Обсуждение: Performance Problem Index Ignored, but why

I've the task of porting a current Oracle application to PostgreSQL.

Database: 7.2.1
OS: Linux 2.4.9-13smp

I've an odd thing happening with a query.  Using a simple table:

            Table "state_tst"
Column  |         Type         | Modifiers
---------+----------------------+-----------
id      | integer              | not null
v_state | character varying(2) |
f_state | character(2)         |
Indexes: st_f_state_idx,
    st_v_state_idx
Primary key: state_tst_pkey

id is a sequence number and primary key, v_state and f_state are 2
character U.S. States.  I created v_state as varchar(2) and f_state as
char(2) to test if the query explained/performed differently (it
doesn't).

CREATE INDEX st_v_state_idx ON state_tst USING btree (v_state);
CREATE INDEX st_f_state_idx ON state_tst USING btree (f_state);

Load the table using a copy from ...

vacuum verbose analyze state_tst;

Total rows: 14309241

Queries using either f_state = or v_state =  explain (and appear to
execute) using a sequential scan.  Resulting in 60 - 80 second query
times.

Can I force the use of an index?  Or do I have something wrong?  Any
ideas?

pg_test=# explain select  count(*) from state_tst where f_state = 'PA';
NOTICE:  QUERY PLAN:

Aggregate  (cost=277899.65..277899.65 rows=1 width=0)
  ->  Seq Scan on state_tst  (cost=0.00..277550.51 rows=139654
  width=0)

EXPLAIN

I'm not sure how well this works in Postgres.  For Informix, we could trick
the optimizer into using an index with something like:

Select colname from table where colname !=NULL.

Specifically mentioning the column in the query was the trick.  In esql/C,
there were return parameters that then told you how many rows were found.


>CREATE INDEX st_v_state_idx ON state_tst USING btree (v_state);
>CREATE INDEX st_f_state_idx ON state_tst USING btree (f_state);
>
>Load the table using a copy from ...
>
>vacuum verbose analyze state_tst;
>
>Total rows: 14309241
>
>Queries using either f_state = or v_state =  explain (and appear to
>execute) using a sequential scan.  Resulting in 60 - 80 second query
>times.
>
>Can I force the use of an index?  Or do I have something wrong?  Any




>ideas?
>
>pg_test=# explain select  count(*) from state_tst where f_state = 'PA';
>NOTICE:  QUERY PLAN:
>
>Aggregate  (cost=277899.65..277899.65 rows=1 width=0)
>   ->  Seq Scan on state_tst  (cost=0.00..277550.51 rows=139654
>   width=0)
>
>EXPLAIN
>
>---------------------------(end of broadcast)---------------------------
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