Hello,
I am confused. I have a table that has an incrementing primary key
id.
When I select max(id) from table is returns almost instantly but
when I select min(id) from table it takes longer than I want to
wait.
Shouldn't postgresql be able to quickly find the minimum id value
in the index?
pmacct=# explain select max(id)
from netflow;
QUERY
PLAN
--------------------------------------------------------------------------------------------------------------------------
Result (cost=1.13..1.14 rows=1 width=0)
InitPlan 1 (returns $0)
-> Limit (cost=0.71..1.13 rows=1 width=8)
-> Index Only Scan Backward using netflow_pkey on
netflow (cost=0.71..3799108784.10 rows=9123246080 width=8)
Index Cond: (id IS NOT NULL)
(5 rows)
pmacct=# explain select min(id)
from netflow;
QUERY
PLAN
-----------------------------------------------------------------------------------------------------------------
Result (cost=1.13..1.14 rows=1 width=0)
InitPlan 1 (returns $0)
-> Limit (cost=0.71..1.13 rows=1 width=8)
-> Index Only Scan using netflow_pkey on netflow
(cost=0.71..3799108784.10 rows=9123246080 width=8)
Index Cond: (id IS NOT NULL)
(5 rows)
pmacct=# \timing
Timing is on.
pmacct=# select max(id)
from netflow;
max
-------------
17547256873
(1 row)
Time: 0.626 ms
pmacct=# select min(id)
from netflow;
^CCancel request sent
ERROR: canceling statement due to user request
Time: 339114.334 ms
Table
"public.netflow"
Column | Type
|
Modifiers
----------------+-----------------------------+---------------------------------------------------------------------
id | bigint | not
null default nextval('netflow_id_seq'::regclass)
agent_id | bigint |
bytes | bigint |
stamp_inserted | timestamp without time zone | not
null default '0001-01-01 00:00:00'::timestamp without time
zone
stamp_updated | timestamp without time zone |
packets | integer | default
0
port_src | integer | default
0
port_dst | integer | default
0
ip_proto | smallint | default
0
tos | smallint | default
0
ip_src | inet | not
null default '0.0.0.0'::inet
ip_dst | inet | not
null default '0.0.0.0'::inet
Indexes:
"netflow_pkey" PRIMARY KEY, btree (id)
"netflow_ts_key" btree (stamp_inserted)
"netflow_tsu_idx" btree (stamp_updated)
Triggers:
netflow_import AFTER INSERT OR UPDATE ON netflow
FOR EACH STATEMENT EXECUTE PROCEDURE netflow_update()
--
