On 22/06/12 09:02, Maxim Boguk wrote:
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.
Default column storage is to "compress it, and store in TOAST" with large values.
This it what is causing the shift. Try to change the column storage of the column
to EXTERNAL instead and rerun the test.
ALTER TABLE <tablename> ALTER COLUMN <column name> SET STORAGE EXTERNAL
Default is EXTENDED which runs compression on it, which again makes it hard to
position into without reading and decompressing everything.
http://www.postgresql.org/docs/9.1/static/sql-altertable.html Let us know what you get.?
Jesper
