Performance of a large array access by position (tested version 9.1.3)

Hi all,

May be I completely wrong but I always assumed that the access speed to the
array element in PostgreSQL should be close to constant time.
But in tests I found that access speed degrade as O(N) of array size.

Test case (performed on large not busy server with 1GB work_mem to ensure I
working with memory only):

WITH
t AS (SELECT ARRAY(SELECT * FROM generate_series(1,N)) AS _array)
SELECT count((SELECT _array[i] FROM t)) FROM generate_series(1,10000) as
g(i);

Results for N between 1 and 10.000.000 (used locally connected psql with
\timing):

N: Time:
1 5.8ms
10 5.8ms
100 5.8ms
1000 6.7ms
--until there all reasonable
5k 21ms
10k 34ms
50k 177ms
100k 321ms
500k 4100ms
1M 8100ms
2M 22000ms
5M 61000ms
10M 220000ms = 22ms to sinlge array element access.

Is that behaviour is correct?

PS: what I actually lookin for - constant fast access by position
tuplestore for use with recursive queries and/or pl/pgsql, but without
using C programming.

--
Maxim Boguk
Senior Postgresql DBA.

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