I have the following setup:
A table called hand:
Table "stage.hand_meta"
Column | Type |
Modifiers
---------------+--------------------------+-------------------------------------------------------------
hand_id | integer | not null default
nextval('hand_meta_hand_id_seq'::regclass)
hand_no | bigint | not null
site_id | smallint | not null
game_id | smallint | not null
time | timestamp with time zone | not null
tournament_id | bigint |
Indexes:
"hand_meta_pkey" PRIMARY KEY, btree (hand_id) CLUSTER
"hand_meta_hand_no_site_unq" UNIQUE, btree (hand_no, site_id)
"hand_meta_time_idx" btree ("time")
"hand_meta_tournament_id_idx" btree (tournament_id)
Referenced by:
TABLE "handhistory_plain" CONSTRAINT
"handhistory_plain_hand_id_fkey" FOREIGN KEY (hand_id) REFERENCES
hand_meta(hand_id)
TABLE "handhistory_staged" CONSTRAINT "staged_hand_hand_id_fkey"
FOREIGN KEY (hand_id) REFERENCES hand_meta(hand_id)
Getting the max hand_id (primary key) results in using an index:
feiketracker=> explain analyze select max(hand_id) from stage.hand;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------
Result (cost=0.03..0.04 rows=1 width=0) (actual time=0.379..0.383
rows=1 loops=1)
InitPlan 1 (returns $0)
-> Limit (cost=0.00..0.03 rows=1 width=4) (actual
time=0.337..0.340 rows=1 loops=1)
-> Index Scan Backward using hand_meta_pkey on hand_meta
(cost=0.00..82667.12 rows=2479440 width=4) (actual time=0.319..0.319
rows=1 loops=1)
Index Cond: (hand_id IS NOT NULL)
Total runtime: 0.823 ms
(6 rows)
Now, if i create a view which left outer joins another table and
select max hand_id it uses a seq_scan, which I think it should'nt use,
as it only needs to query hand_meta and then use the index:
feiketracker=> create view seqscan_example as (select * from hand_meta
left join handhistory_plain using(hand_id));
CREATE VIEW
Time: 72.736 ms
feiketracker=> explain analyze select max(hand_id) from seqscan_example;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=49261.00..49261.01 rows=1 width=4) (actual
time=34672.052..34672.054 rows=1 loops=1)
-> Seq Scan on hand_meta (cost=0.00..43062.40 rows=2479440
width=4) (actual time=0.180..16725.109 rows=2479440 loops=1)
Total runtime: 34672.874 ms
(3 rows)
feiketracker=> select version();
version
------------------------------------------------------------------------------------------------------------------------------------
PostgreSQL 9.0.6 on armv5tejl-unknown-linux-gnueabi, compiled by GCC
gcc (GCC) 3.4.4 (release) (CodeSourcery ARM 2005q3-2), 32-bit
(1 row)
I cannot think of a reason to use a seqscan, the left join should
indicate all results from hand_meta should be used, hand_id is the
primary key, so selecting max(hand_id) from the table or the view
should result in the same execution plan or am I thinking wrong?
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