Thread: Postgres does not utilize indexes. Why?

Hi everybody!

I can�t make use of indexes even I tried the same test by changing different settings in
postgres.conf like geqo to off/on & geqo related parameters, enable_seqscan off/on & so on. Result
is the same.

Here is test itself:

I�ve created simplest table test and executed the same statement �explain analyze select id from
test where id = 50000;� Few times I added 100,000 records, applied vacuum full; and issued above
explain command.
Postgres uses sequential scan instead of index one.
Of cause Time to execute the same statement constantly grows. In my mind index should not allow
time to grow so much.

Why Postgres does not utilizes primary unique index?
What I�m missing? It continue growing even there are 1,200,000 records. It should at least start
using index at some point.


Details are below:
100,000 records:
QUERY PLAN
----------------------------------------------------------------------------------------------------
 Seq Scan on test  (cost=0.00..2427.00 rows=2 width=8) (actual time=99.626..199.835 rows=1
loops=1)
   Filter: (id = 50000)
 Total runtime: 199.990 ms

200,000 records:
QUERY PLAN
-----------------------------------------------------------------------------------------------------
 Seq Scan on test  (cost=0.00..4853.00 rows=2 width=8) (actual time=100.389..402.770 rows=1
loops=1)
   Filter: (id = 50000)
 Total runtime: 402.926 ms


300,000 records:
QUERY PLAN
-----------------------------------------------------------------------------------------------------
 Seq Scan on test  (cost=0.00..7280.00 rows=1 width=8) (actual time=100.563..616.064 rows=1
loops=1)
   Filter: (id = 50000)
 Total runtime: 616.224 ms
(3 rows)

I've created test table by script:

CREATE TABLE test
(
  id int8 NOT NULL DEFAULT nextval('next_id_seq'::text) INIQUE,
  description char(50),
  CONSTRAINT users_pkey PRIMARY KEY (id)
);

CREATE SEQUENCE next_id_seq
  INCREMENT 1
  MINVALUE 1
  MAXVALUE 10000000000
  START 1
  CACHE 5
  CYCLE;

I use postgres 7.4.2






=====
Thanks a lot
Igor Artimenko
I specialize in
Java, J2EE, Unix, Linux, HP, AIX, Solaris, Progress, Oracle, DB2, Postgres, Data Modeling



__________________________________
Do you Yahoo!?
New and Improved Yahoo! Mail - Send 10MB messages!
http://promotions.yahoo.com/new_mail

Artimenko Igor wrote:
>   id int8 NOT NULL DEFAULT nextval('next_id_seq'::text) INIQUE,

ID column is bigint, but '50000' is int, therefore the index does not
match. You need to cast your clause like this:

select id from test where id = 50000::int8

Also, issue VACUUM ANALYZE, so Postgres knows about the structure of the
data.

--
Michal Taborsky
http://www.taborsky.cz

> test where id = 50000;” Few times I added 100,000 records, applied

    cast the 50000 to int8 and it will use the index