the trend. I tried this with the following table:

CREATE TABLE "itemresponse" (
"testoccasionid" INTEGER NOT NULL,
"itemorder" INTEGER NOT NULL,
"placeholdertypeid" SMALLINT DEFAULT 1 NOT NULL,
"response_datatype" SMALLINT NOT NULL,
"response" TEXT,
CONSTRAINT "itemresponse_new_idx" PRIMARY KEY("testoccasionid",
"itemorder", "placeholdertypeid")
) WITHOUT OIDS;

SELECT * FROM itemresponse WHERE testoccasionid=1751
--returns 20 records

I tried this with 10^2, 10^3, 10^4, 10^5, 10^6, 10^7 records in the
table.
To my surprise neither the time for the select nor the time for the
insert (1000 additional records) increased measurably.
Can it be real or is it an artefact?

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On a more general note I think it would be usefull to make a
'theoretical' graph to illustrate the behaviour of an index. Probably
there is already one but I didn't find it.
Say there is a table:

CREATE TABLE "test" (
"id" INTEGER NOT NULL,
CONSTRAINT id_idx PRIMARY KEY("id")
) WITHOUT OIDS;

and there are 0, 10^1, 10^2, 10^3, 10^4, 10^5, 10^6, 10^7, 10^8, 10^9
records in it

- Select id from test Where id=99 - time in whatever unit
- Insert Into test (id) Values (99) - time in whatever unit
- Select count(id) from test - time in whatever unit
- Table size - kb=?
- Index size - kb=?
- omit or add whatever makes/doesn't make sence here (eg. memory
required to do the select?, time to vacuum?)

and the same thing without an index on the table. I think it would make
a good addition to the manual.

Its just a thought, let me know what you think.
Balázs